United States

Department of

Agriculture

u

cfiI

Forest Service

Southern Forest

Research Ndte

Experiment Station

SO-326

August 1999

Early Impact and Control of Aphid (Chaitophorus popukola Thomas)

Infestations on Young Cottonwood Plantations in the Mississippi Delta

J. D. Solomon

SUMMARY

A very heavy infestation of the aphid Chaitophorus

populicola Thomas developed primarily on growing

shoots in commercial cottonwood plantations and

caused serious injury to terminals. Terminal mortality in

heavily infested fields averaged 92.5 percent, and shoot

dieback averaged 4.3 inches. Many of the surviving terminals were weakened to the extent that slow growth

allowed lateral bud growth to become dominant. After

one growing season, trees in heavily infested fields generally had more stem deformities in the form of heavy

branches and forks than did those in lightly infested

fields. Four insecticides, acephate, carbaryl, chlorpyrifos, and diazinon, gave excellent control of the aphids.

Additional keywords: Populus deltoides, terminal

mortality, dieback, stem deformities, insecticides.

INTRODUCTION

Cottonwood (Populus deltoides Battr. ex Marsh.) is a

good source of veneer, lumber, and pulp and is the

fastest growing tree species in the South (Krinard and

Johnson 1984). Several companies and individuals are

planting and growing cottonwood on a commercial scale.

More than 50,000 acres of cottonwood plantations have

been established in the South-mainly in the lower Mississippi River Valley (Krinard and Burkhardt 1984). Establishment costs for cottonwood plantations, including

site preparation (with land clearing), planting, and firstyear cultivation, are estimated at between $300 and

$400 per acre (Anderson and Krinard 1985), compared

to about $100 per acre in 1970 (Dutrow et al. 1970).

These large early investments demand that losses from

pests be controlled.

One of the little known pests of cottonwood is the

aphid, Chaitophorus populicola Thomas (Morris and

Oliveria 1976). This aphid is widely distributed from

Canada south to the Gulf Coast and west to the Rocky

Mountains (Richards 1972). It has caused noticeable

leaf drop in aspen stands (Furniss and Carolin 1977).

But little information has been documented on this insect, and even less is known of its potential for injury and

impact in commercial cottonwood plantations.

Light aphid populations have been observed often in

cottonwood plantations and moderate populations

sometimes occur, but heavy, threatening infestations

have occurred infrequently. However, in 1976 a very

heavy aphid infestation occurred in young cottonwood

plantations in the lower Mississippi Delta. This heavy

infestation provided an opportunity to study the effects of

aphid feeding on terminal mortality and subsequent tree

form. In 1980, small scale trials with selected insecticides were conducted to control aphid infestations in

cottonwood.

MATERIALS AND METHODS

The study was conducted in first-year commercial cottonwood plantations in lssaquena County, Mississippi.

The plantations were established with a random clone

mixture of five Stoneville select clones (clones 66, 67,

74, 92, and 109). The plantations were cultivated four to

five times during the first growing season for weed control.

The study plantations were visited periodically, beginning in early spring. After the aphid population began to

build up in late summer, the plantations were visited at lto 2-week intervals to assess the aphid infestation. During each visit, the number of aphids was counted on

single leaves selected from the terminals of 10 or more

randomly selected trees per field.

Southern Forest Experiment StationTT-10210 U.S. Postal Services Bldg., 701 Loyola Avenue, New Orleans, La. 70113

Forest Service, U.S. Department of Agriculture.

Serving Alabama, Arkansas, Louisiana, Mississippi, E. Oklahoma, Tennessee, E. Texas, Puerto Rico, U.S. Virgin Islands

1
During the following spring on April 1 and 2, just after

growth had begun, the plantations were sampled to estimate terminal mortality and extent of dieback. Eleven

fields that had been lightly to heavily infested the previous year were selected for sampling.

Five lo-tree plots were randomly established in each

field. Thus, a total of 50 trees was sampled in each field.

The terminal of each tree was examined and recorded

as dead or alive. Also, the extent of terminal dieback was

measured. Because shoot growth had just begun and

was 2-6 inches long when the examinations were made,

an assessment was made of trees having live terminals

but obviously weakened and with laterals showing evidence of becoming dominant.

During the following winter, one full growing season

after the aphid infestation, the same 11 fields (but not the

same trees) were again evaluated to assess the effect of

aphid-caused terminal mortality on developing tree form.

Once again, five plots were randomly chosen in each

field, and 10 trees were selected in each plot for examination. Sample trees were examined for evidence of the

previous year’s terminal mortality (junction of first- and

second-year growth), bole crooks, heavy branching, and

forking. Crooks were rated 1 or 2 based on degree of

crook, where 1 = crook deviating 1 to 3 inches from a

straight line, and 2 = crook deviating 3 to 5 inches from

a straight line. Degree of branching was rated 1 or 2,

where 1 = heavy branch(es) l/2-3/4 the diameter of the

adjacent trunk, and 2 = heavy branch(es) more than 314

the diameter of the adjacent trunk. Extent of forking was

rated as 1 or 2, where 1 = fork with one branch becoming dominant, and 2 = fork with no evidence of one

branch becoming dominant.

Four widely used insecticides were tested in 1980 for

efficacy in controlling aphid populations. Two rates of

acephate, carbaryl, and chlorpyrifos and one rate of diazinon were evaluated. The trees were in their first growing season and were 3-5 feet tall. Ten aphid-infested

trees were selected at random for each treatment and

untreated check. Foliar applications were made with a

2-gal hand sprayer calibrated at 20 lb/in* to deliver 11 gal

per acre. Prespray counts were made of all living aphids

on 3 randomly selected leaves collected from the terminals of each of the 10 trees in each treatment. Postspray

counts of surviving aphids were made 24 hours after

spray application to evaluate efficacy of insecticide treatments.

All percentage data were transformed by arcsin before

an analysis of variance was performed. Duncan’s new

multiple range test was used to compare treatment

means for significance at the 0.05 level.

RESULTS AND DISCUSSION

An aphid, C. populicola, infestation began to build up

in mid-summer of 1976 and became very heavy in some

fields during late summer and early fall. Sample counts

of aphids per terminal leaf, made from late August

through early October, averaged 360 + 102 (X + SD) in

four fields designated as heavily infested; 210 + 64 in six

fields designated as moderately infested; and 70 in a

single field designated as lightly infested. Also, the number of aphids on the terminal stems was estimated at

over 200 per linear inch of stem in heavily infested fields,

100 in moderately infested fields, and under 40 in the

lightly infested field.

The highest aphid population occurred on the young

leaves and tender shoot growth of the apical 6-12

inches of terminals and branch ends (fig. 1 A). Honeydew

excreted by the aphids, and subsequently black sooty

mold, became very noticeable on the foliage and particularly on the lower leaves. Injured shoots became heavily blistered, scabby, pitted, and rough. Heavily injured

terminals dropped their leaves prematurely, 4 to 8 weeks

before the end of the growing season. Toward the end of

the growing season, in October and November, some

shoots exhibited signs of drying and shriveling.

When growth started the following spring, dead terminals became very noticeable (fig. 1 B). Terminal mortality

assessed in the 11 infested fields ranged up to 98 percent and averaged significantly higher in heavily infested

fields, followed by the moderately and lightly infested

fields (table 1). The extent of terminal dieback ranged

from 2.0 to 4.7 inches among the 11 fields and averaged

significantly greater in the heavily infested fields. It was

interesting to note that the three fields having the highest

terminal mortality, 96, 94, and 98 percent, also had significantly greater terminal dieback, 4.5, 4.4, and 4.7

inches, respectively. Many of the terminals that were not

killed were weakened to the extent that slow growth

during early spring allowed lateral bud growth to quickly

become dominant. The percentage of trees (those having live terminals) with laterals become dominant on

them ranged from 0 to 24 percent among fields. But

there were no significant differences in lateral branch

response among fields, probably because of the wide

variation in counts or the early timing of the observations. Thus, lateral branch response was not a reliable

measure of aphid injury.

Evaluation of the plantations one full growing season

after the aphid infestation revealed generally poor tree

form in the heavily infested fields (table 2). The proportion of trees with crooks ranged from 18 percent in the

lightly infested field to 52 percent in the heavily infested

fields, yet the wide variation in the amount of crook

among fields prevented any significance. One of the

heavily infested fields mentioned above, which had

96 percent terminal mortality the previous year, had the

highest total crook (68 percent), including 14 percent of

the more serious (degree 2) crook. The proportion of

trees developing heavy branches ranged from 22 to

66 percent among individual fields and averaged significantly greater in heavily infested fields than in the moderately and lightly infested fields. The two fields having

the highest terminal mortality, 96 and 98 percent, also

J. D. Solomon is Entomologist at the Southern Hardwoods Laboratory, maintained by the Southern Forest Experiment Station, Forest ServiceUSDA, in cooperation with the Mississippi Agricultural and Forestry Experiment Station and the Southern Hardwood Forest Research Group.

2
Table 1 .-Mean terminal mortality, dieback, and lateral branch dominance caused by heavy aphid

infestation in young cottonwood plantations in the Mississippi Delta 1 2

Degree of Trees with dead

infestation terminals (%)

Extent of terminal

dieback (in)

Trees having

live terminals,

with lateral branch

becoming dominant (%)

Light

Moderate

Heavy

22.0 a

64.0 b

92.5 c

2.30 a

2.65 a

4.25 b

9.0 a

16.0 a

10.0 a

‘Trees were examined on April 1 and 2, when new shoot growth was 2-6 in long, following late

season aphid infestation the previous year.

*Means within a column followed by the same letter are not significantly different (P = 0.05) as

determined by Duncan’s new multiple range test (DNMRT).

Figure 1 .-Aphid (Chaitophorus populicola) infestation and injury to cottonwood. (A) Terminal heavily infested with aphids. (6) Terminals with

dieback. Note the rough, scabby, pitted bark.

exhibited the most total forking (22 and 20 percent),

including the most degree 2 forking (6 and 10 percent).

Forking appears to be the most serious stem deformity

affecting bole form in young cottonwood plantations

(Solomon 1985).

Because this study was pursued for only 1 year after

the aphid-caused terminal mortality, the question

arises-would these stem deformities be sustained,

worsen, or become less significant in subsequent years?

A recently completed 5-year study on insect impact indicates that the response of trees to these stem deformities depends on whether the pest injury is repeated in

subsequent years, site quality, rainfall, and other factors

(Solomon 1985). Given optimum combinations of these

conditions, a high proportion of trees will outgrow or

overcome many of these stem deformities, while poor

combinations of the same conditions will result in the

stem deformities being sustained or worsened in a high

proportion of trees. Although of minimal importance in

pulpwood production, bole form assumes great importance to landowners producing sawlogs and veneer

logs.

Each of the four insecticides, acephate, carbaryl,

chlorpyrifos, and diazinon, gave excellent control of the

aphids (table 3). There were no significant differences in

percentage of aphid reduction among insecticides and

rates, but all were significantly different from the check.

No phytotoxicity was observed for any of the treatments.

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Table 2.-Effects of aphid-caused terminal mortality on tree form evaluated one growing season after

aphid infestation in young cottonwood plantations in the Mississippi Delta 1

Mean percentage of trees withDegree of

infestation

Heavy branching

Crook (degree)2 (degree)3 Forking (degree)4

1 2 Total 1 2 Total 1 2 Total

Light 18.0 0.0 18.0 a 8.0 14.0 22.0 a 2.0 2.0 4.0 a

Moderate 39.3 1.3 40.6 a 19.3 18.7 38.0 a 7.0 3.0 10.0 a

Heavy 52.0 3.5 55.5 a 25.0 27.5 52.5 b 11.5 6.0 17.5 b

‘Means within a column followed by the same letter are not significantly different (P = 0.05) as

determined by DNMRT.

2Degree 1 = Crook deviating l-3 in from a straight line; degree 2 = crook deviating 3-5 in from a

straight line.

aDegree 1 = Heavy branch(es) l/2-3/4 the diameter of the adjacent trunk; degree 2 = heavy

branch(es) more than 3/4 the diameter of the adjacent trunk.

4Degree 1 = Fork with one branch becoming dominant; degree 2 = fork with no evidence of one

branch becoming dominant.

Table 3.4nsecticides tested for control of aphids on young cottonwood trees in their first growing

season in the Mississippi Delta

Chemical and Rate

formulation (lb ai/acre)

Mean number of aphids per leaf

Prespray Postspray % reduction’

Acephate 75s 0.75 491.5 0.0 100.0 a

Acephate 75s 0.50 268.1 0.0 100.0 a

Carbaryl 80s 1 .oo 267.0 0.0 100.0 a

Carbaryl 80s 0.50 402.0 0.0 100.0 a

Chlorpyrifos 4E 0.25 311.8 2.9 99.1 a

Chlorpyrifos 4E 0.125 417.0 2.2 99.5 a

Diazinon AG-500 0.50 400.5 1.2 99.7 a

Check . 188.0 257.8 0.0 b

‘Means within a column followed by the same letter are not significantly different (P = 0.05) as

determined by DNMRT.

CONCLUSIONS

The results show that high aphid infestations averaging over 300 aphids per terminal leaf and 200 per inch of

terminal stem can cause high rates of terminal mortality

in young cottonwood plantations. Heavy terminal mortality, in turn, can result in more crooks, heavy branches,

and forks that adversely affect bole form, which is so

important to landowners producing sawlogs and veneer

logs. Efficacy tests show that acephate, carbaryl, chlorpyrifos, or diazinon can be used to control heavy populations of aphids.

LITERATURE CITED

Anderson, Walter C.; Krinard, Roger M. The investment

potential of cottonwood sawtimber plantations. In:

Proceedings, 3rd biennial southern silvicultural research conference; 1984 November 7-8; Atlanta, GA,

Gen. Tech. Rep. SO-54. Washington, D.C.: U.S. Department of Agriculture, Forest Service; 1985: 190-

197.

Dutrow, G. F.; McKnight, J. S.; Guttenberg, S. Investment guide for cottonwood planters. Res. Pap.‘SO-59.

New Orleans, LA: U.S. Department of Agriculture,

4

Forest Service, Southern Forest Experiment Station;

1970. 15 p.

Furniss, Ft. L.; Carolin, V. M. Western forest insects.

Misc. Publ. 1339. Washington, D.C.: U.S. Department

of Agriculture, Forest Service; 1977. 645 p.

Krinard, R. M.; Burkhardt, E. C. Growth through 13 years

of two pruned cottonwood plantations spaced for sawtimber production. Southern Journal of Applied

Forestry. 8(4): 221-223; 1984.

Krinard, R. M.; Johnson, R. L. Cottonwood plantation

growth through 20 years. Res. Pap. SO-212. New Orleans, LA: U.S. Department of Agriculture, Forest

Service, Southern Forest Experiment Station; 1984.

11 p.

Morris, R. C.; Oliveria, F. L. Insects of periodic importance in cottonwood. In: Eastern cottonwood and related species: Proceedings of a symposium; 1976

September-October; Greenville, MS. Baton Rouge,

LA: Louisiana State University; 1976: 280-285.

Richards, W. R. The Chaitophorinae of Canada (Homoptera: Aphidae). Memoirs No. 87, Entomological

Society of Canada; 1972. 109 p.

Solomon, J. D. Impact of insects on growth and development of young cottonwood plantations. Res. Pap. SO217. New Orleans, LA: U.S. Department of Agriculture, Forest Service, Southern Forest Experiment

Station; 1985. 6 p.