Ecology of the Squash and Gourd Bee,

Peponapis pruinosa, on Cultivated

Cucurbits in California

(Hymenoptera: Apoidea)

PAUL D. HURD, JR., E. GORTON LINSLEY,

and A. E. MICHELBACHER

SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY • NUMBER 168
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SMITHSONIA N CONTRIBUTION S TO ZOOLOGY • NUMBE R 168

Ecology of the Squash and Gourd Bee,

Peponapis pruinosa, on Cultivated

Cucurbits in California

(Hymenoptera: Apoidea)

Paul D. Hurd, Jr., E. Gorton Linsley,

and A . E. Michelbacher

SMITHSONIAN INSTITUTION PRESS

City of Washington

1974
ABSTRACT

Hurd, Paul D., Jr., E. Gorton Linsley, and A. E. Michelbacher. Ecology of the

Squash and Gourd Bee, Peponapis pruinosa, on Cultivated Cucurbits in California (Hymenoptera: Apoidea). Smithsonian Contributions to Zoology, number

168, 17 pages, 4 figures, 8 tables, 1974.—This investigation focuses primarily on

those ecological aspects involved in the natural history of the squash and gourd

bee, Peponapis pruinosa, as it occurs in commercial squash and pumpkin

plantings of the Sacramento Valley of Calitornia. Information is presented on

this species' nesting habits, nest site selection, burrow construction, distribution

and density of nest cells, diurnal and seasonal nesting activities, competition at

the nesting site, diurnal and seasonal field activity, number of annual generations, and the results of transporting and introducing the adult bees into squash

and pumpkin plantings in geographical settings well removed from their natal

area.

OFFICIAL PUBLICATION DATE is handstamped in a limited number of initial copies and is recorded

in the Institution's annual report, Smithsonian Year. SI PRESS NUMBER 5002. SERIES COVER

DESIGN: The coral Montastrea cavernosa (Linnaeus).

Library of Congress Cataloging in Publication Data

Hurd, Paul David, 1921-

Ecology of the squash and gourd bee, Peponapis pruinosa, on cultivated cucurbits in California

(Hymenoptera: Apoidea).

(Smithsonian contributions to zoology, no. 168)

1. Peponapis pruinosa. 2. Insects—California. I. Linsley, Earle Gorton, 1910- joint author.

II. Michelbacher, A. E., joint author. III. Title. IV. Series: Smithsonian Institution. Smithsonian contributions to zoology, no. 168.

QL1.S54 no. 168 [QL568.A6] 591'.08s [595.7'99] 73-16424

For ule by the Superintendent of Documents, U.S. Government Printing Office

Washington. D.C. 20402 - Price 60 cents
Contents

Page

Introduction 1

Acknowledgments 2

Methods of Study 2

Nest Site Observations 2

Flower Site Observations 2

Study Sites 2

Results of Study S

General Nesting Habits 3

Selection of the Nesting Site 3

Burrow Construction 4

Distribution and Density of the Cells 6

Diurnal and Seasonal Nesting Activities 6

Activity at the Nesting Site 7

Competition at the Nesting Site 11

Diurnal and Seasonal Field Activity 11

Number of Annual Generations 14

Oxford Tract Experiments 14

Summary and Conclusions 16

Literature Cited 17

Ecology of the Squash and Gourd Bee,

Peponapis pruinosa, on Cultivated

Cucurbits in California

(Hymenoptera: Apoidea)

Paul D. Hurd, Jr., E. Gorton Linsley,

and A . E. Michelbacher

Introduction

The principal pollinators of plants of the genus

Cucurbita (gourds, squashes, and pumpkins) are

bees. Of these, the native squash and gourd bees

belonging to the genera Peponapis and Xenoglossa

are by far the most important, and their evolutionary history has been closely interrelated with

that of their cucurbit hosts (Hurd, Linsley and

Whitaker, 1971). These bees are solely dependent

upon Cucurbita for pollen and in the process of

gathering pollen and nectar from the flowers, pollination is accomplished. The bees are well adapted

for the collection of the large pollen grains of

Cucurbita and their diurnal flight period is synchronized with the opening cycle of the flowers.

Eleven species of squash and gourd bees occur

in the United States, six of Peponapis and five

of Xenoglossa (Hurd and Linsley, 1964). Two

members of each genus occur in southern California. One is typically a desert inhabitant (Peponapis timberlakei Hurd and Linsley), one primarily

a desert species extending to the coast (Xenoglossa

Paul D. Hurd, Jr., Department of Entomology, National Museum of Natural History, Smithsonian Institution, Washington, D.C. 20560. E. Gorton Linsley and A. E. Michelbacher,

Department of Entomological Sciences, University of California, Berkeley 94720.

angustior Cockerell), and two largely cismontane

and coastal (Peponapis pruinosa (Say) and Xenoglossa strenua (Cresson). The first two species

range from New Mexico and Arizona into Mexico

and southern California and are attached primarily

to the gourds Cucurbita digitata and C. palmata.

However, Xenoglossa angustior also takes pollen

from the gourd Cucurbita foetidissima and the cultivated squashes and pumpkins and has apparently

extended its range into the San Joaquin Valley

(Kern, Fresno, Tulare, Madera, Merced, and

Stanislaus counties), as these plants became available through farming and other activities of man.

Peponapis pruinosa and Xenoglossa strenua are

widely distributed species ranging transcontinentally from the Atlantic Coast to southwestern

United States and Mexico, taking pollen from the

widespread gourd Cucurbita foetidissima and all

of the cultivated species of squash and pumpkin.

Both species apparently spread eastward from the

Great Plains region with the development of agriculture, but only P. pruinosa has extended its

range significantly from southern California north

of the Tehachapi Mountains, the northern limit of

the presumed range of its wild host in California

before the advent of man. As presently known, its

California distribution includes coastal southern

California and the western desert margins and the

1
SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY

San Joaquin and Sacramento valleys of central

and northern California to at least as far north

centrally as Anderson, Shasta County, as far west

as Willits, Mendocino County, and east as far as

Susanville, Lassen County. It occurs most abundantly in valleys, not only where squashes and

pumpkins are grown, but virtually everywhere that

its native host, C. foetidissima, occurs. Although

Peponapis pruinosa is a relatively recent invader

in the northern part of California, it is well established and adapted in this region. Except in the

central and southern San Joaquin Valley, where

Xenoglossa angustior is present, the females have

no squash or gourd bee competitors for Cucurbita

pollen.

Although squash and gourd bees are the principal pollinators of cultivated Cucurbita, other

bees often play an important role. The most important of these is the introduced honeybee, not

only in areas where squash and gourd bees do not

occur, but also during the early- and late-season

periods when adult squash bees are not active.

When both squash bees and honeybees are present

in numbers, however, the former have a competitive advantage because of their somewhat larger

size, more rapid flight, and earlier period of diurnal

pollen-collecting activity.

ACKNOWLEDGMENTS.—We wish to express our

appreciation to Rea and Hal Gardner, Tim

Tokuno, and Robert Starkey, all of Gridley, California, for their cooperation in making available

to us their fields and gardens for the conduct of

these investigations. The studies would not have

been possible without free access to their properties.

For technical assistance and advice we are indebted to R. F. Smith and John Chemsak of the

Department of Entomological Sciences of the University of California, Berkeley. Juanita M. Linsley

(Mrs. E. G.) and Martha M. Michelbacher (Mrs.

A. E.) ably assisted in field studies and the recording of field notes. In the preparation of the manuscript valuable service was rendered by Celeste

Green and Kathleen Rentz, both staff members

of the Department of Entomological Sciences, University of California. Special thanks are due also

to John C. Kollenbaum, who supervised the care

of the experimental squash and pumpkin plantings on the Oxford Tract of the University of

California, Berkeley.

Methods of Study

NEST SITE OBSERVATIONS.—The seasonal pattern

of burrow construction was determined by periodic

counts of burrow entrances and tumuli in specified

areas of the nesting sites. The general level of

activity over the nest area was judged subjectively

at various times during the day throughout the

season. Time intervals for activity spent away from

and within the burrow were recorded for individual females, and the return of foraging females

noted for the presence or absence of pollen loads.

Such observations were made at various times of

day and during different portions of the season.

An indication of the distribution and density of

cells in the nest site was obtained during the winter

by excavating square-foot columns of soil, dissecting out the cells, and recording their number at

various levels below the surface. This also provided

data on survival as well as loss from mold or

disease.

FLOWER SITE OBSERVATIONS.—Population trends

and abundance of bees at the flowers of Cucurbita

were determined at various times of day throughout the season by two methods. The first involved

recording the number of bees by sex in each of

100 flowers selected at random. The second involved recording by sex the number of bees observed in randomly selected flowers during a 15-

minute interval. Experience revealed that early

in the season, data obtained by the two methods

were virtually identical since approximately 15

minutes was required to count the bees in 100

flowers, although at times a survey of 100 flowers

took somewhat longer. As the season progressed

and the density of the flowers increased, less time

was required to examine 100 flowers and 15-minute

counts yielded more individuals and more consistent data. The contrast became even greater

toward the end of the season, with a decline in

the bee population and the resultant reduction in

the time required to count the bees in 100 flowers.

Study Sites

Two prinicpal areas were utilized in the studies

reported here. A number of sites were selected in

the vicinity of Gridley, Butte County, California,

in the center of the Sacramento Valley. This is a

region having a deep rich soil, is largely under
NUMBER 168

irrigation, and is devoted to the growing of

orchard, field, and truck crops. It is an ideal area

for the study of Peponapis pruinosa because of the

dense concentration of host plants. The Gridley

area is not only a center for the growing of pumpkins for the canning industry, but contains numerous commercial plantings of both summer and

fall type squashes richly supplemented by many

home plantings. The large populations of squash

bees have been under observation since 1962 and

give every evidence of having been in existence

for many years previously.

The second study area is located at Berkeley,

Alameda County, on the Oxford Tract Agricultural Experiment Station plot adjacent to the

campus of the University of California. This site

was selected because continuous observations and

experiments could be conducted on planned Cucurbita plantings. Further, the climate of Berkeley is

modified by coastal influences and the cooler summer and warmer winter provide a useful contrast

with the continental climate at Gridley.

Results of Study

GENERAL NESTING HABITS.—Peponapis pruinosa

is a gregarious, ground-nesting bee. The burrows

are constructed in flat ground. The main shaft is

vertical or nearly so with single cells at the end

of downward-slanting laterals. At our nest sites the

burrows were constructed similarly to those described and figured by Mathewson (1968), but

were generally deeper.

SELECTION OF THE NESTING SITE.—Nesting sites of

Peponapis were discovered at Gridley in 1969 on

lawns near homes in a rural area. At the first site

the burrows were located in a lawn which had

been maintained for V2. years and in recent times

has been watered for four hours once a week. Certain areas in the lawn appeared to be more attractive for nesting than others. These were areas of

deep, relatively undisturbed soil with a sustained

supply of moisture. Lawn grass also attracts the

bees which avoid old dense stands of Bermuda

grass with nearly impenetrable mats of underground stolons or rhizomes.

In the first nesting site, the lawn covered 1600

square feet, but the burrows were mostly concentrated in one section. On 10 July 1969, 10

square-foot areas were selected at random throughout the lawn, the grass clipped, and the number

of burrows in each sample was counted. They

ranged from 0 to 11 per square foot with an average of 5.3. The variation primarily reflected variations in the density of old stands of Bermuda

grass.

At the second nesting site, burrows extended

well beyond the lawn area. Here the owner watered

the lawn each evening. As with the first site, nesting was uneven and in the lawn there was an area

where the concentration of nests was much greater

than elsewhere. In this respect the two nesting

sites closely paralleled each other. Unlike the first

location where the burrows were confined to areas

of dense grass, the bees were nesting in bare ground

in considerable numbers. However, there may be

a tendency for the females to seek some protection as burrows were found more concentrated

under shrubs, boards (see also Mathewson, 1968),

and at the base of clumps of grass. On 11 July 1969,

under the dead branches of a small juniper shrub

10 burrows were found in a square-foot area (Figure 1). In the bare open ground 10 square-foot

random samples were taken. In an arbitrarily selected site under a 12-inch plank, 10 burrows were

observed in a square-foot sample (Figure 2). Here

the number of burrows ranged from 0 to 7 with

an average of 1.8 per square foot. Thus, the evim tm

. : . *• • •

FIGURE 1.—Burrow entrances to nests of Peponapis pruinosa

(Say) located under dead branches of a small juniper shrub.
SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY

dence strongly indicates that objects that offer

some protection influence females seeking burrow

sites.

According to homeowners, the same nesting sites

had been used by the bees over an extended period

of years. Further, over an extended period of years

the areas of highest burrow concentration have

remained essentially the same. Both nesting sites

occur in a region of host abundance, consisting

mostly of pumpkin fields. These plantings were

either adjacent or close to the nesting sites.

BURROW CONSTRUCTION.—During late fall of 1970

a two-foot-wide cement walk was laid at the second

nesting site, separating the lawn from the bareground nesting area. The walk exerted a profound

influence on the nesting habit of the bee. In 1971

the females tended to concentrate their burrows

along the margins of the walk, especially on the

lawn side.

On 31 July 1971, the number of burrows per

square foot in the high density portion of the lawn

was determined from 15 random samples. The

number of burrows per sample ranged from 0 to

9 with an average of 4.46 per square foot.

On 30 August 1971, two square-foot samples

from the lawn side of the walk were examined and

in each there were 19 burrows. In each sample the

majority of burrows were located within three to

FIGURE 2.—Burrow entrances to nests of Peponapir pruinosa

(Say) located under 12-inch board planking.

four inches of the edge of the walk and were close

together, and some of them were interconnected.

That this situation occurred was indicated by

marked bees entering one burrow and emerging

from another.

Observations conducted on 31 August 1971 revealed that the nesting site extended well beyond

the lawn and the bare-soil area. In the early morning many bees were observed flying in with pollen

to burrows in an adjacent area covered with a

coarse grass (probably water grass).

Undoubtedly many nesting sites exist in the

region. For example, on 31 July 1971, another

site was discovered in the lawn of a farmstead located in a pumpkin field in the same restricted

area.

Further evidence that lawns are attractive as

nesting sites was obtained at Berkeley. Following

experimental introductions it was found that

marked females released in squash and pumpkin

plantings constructed their burrows in a lawn

across the street from the experimental plot

(Michelbacher, Hurd, and Linsley, 1971).

Burrow construction activity occurred and working females were observed from before daylight

until late evening. There is a possibility that it

might even extend throughout the night. During

the summer, nests in all stages of development are

present. Often tumuli are associated with them.

Sometimes the entrances are plugged or covered

with a mound, but many were free of excavated

soil. When females plug their burrows, they usually

do so about one-fourth of an inch below the surface.

On 31 July and 30 August 1971 and again on

1 July and 7 July 1972, several surveys of burrows

in a nest site were conducted (Table 5). In general the majority of burrows had tumuli and the

presence of a tumulus is an indication of recent

nest construction.

In the afternoon and early evenings there appears to be an increased activity in nest construction and repair. This may be because the bees

are no longer engaged in pollen collecting and

thus have more time to spend on nest development. This is indicated by an increase in fresh

tumuli deposited about the openings of the burrows which, due to moisture, is darker in color

than older deposits, and consequently is easy to

distinguish.
NUMBER 168

TABLE 1.—Burroius per square foot of surface in a

nesting site of Peponapis pruinosa at Gridley,

California, 10 July 1969

Sample Number

number of nests Condition of ground cover

1

2

3

4

5

6

7

8

9

10

Total

Average

0

0

8

11

3

10

2

11

0

8

53

5.S

Dense grass with some Bermuda grass

Thickly covered with Bermuda grass

Rye grass

Rye grass

Rye grass

Rye grass

Grass very dense and rank

Rye grass

Very dense with considerable Bermuda

grass

Rye grass

The practice of watering the lawn each evening

apparently exerted little or no adverse influence

upon the nesting bees. It is also possible that the

water tends to wash away the tumuli, although in

some cases the tumuli melt into chimney-like structures around the entrance to the burrows. The

morning alter watering, females returning with

pollen freely entered burrows with and without

tumuli. Apparently, they do not find it necessary

to make any repairs to the nest. Where tumuli

were present, it is certain that some of these had

been worked on after watering. Whether any of

this was in response to the watering is not known.

It has been determined, however, that the bees

will work on their burrows until at least 2000.

On 30 August 1971, the nest site was visited at

1905 during a light drizzle. The lawn had been

watered lightly at 1730 and because of darkness,

observations were made by flashlight. On arrival,

several bees were seen in their burrows. In order

to secure some measure of the amount of bee activity in their nests, two 15-minute observation

periods were conducted. In the first, which lasted

from 1910 to 1925, 11 bees were seen in their nests,

and in the second, 1925 to 1940, 7 bees were observed. Of interest, some bees were seen in their

burrows where there was no tumulus at the entrance. The use of lights did not appear to have

an adverse effect on the bees.

301-

25

O

"S

i. 15

E

3

X X

X X

o « living cells

• = empty oM cells

x = total of o+ »

o • • •

• O • OX * X

X o o o o o

• o o X

• o x • o x x

• ••O X X O

X o o • o o •

o • • x • «

• o • o o o • o o x x x

o *•*•«. • • *

, » • • • H •

* w *

10 15

l I

Soil Depth in Inches

20 25

FIGURE 3—Distribution at different levels of Peponapis pruinosa (Say), nest cells in a square-foot

column of soil.
SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY

The nesting site was revisited the next morning, 31 August, at 0440. The temperature at 0455

was 16°C and at 0515 it dropped to 15°C. A cool

breeze developed. Using flashlights, a search of

burrows for bee activity was conducted. From 0445

to 0500, three bees were seen and from 0500 to

0515 only two individuals were observed. This

low level of activity came as a surprise. It appears

that very early in the morning bee activity is near

a standstill. Foraging flight began at approximately

0508.

Where the soil is extra moist, the tumuli which

are developed are deposited as small pellets. Such

tumuli stand out in marked contrast to the more

normal ones made of fine soil particles.

When the tumulus formed a cone about the entrance to the burrow, the soil along the inner

surface of the cone was packed and smoothed in a

uniform manner.

DISTRIBUTION AND DENSITY OF THE CELLS.—In

order to secure some information on the distribution and number of cells, a square-foot column

of soil was excavated at a nest site at Gridley on

16 March and 17 March 1970. The cells were dissected out and the data obtained plotted in Figure

3. There were 176 cells with live individuals and

231 that were empty. Cells were encountered at

depths from about 3i/£ to 27 inches with the greatest concentration between 5 and 12 inches. Further

information on the abundance of cells was obtained 9 December 1971. Prepupae were wanted

for introduction investigations being conducted in

New Zealand. These were excavated in the bare

soil area at the nesting site where this zone was

separated from the lawn by a cement walk. Again

no cells were found in the first 3 inches of soil.

From this level the number of cells increased and

cells were found very abundant in the 8- to 12-inch

zone. As expected, the greatest concentration of

cells was next to the walk. This is the area where

the burrows were found in most abundance during the active nesting season. In all, 6 to 7 square

feet of soil were examined and 530 usable prepupae

were collected. In addition some were killed in

digging and many more probably escaped detection. Further, no attempt was made to recover

the prepupae at the lower level. Some individuals

infected with a mold or a disease were encountered.

They were usually in clusters and were relatively

few in comparison to the healthy individuals.

DIURNAL AND SEASONAL NESTING ACTIVITIES.—In

northern California, Peponapis pruinosa begins

field activity at about one-half to three-quarters

of an hour before sunrise. For example, on 31

August 1971, at a nesting site near Gridley, the

first bee was heard at 0508 hours. At this time

there was just sufficient light to vaguely distinguish the color of larger objects in the area.

On a number of occasions, nesting sites were

under observation in early morning to determine

when the first bees initiated foraging activity. Also,

observations were made in the fields to determine

the time of arrival of the bees at the flowers (Table

2). Field activity closely followed the time the

bees left their foraging area. Once activity is

initiated, it rapidly increases and in a few minutes

a constant hum develops about the nesting site.

This surge of activity is paralleled by expanding

activity at the flowers. The flight about the nesting areas is most impressive. This is due to the

TABLE 2.—Time of earliest flight activities of Peponapis pruinosa at nest site

and in the field at Gridley, California

Date

6-25-69

7-10-69

8-12-69

8-31-71

9-11-69

9-26-69

Sunrise

0448

0500

0540

0545

0552

0604

Time

0415

0410

0410

0508

0500

0515

Temperature

(°C)

14°

17°

16°

15°

17°

13°

First

bee

heard

0417

0405

0446

0508

0510

0542

Nesting site

First bee

returns

with pollen

0430

0419

0455

0520

_

0549

Elapsed

time

(minutes)

13

14

9

12

_

7

First

bee

heard

0417

0417*

0451

_

0515

0546

Field

First

bee in

flower

0417

0415

0453

_

0520

0549

Foraging start

(minutes

before sunrise)

31

55

54

37

42

22

• A steady hum over the flowers.
NUMBER 168

great concentration of burrows and the resulting

numbers of females flying in and out of their

burrows. Literally hundreds of females are involved and they appear to ignore persons standing in the area. Although there is a hivelike level

of activity, a female has never been observed to

attack a person. This, however, may not be of

much comfort to a homeowner if the nesting site

should occupy his lawn. It can produce an annoying situation, especially if there are children. If

a child should capture a female bee, there is every

likelihood of its being stung. Most parents would

hesitate to allow their children to venture into

this flight of bees, and unless informed are likely

to undertake eradication of the bees.

The beginning of seasonal adult activity varies

with the locality. Thus in the more coastal regions

of central California the start of the season is later

than in the hotter interior valleys. The bees are

out in numbers in the Gridley area long before

this situation prevails in the Berkeley region. Also,

the start of activity period in a given locality varies

with the season. As the season advances, the start

of the foraging period begins later in the morning. The season of 1971 was delayed and a survey

of two nesting sites at Gridley on 19 and 20 June

revealed little or no activity. Although many bees

had emerged, nest construction was not underway

to any appreciable degree. It would have been

impossible at that time to secure information of

the type presented in Table 2.

At the time the bees commence their foraging

activity, it is too dark to see what is occurring

without the use of lights. In general, the lights

appear to exert little influence on the bees. However, on rare occasions they react to them. On

10 July 1969, at 0400, a male bee flew to the lamp

at a nesting site. At 0403 two females came out

of their burrows but returned again. This was

followed at 0405 by a female which emerged and

flew away. At the same time another flew to the

light and then away. On this day it was barely

light enough to see in the flowers at 0420.

ACTIVITY AT THE NESTING SITE.—Upon emerging

the females leave the nesting area and seek nectar

and protection in the flowers of their host. There

is a period before the female is conditioned to

build a nest and start pollen collection. The length

of the prenesting period has not been fully determined. Studies by the present investigators indicate that it is of approximately two to three weeks'

duration. Mathewson (1968) investigated this bee

under Rhode Island conditions and reported that

both sexes are abundant on Cucurbita blossoms

during the 21 days following general emergence,

but there is no evidence of nest building or pollen

transport during this interval. It is this period that

has been utilized in our introduction experiments

since it is believed that a minimum of disruption

occurs if females are moved to new areas before

the nesting instinct develops and pollen collecting is initiated (Michelbacher, Hurd, and Linsley,

1968, 1971).

Thus in early season few if any bees may be

found in the nesting site, while large numbers of

both sexes are present in the field throughout the

day, since burrow construction lags two to three

weeks behind emergence. The 1971 season was a

late one which resulted in a delayed appearance

of the bees in comparison to other more normal

years. A survey on 19 and 20 June showed an

abundance of individuals in host flowers (see

Tables 6 and 7). Some of the females had some

pollen on the legs, but the amount was very small,

and they were not flying from flower to flower.

There was no evidence that they appeared to be

interested in collecting pollen, and none was seen

with a full pollen load. Observations at nesting

sites revealed no burrow construction. The 1970

season was a more normal one and yet as late as

7 July only one female was observed in a nesting

area. The 1972 season was early. In one of the

nesting sites, burrow activity was well underway

by 1 July. On 2 July, many burrows were under

construction and the intensity of this activity was

evident by the abundance of burrows with tumuli.

On this date, summer squash was flowering freely

and in production, but pumpkin, which is the

main host, had begun blooming in only one field.

Just after dawn on 2 July, bees were most active

about the nesting site, but their behavior was

different from that encountered later in the season

when pollen was in abundance. Although some

of the females were returning to the nesting site

with pollen, it soon became evident that there

was not sufficient pollen and probably nectar, from

squash flowers, to support the bee population

present in the nesting site. Often the pollen loads

appeared to be light, and the supply of pollen

was apparently so limited that there was no evi-
8

dence of rhythm to foraging flights. Many of the

females were working in their burrows, while

other remained in the flowers or were seeking

nectar.

Between 2 July and 7 July, the number of burrows more than doubled in the nesting areas

(Table 5). The available pollen still appeared to

be less than the bees could have utilized. At 0818

most of the females were coming in with pollen,

although there were some with none. Between

0835 and 0915 females with light to full pollen

loads were observed. Sometimes unequal distribution of the pollen on the scopa of individual bees

was noted. Of the females returning to the nesting

site between 1010 to 1030, 43 carried pollen, while

46 had none. In a similar survey from 1140 to

1200, 1 individual arrived with pollen and 57 were

without.

On 8 July, observations on the number of females returning to the nesting site with and without pollen were continued with the following results:

Time

0458-0513

0515-0530

0710-0725

Females

with

pollen

43

53

95

Females

without

pollen

3

10

3

Temperature

(°C)

13°

14°

18°

Since in each period some bees flew in without

pollen, it is very possible that these represented

individuals that were in the early stages of nest

construction. Further it should be noted that the

number of females entering their burrows represent

but a small proportion of the individuals milling

over the nesting site. However, the figures presented above do give a good index of the activity

and clearly show that it increased with a rise in

temperature.

Once nesting is initiated, activity in the nesting

site increases rapidly and nests can be found in

all stages of development, with females carrying

pollen, others working on their nests, and some

hovering as if searching for a site to build a nest.

Such a situation was encountered at nesting sites

on 24 and 25 June 1969.

Where the soil is moist, a female is capable of

constructing a burrow in a relatively short period.

On 9 July 1969, a female commenced a burrow

at 1035 and by 1105 the excavation penetrated to

the depth of one inch and a well-developed tumuSM1THSONIAN CONTRIBUTIONS TO ZOOLOGY

lus was formed. By 1250 the burrow entrance was

covered with a mound. Even by the middle of

July the great majority of females were collecting

pollen and this activity along with nest construction appeared to be the principal occupation. Male

bees are rarely encountered at the nesting area,

except for individuals that have just emerged.

In an attempt to obtain comparative information on the length of time females spend in their

burrows and in foraging, on 1, 30, and 31 August

1971, a number of nests were selected at random

for observation. These were watched during the

morning, spanning the period of major foraging

activity (Table 3). The 1 August survey was conducted from 0458 to 0730. Thirteen bees were involved and the foraging flights ranged from 1 to

25 minutes, with an average of 8.8 minutes. The

time spent in the burrow (excluding the one individual that never left the nest) ranged from 2

to 55 minutes, with an average of 7.6 minutes.

The survey on 30 August ran from 0730 to 1000

and involved 6 bees. The foraging flight ranged

from 1 to 31 minutes, with an average of 7.6

minutes. The time they remained in the burrow

varied from 1 to 45 minutes, with an average of

8.1 minutes. The 31 August survey included 7

individuals and surveillance was from 0530 to

0730. The foraging flight ranged from 4 to 20

minutes, with an average of 13.3 minutes. The

period in the burrow fluctuated between 4 and 31

minutes, with an average of 9.2 minutes.

The results obtained on the three surveys were

in rather close agreement. A considerable variation occurred in both the length of time spent in

the burrows and on foraging flights. On the 31

August survey, however, the length of the foraging

period was distinctly greater than that noted on

the two previous ones. The time spent in the burrows was also longer but the difference was less

marked. It is believed that the decreased activity

was in response to the relatively lower temperature

that occurred on the morning of 31 August. The

temperature at 0455 was 16°C, at 0515 it had

dropped to 15°C, and at 0730 it was 17°C

Some evidence that age is a factor influencing

the length of the foraging flight was obtained on

the 31 August survey. Bee number 4 was an old

bee and this was indicated by the fact that her

thorax was badly rubbed, the wings frayed, and

the mandibles were well worn apically. Foraging
NUMBER 168

TABLE 3.—Length of time (in minutes) spent by female Peponapis pruinosa in

foraging and in burrows at GritUey, California, observed in 10 time periods

Individual

number

1

2

3

4

5

6

7

8

9

10

11

12

13

1

2

3

4

5

6

1

2

3

4

5

6

7

12

23

17

23

6

15

2

7

25

19

O

#

18

7

9

7

9

9

0»

8

14

11

9

20

12

4

11

7

13

6

9

24

3

7

7

9

9

5

6

9

20

8

31

•

6

15

13

11

26

10

16

10

7

9

4

4

5

5

5

10

9

9

5

3

2

8

9

12

11

20

16

19

18

5

2

24

3

5

8

13

10

4

7

7

2

8

7

10

8

8

25

5

7

7

8

2

26

4

7

2

1

8

8

17

4

Foraging

13

9

2

3

5

8

7

7

9

7

5

5

3

9

7

1

5

10

8

1 August 1971 (0458

3

15

15

13 8 6

3 9 9

10 6 12

55

27

6

16

3

3

to

53

11

28

14

9

3

2

4

5

5

4

30 August 1971 (0730 to

7

12

4

25

5

31 August 1971 (0530

4

10

5

7

7

30

7

4

18

10

20

4

to

4

5

3

4

6

4

5

0750)

3

4

3

5

6

6

3

3

4

4

/«»)

4

3

2

20

4

0730)

14

27

9

2

5

6

6

2

5

3

5

6

2

5

5

3

4

7

1

6

5

5

23

4

31

4

/n

5

6

5

4

7

2

11

2

16

2

2

3

4

6

19

burrow

4

3

5

4

4

5

1

3

5

5

4

3

6

3

4

6

9

2

45

5

3 7

4 10

4 6

10

• Showed a number of times but never left its burrow.

•• Left the burrow at 0758 and never returned.

flights of this individual were longer than for any

of the others (Table 4) and averaged 33 minutes

as compared to 13.3 minutes for the others.

There are probably a number of reasons which

might account for the wide variation observed in

the length of the foraging flight and the duration

of the period spent in the burrow. The time

utilized on foraging is probably influenced by age

of individual, distance the bees had to go to obtain pollen, weather conditions, and the proportion

of this time spent in seeking nectar. Apparently

little time is required by a bee to deposit her pollen

load. After a pollen trip, however, it may be necessary for the bee to spend some time on nest construction or modification. On some occasions bees

could be seen working on their burrows, and this

TABLE 4.—Numbers of female Peponapis pruinosa

returning to burrows from early afternoon to early

evening at Gridley, California, 30 August 1971

Time period

1300-1315

1315-1330

1330-1345

1345-1400

1400-1415

1645-1700

1700-1715

1715-1730

Number of females

with

pollen

2

0

0

0

0

0

0

0

without

pollen

16

12

9

10

9

3

2

0

Temperature

time

1300

1415

1655

1740

•c

26"

27°

26°

23° •

* Light rain starting to fall.
10 SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY

activity tended to materially lengthen the time.'

spent in the burrow.

Sometimes a female would apparently enter a

burrow other than her own. When this happened

the visitor usually had no pollen and left the

burrow in about a minute. In other cases, competition for possession of the burrow would result

and interruption of the usual trip pattern occurred.

With some burrows with nest openings separated

by less than an inch and obvious "landmarks" often

absent, it was not surprising that some confusion

would result. However, interference occurred even

in instances where the entrances to burrows were

more widely separated. Females often did not fly

directly to their burrow. For example, one bee

under observation on her last four trips would

light as much as two or more feet away from her

burrow. She would then begin to explore and

search for her burrow. In doing so she examined

a number of burrows but did not try to enter

them. She continued her search and always tended

to move in the general direction of her burrow.

Once it was located, she readily entered it. So

characteristic were her movements that she could

be easily recognized on arrival at the nesting site.

Other individuals displayed equally characteristic

behavior.

At least within limits, the activity of the bees

increases with a rise in temperature. On 1 August

1971 as the sun rose over the horizon, the nesting

site was gradually but increasingly exposed to it.

By 0710 about one-third of the area was in sunlight

and the flight of the bees therein was noticeably

faster. Such observations have been substantiated

on a number of occasions. The increase in flight

tempo is most marked on days when the early

morning temperature is relatively low. Under such

conditions, the level of activity is lower compared

to later in the morning when the temperature

readies lf)°C and above. There also seemed to be

a rhythm to flight activity as indicated visually

and by changes in the constant hum that develops

over the nesting site.

Near the close of a nest survey at 1000 on 30

August it was noted that on occasions a bee was

returning to her burrow without pollen. This

would be expected as the day progressed, as less

and less pollen was available for collection, and

the females more actively seek nectar. In order to

obtain more information on this subject, burrows

selected at random were placed under observation

at 1150 on 30 August 1971. Although there was

considerable activity in the nesting site, this had

diminished to a point where watching a few burrows would yield little information. To secure

usable data, it was found necessary to make observations on a much larger area. From 1230 to 1245,

although greatly reduced, there was still quite a

hum from the bees over the nesting site. By 1300

it was markedly reduced and continued to decrease

as the afternoon advanced. Between 1212 and 1255

three bees were observed coming into the nesting

site with pollen, but no count was made of those

returning without pollen. Starting at 1300 a record

was made of the bees with and without pollen

coming in and entering their burrows. The information based on 15-minute intervals is presented

in Table 4. No bees with pollen were observed

after the 1300 to 1315 period. A few bees without

pollen continued to fly in but the number tended

to decrease with the advance in time. The last bees

were observed to fly in during the 1700 to 1715

interval. None were observed in the 1715 to 1730

period so observations were discontinued. Observations made at a second nesting site developed

information that substantiated that shown in

Table 4.

With the exceptions observed on 1 and 7 July

1972, recorded below, no female with pollen was

seen to enter her burrow later than the 1300-1315

period or without pollen after 1700-1715.

Time

1445-1500

1500-1515

1700-1715

1715-1730

1730-1745

1745-1800

1800-1815

1815-1830

1830-1845

1945*

1700-1730

1730-1745

Number of

females

1 July 1972

5

5

0

3

13

22

45

44

38

7 July 1972

6

13

Temperature

(°C)

37° 1515

36° 1700

36° 1745

34" 1800

32° 1845

29° 2000

30° 1707

* Area visited 1945, no bees seen. During peak of flight as

many as 5 or 6 individuals seen in flight at one time. Most

flights within a foot of the surface and appeared to be concentrated over thickest stand of grass. Here bees were observed to alight.
NUMBER 168

Time

1745-1800

1800-1815

1815-1830

1830-1845

1845-1900

1900-1915

1915-1930

Number of

females

7 July 1972

12

5

11

6

1

4

0

Temperature

(•C)

29° 1815

26° 1930

None of the females in the above tabulation

had pollen, and the data clearly show that the

females were active much later into the evening

than indicated in Table 4. There are several possible explanations for this behavior. Early in the

season there is a possibility that females working

on their nests find it necessary to seek nectar for

nourishment, or the individuals involved are those

that have just emerged and are stimulated to flight

in a search for nectar. The flight at dusk, although

unusual, is not unparalleled, for in the Cape Region of Lower California, Mexico, the squash bee

Xenoglossa mustelina (Fox), on a number of occasions, has been observed flying at dusk and visiting

the flowers of Tecoma stans, apparently seeking

nectar (Hurd and Linsley, 1967).

The flight of Peponapis pruinosa at dusk is limited to early in the season, and the information in

the above tabulation shows that the flight on 7

July was of less magnitude than that of 1 July.

Further, it should be noted that the number of

TABLE 5.—Surveys of burrows in a nest site of

Peponapis pruinosa at Gridley, California

Time period

1 Jul 72

1015-1030

1340-1355

1700-1715

1955-2010

7 Jul 72

0855-0910

1430-1445

31 Jul 72

1345-1400

1600-1615

30 Aug 71

1020-1035

1645-1700

Female

active and

seen in

burrow

5

1

1

0

2

11

21

20

3

5

Tumulus

and

plugged

3

5

3

1

3

4

29

13#

2

10##

Tumulus

and

not

plugged

25

36

42

30

129

142

67

105

59

68

No

tumulus

16

14

35

16

57

50

58

62

53

82

Total

burrows

49

56

81

47

191

207

175

200

126

165

•10 with entrance covered with mound.

• • 7 with entrance covered with mound.

11

individuals involved in the flight at dusk is small

as compared to the flight activity that takes place

in the morning.

Often in early morning the bees were very sluggish, and some experienced difficulty in orienting

to their nests. For example, on the morning of

31 August 1971, many bees returning to the nest

site did not appear to find their burrows readily

and some even dropped pollen, which accumulated

noticeably on the soil about burrow entrances.

At 0810 on 9 July 1969, bees returning with pollen

also seemed to have difficulty orienting to their

nests, although it is possible that the presence of

observers in the nesting site may have interfered

with the "landmarks" utilized by the bees. The

disturbance of landmarks was illustrated when a

homeowner who had allowed the grass to grow to

the height of four to six inches cut the grass

and incoming bees appeared so confused that they

became "stacked" in flight over the nesting area.

COMPETITION AT THE NESTING SITE.—As far as

could be determined, very little intraspecific competition takes place in a nesting site. What little

competition is evident is largely among female

bees rather than with other organisms. In those

areas where the burrows are very concentrated,

females occasionally interfered with one another

and there was disruption of normal behavior. Instances were observed where a visitor would be

rejected from an invaded burrow. However, no

violent action was ever observed. In general, a

rather harmonious relation existed, considering the

large number of individuals populating the nesting

sites.

Other organisms such as pillbugs (Armadillidium) and a small species of millipede were observed to enter the burrows, but this type of

activity appeared to be tolerated by the bees. Once

a large pillbug was seen to enter an occupied burrow, but the bee emerged and took off in the usual

manner, although it is possible that interconnecting burrows were involved and that the bee may

have bypassed the pillbug.

DIURNAL AND SEASONAL FIELD ACTIVITY.—In the

Gridley area, bees begin to emerge in late May and

field activity continues into October. Once emergence is initiated, it mounts rather rapidly and

bees are abundant before there are sufficient host

flowers to adequately accommodate all of them.

At the time of the earliest emergence, some home
12

SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY

and commercial plantings of summer squash (zucchini, crookneck, and scallop) are in bloom, but

not pumpkin, which is the major host. As a result,

the available flowers are crowded with bees and

many seek nectar from the flowers of morning

glory (Convolvulus arvense) blackberry, cucumber,

watermelon, and other melons. There are probably other hosts that serve this purpose but they

were not observed. At this time, pollen is not an

important item because there is a two- to threeweek lag before the females are conditioned for

nesting and pollen collecting. By then the available

sources of pollen have increased to a sufficient level

to maintain the bees.

In early season, males are particularly abundant

and practically 100 percent of the flowers are likely

to be visited.

Males spend their entire life in the field away

from the nesting site. Much of the time is spent

in the protection of the flowers. In the morning,

they may be seen "cruising" the flowers. Their

activity in the morning may lag somewhat behind

the time that females commence their foraging

activity. Often they may still be found sleeping

in the closed flowers of the previous day. If disturbed, they reacted in a very sluggish manner.

After the cruising period, the males tend to conTABLE 6.—Summary of numbers of Peponapis

pruinosa collected in 100 flower samples in fields

at Gridley, California (under "Host," Z=zucchini

squash, C = crookneck squash, P = pumpkin)

Time period

17 Jun 69

0815-0850

0850-0925

1045-1115

19 Jul 71

0830-0845

0855-0910

1210-1225

1235-1300

1515-1530

1540-1600

21 Jul 71

0750-0810

0815-0835

Host Males

Total

males and

Females females

285

396

696

325

335

202

432

165

339

314

334

90

101

180

29

35

22

53

21

29

183

194

375

479

876

354

370

224'

485*

186*

368*

497

528

Time period

24 Jun 69

0800-0820

0820-0835

0850-0905

0925-0940

25 Jun 69

0700-0725

0725-0747

1 Jul 72

0800-0815

1155-1210

1210-1225

1530-1600

2 Jul 72

0616-0639

0719-0736

0736-0751

7 Jul 72

0755-0810

0930-0944

1207-1230

8 Jul 72

0545-0602

9 Jul 69

0855-0915

0915-0937

10 Jul 69

0525-1550

0705-0718

0718-0726

0930-0942

11 Aug 69

0807-0828

0828-0845

1025-1045

1045-1105

1245-1320

10 Sep 69

0750-0800

0837-0852

0853-0907

25 Sep 69

0805-0820

0820-0834

0835-0850

0915-0925

0930-0937

0945-0956

Host

I.

C

P

P

7.

C

Z

Z

C

P

P

7.

C

P

P

P

P

P

P

P

c

z

p

p

p

p

p

p

p

p

p

p

p

p

p

p

p

Males

210

287

117

209

247

150

56

16

35

255

214

35

26

90

138

152

165

76

113

188

14

5

67

70

54

47

35

50

7

5

4

0

0

0

0

0

2

Females

93

138

149

176

105

152

19

2

1

29

87

94

81

66

41

7

67

15

24

38

19

14

4

4

5

2

0

3

0

1

1

1

5

4

0

0

0

Total

males and

females

303

425

266

385

352

302

75

18#

36*

284*

301

129

107

156

179

159»

232

91

137

226

33

19

71

74

59

49

35

53

7

6

5

1

5

4

0

0

2

• Closed flowers.
NUMBER 168 13

centrate in the flowers at about the time they

begin to close. Mating mostly occurs in the flowers

during the morning hours while the flowers are

still open.

Females, although less abundant than males, are

found in the flowers throughout the day in the

early season, and this situation continues until the

nesting instinct comes into play. Surveys conducted

throughout the day on 19 June 1971 (Table 6)

clearly illustrate this point. In the 1540-1600 survey, the females were as abundant as they were

earlier in the day, although females often leave

closed flowers and seek other protection on very

hot days. As nesting activity increases, fewer and

fewer are seen in the field as the day advances.

Some will usually be found, however, and this is

consistent with observations at the nesting site

which have shown them flying in as late as 1700

to 1715 (Table 4). Also, there is always the possibility that a female may have emerged very late

in the season and is not yet ready to take up nesting activity.

During the prenesting period, females as well

TABLE 7.—Summary of numbers of Peponapis

pruinosa observed in 15-minute intervals in flower

samples at Gridley, California (under "Host,"

Z = zucchini squash, C = crookneck squash, P =

pumpkin)

Time period

19 Jun 71

0745-0800

0806-0821

0830-0845

0855-0910

20 Jun 71

0455-0510

0515-0530

0550-0605

0612-0627

1 Jul 72

0800-0815

0815-0830

0845-0900

0900-0915

1155-1210

1210-1225

1530-1545

Host

I.

C

Z

c

z

c

z

c

z

c

z

c

z

c

p

Males

415

416

325

335

196

256

461

450

56

63

34

71

16

35

115

Females

28

35

29

35

9

22

28

27

19

60

9

30

2

1

13

Total

males and

fetnales

443

451

354

370

205 •

278*

489

477

75

123

43

101

18»«

36»#

128"

Time period

2 Jul 72

0616-0631

0719-0734

0735-0750

7 Jul 72

0755-0810

0930-0945

1207-1222

8 Jul 72

0545-0600

12 Aug 69

0501-0516

0516-0531

0531-0546

0546-0601

25 Aug 67

0500-0515

0515-0530

0530-0545

0545-0600

0600-0615

0615-0630

0630-0645

0645-0700

0700-0715

11 Sep 69

0520-0535

0535-0550

0550-0605

0605-0620

0620-0635

26 Sep 69

0550-0605

0605-0620

0620-0635

0752-0807

0807-0822

Host

P

Z

c

p

p

p

p

p

p

p

p

p

p

p

p

p

p

p

p

p

p

p

p

p

p

p

p

p

p

p

Males

145

33

26

90

148

97

126

24

34

71

78

7

25

114

94

86

72

70

86

42

3

0

4

3

4

0

0

0

0

0

Females

56

90

81

66

44

4

52

0

2

8

7

11

4

12

7

8

8

5

7

4

7

10

16

17

12

7

2

5

12

6

Total

males and

females

201

123

107

156

192

101»»

178

24

36

79

85

18

29

126

101

94

80

75

93

46

10

10

20

20

16

7

2

5

12

6

* Many bees still in closed flowers of yesterday bloom.

* Closed flowers.

as males may remain in the closed flowers until

alter sunrise. For example, on 20 June 1971, a

field was visited in early morning and many of

the bees were still in the previous day's flowers

although sunrise had occurred at 0450. Between

0455 and 0510 the current day's new flowers were

examined and 196 males and 9 females were

counted, while closed flowers of the previous day
14 SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY

produced 97 males and 19 females. When the two

counts are added together, the sums are 293 males

and 28 females, numbers that approximate those

recorded in an 0550 to 0605 survey, which yielded

461 males and 28 females (Table 7).

In the early season, before females are abundant,

males, because of their large numbers, are probably very important in carrying pollen from staminate to pistillate flowers. Those seeking nectar in

staminate flowers were almost invariably covered

with pollen, which adhered to their legs as well

as the rest of the body, and such individuals were

commonly seen taking nectar from pistillate flowers, assuring pollen transfer.

The largest number of bees found in a single

flower was 21; on 17 June 1969, 19 males and 2

females were observed in a staminate pumpkin

flower. In a survey conducted on 25 August 1967,

numerous flowers were found with 6 or more bees.

Where this occurred, at least one of the individuals

was a female. In early season it is not unusual to

find an average of 1 or more females per flower

(Table 6). Often they are associated with a number

of males.

In both early and late season, examinations of

closed flowers usually revealed a few dead bees.

The number is never large and the individuals

usually appeared to be in perfect condition. Since

the area is devoted to a wide range of crops, insecticide applications to those are suspected as

the cause of death. However, no evidence of mass

destruction of squash bees by pesticides was encountered.

The adult population declines as the later season advances, and as the end of the season approaches, male bees begin to disappear (Tables

6, 7). The phasing out of the males is more clearly

shown in the data discussed later, relative to the

investigations conducted on the Oxford Tract.

NUMBER OF ANNUAL GENERATIONS.—At first it

was believed that under California conditions two

generations of Pcponapis pruinosa might occur.

As the facts developed, there appears to be but a

single somewhat bimodal generation. The question

arose because relatively young individuals continued to make their appearance throughout the

summer. As late as 12 August 1969, two freshly

emerged males were observed in a nesting site.

The day before a freshly emerged male crawled

up a grass stem, unable to fly. On that date most

of the females coming into the nest area had worn,

frayed wings, but some appeared to be newly

emerged. Young individuals continued to make

their appearance. On 10 September 1969, out of

139 bees netted, at a nesting site, only 12 were

old bees with badly worn wings and mandibles.

The remaining bees appeared to be freshly emerged

females (Table 8). It was this type of information

that led to speculation that there was a second

generation or at least the beginning of one. However, it now appears more likely that there is but

one extended generation, a provision that may

insure the survival of the species during periods

of adverse environmental conditions.

OXFORD TRACT EXPERIMENTS.—Additional information on the behavior of Pcponapis pruinosa was

obtained in experiments conducted during 1971

at the Oxford Tract of the University of California,

Berkeley. An important advantage of this site was

the fact that continuous observations were possible.

A number of varieties of squash and pumpkin

were planted in the experimental plot. The 1971

season was much delayed and it was not until 25

June that the earliest varieties were coming into

bloom. On 28 June one female squash bee taking

nectar and two males were seen. Another male was

encountered on 2 July. No further observations

were made until 20 July, when, after an extensive

search, a single female seeking nectar was noted.

This was unexpected since in the previous year

a large population had been introduced and appeared to be established. However, because of the

limited population of bees, observations of 400

flowers were utilized, rather than 15-minute counts,

until the end of August. Most of the flower counts

were made between 0615 and 0645.

At this time some of the host varieties were

TABLE 8.—Numbers and condition of female

Peponapis pruinosa corning into nest area, Gridley, California, 10 September 1969

Time

period

0840-OWO

124.r>-134r>

1415-1515

Temperature

(°C)

37°

37°

Old

bees*

3

7

2

Young

with

pollen

68

0

0

bees**

withoui

pollen

40

15

4

* Wings badly worn ami stumps for mandibles.

** Wings, mandibles, and pubescence intact.
NUMBER 168 15

(-1

- , n n

ii

Si

July Aug 15 i 0

Sept 15hi 1

• Moles

0 Females nectar

1 Females pollen

I

July Aug 13 Sept

FIGURE 4. Seasonal trends in populations of Peponapis pruinosa and Apis mellifera at flowers

in experimental Cucurbita plantings at Oxford Tract, University of California, Berkeley.

reaching maturity, and there was a considerable

decline in the number of flowers available. This

fact necessitated lowering the flower counts to 200,

the number used from 3 September to the end of

the season. The results obtained, including that

secured on 20 July, are presented in Figure 4.

Despite the slow start, a relatively large adult

population developed. The prenesting period was

rather long, as indicated by the length of time that

the females were seeking mainly nectar. It was

not until early August that pollen collecting by

the females came into prominence. Until the end

of August male bees were encountered in much

greater abundance than females. Then the male

population dropped off sharply and only isolated

individuals occurred thereafter, substantiating the

seasonal information obtained at Gridley. The

female population continued uninterrupted, but

after 15 September it declined and none were

found after 29 September.

In late September it was suspected that the females would visit flowers later in the day than

during midseason. Based upon the behavior of

other species of squash bees, it was believed that

seeking nectar becomes more important than further gathering pollen (Michelbacher and Hurd,
16 SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY

1968). With this in mind, the planting was visited

between 1145 and 1215 on 26 September. One

hundred and fifty-one flowers were examined and

4 females taking nectar were observed. In addition

56 honeybees and several females of Agapostemon

texanus californicus were seen. On 27 September

the experimental plot was visited at the usual

time (0615-0650). It was clear, calm, and cool and

only a single honeybee was seen to enter a flower.

There was no evidence of pollen disturbance. The

field was again visited between 0730 and 0800.

Three females were encountered drinking nectar

along with 21 honeybees. The temperature at 0800

was 14°C. The temperature at the normal survey

period was well above the threshold (11.1°C) for

Peponapis activity. Evidence again indicated that

in late season, delayed flower visits and the seeking of nectar are more important than pollen

collecting.

On 1 October the plot was again visited between 0740 and 0810 (temperature 0810, 15°C).

No squash bees were seen and there was little or

no disturbance of the pollen. Furthermore, the

flowers were mostly of poor quality. A final survey

on 3 October produced no squash bees.

The information presented in Figure 2 clearly

indicates that in the Berkeley area Peponapis

pruinosa has but a single generation a year.

The experimental plots were again planted in

1972. The season was an early one. It should be

noted that spot checks during early summer revealed a marked increase in the number of

Peponapis encountered as compared to the previous

season.

Although our investigations have concentrated

on the pollination role of Peponapis, the importance of honeybees as pollinators of squash and

pumpkin should not be minimized, even though

they are poorly adapted to manipulate the relatively large-sized pollen of these plants. The abundance of honeybees and the ease of manipulation

favor their effectiveness. They can also be of value

where the fruiting period of squash and pumpkin

extends beyond the active stage of squash bees

or in areas only marginally acceptable to squash

bees. This is well illustrated in the early portion

of the Oxford Tract Experiments. During 1971

the number of honeybees seen was recorded along

with that of the squash bees. These data are presented in Figure 4. A large honeybee population

was present throughout the period of bloom. During July it was the most important pollinator and

was probably responsible for the good set of squash

fruits that was produced during this period. The

bees were frequently seen heavily coated with

pollen as they flew from staminate to pistillate

flowers. There can be but little question that their

activity throughout the flowering period was of

sufficient intensity to result in satisfactory pollination.

In the investigations at Gridley, honeybees were

never encountered in the numbers observed at the

Oxford Tract; more often they were scarce.

Summary and Conclusions

1. Peponapis pruinosa is a gregarious groundnesting bee with a nest site preference for flat,

usually grass-covered ground which is moist during

the nesting season and adjacent to or near an

adequate source of pollen.

2. Like other members of the genus, females

take pollen only from plants of the genus

Cucurbita, the native wild host being Cucurbita

foetidissima, the preferred cultivated hosts being

pumpkins, summer squashes, and winter squashes

in that order.

3. They are the most important pollinators of

these crops.

4. In the Sacramento Valley, California, emergence of adults usually occurs in late May and

activities extend into October.

5. Early in the season, males and newly emerged

females spend the day taking nectar from or resting in the squash flowers, which open before sunrise and wilt toward midmorning on a clear hot

day; both sexes spend the night in wilted flowers,

cutting their way out near sunrise.

6. Mating occurs mostly in the flowers during

the morning hours while the flowers are still open.

7. Approximately two to three weeks after

emergence, females abandon the flowers for "sleeping" and begin burrow construction; shortly afterward they begin foraging for pollen.

8. Burrow construction and maintenance continue throughout the season.

9. In northern California P. pruinosa has few

competitors at the nest site; the incidence of moldy

and diseased cells was low and no major parasites

or predators were encountered.
NUMBER 168

10. Honeybees (Apis mellijera) were the principal pollen competitors but, except at Berkeley,

they were not sufficiently abundant or active

early enough in the morning to provide significant

competition.

11. Experiments at Berkeley proved that Pell

ponapis can be introduced and established by the

transport and release of prenesting adults into an

area with suitable nest sites and immediately available sources of nectar and, when needed, Cucurbita

pollen.

Literature Cited

Hurd, Paul D., Jr., and E. Gorton Linsley

1964. The Squash and Gourd Bees—Genera Peponapis

Robertson and Xenoglossa Smith—Inhabiting

America North of Mexico (Hymenoptera: Apoidea).

Hilgardia, 35:375-477.

1967. Squash and Gourd Bees of the Genus Xenoglossa

(Hymenoptera: Apoidea). Annals of the Entomological Society of America, <>0(5):988-1007.

Hurd, Paul D., Jr., E. Gorton Linsley, and

Thomas W. Whitaker

1971. Squash and Gourd Bees (Peponapis, Xenoglossa)

and the Origin of the Cultivated Cucurbita. Evolution, 25:218-234.

Mathewson, John A.

1968. Nest Construction and Life History of the Eastern

Cucurbit Bee, Peponapis pruinosa (Hymenoptera:

Apoidea). Journal of the Kansas Entomological Society, 41:255-261.

Michelbacher, A. E., and Paul D. Hurd, Jr.

1968. Late Season Foraging Activities of Xenoglossa

gabbii crawfordi Cockerell. (Hymenoptera: Apoidea).

Pan-Pacific Entomologist, 44:58-66.

Michelbacher, A. E., Paul D. Hurd, Jr., and E. G. Linsley

1968. The Feasibility of Introducing Squash Bees

(Peponapis and Xenoglossa) into the Old World.

Bee World, 49:159-167.

1971. Experimental Introduction of Squash Bees

(Peponapis) to Improve Yields of Squash, Gourds

and Pumpkins. Bee World, 52:156-166.

Michelbacher, A. E., Ray F. Smith, and P. D. Hurd, Jr.

1964. Bees are Essential . . . Pollination of Squash,

Gourds, and Pumpkins. California Agriculture, 18:

2-4.

U.S. GOVERNMENT PRINTING OFFICE: 1*74— 546-3S»/lO



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