United States

Department of

Agriculture

Forest Service

Southern

Research Station

Research Paper

SRS-33

Susceptibility of Potted

Sweetgum Seedlings to Insect

Herbivore Damage as Influenced

by Fertilization

Kenneth E. Ward and Mary Anne Sword Sayer
Kenneth E. Ward, Assistant Professor, Alabama A&M University, Department of Plant and Soil Science, Normal, AL 35762;

and Mary Anne Sword Sayer, Research Plant Physiologist,

U.S. Department of Agriculture, Forest Service, Southern

Research Station, Pineville, LA 71360.

The Authors

July 2004

Southern Research Station

P.O. Box 2680

Asheville, NC 28802

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containers.
1

Susceptibility of Potted Sweetgum Seedlings to Insect

Herbivore Damage as Influenced by Fertilization

Kenneth E. Ward and Mary Anne Sword Sayer

Abstract

We report the influence of fertilization on the susceptibility of sweetgum

(Liquidambar styraciflua L.) seedlings to naturally occurring insect

herbivores. Thirteen-week-old potted sweetgum were placed in a pasture

near the margin of a hardwood forest containing scattered sweetgum

trees. Groups of 14 seedlings were treated weekly with either no (0), low

(1.5 g/L), medium (3.0 g/L), or high (6.0 g/L) concentrations of 20-20-20

soluble fertilizer. Seedling performance was assessed by measuring height

and leaf number and rating seedling condition throughout the 11-week

experiment. Types and numbers of insect herbivores were recorded at 2-

to 3-week intervals for each seedling throughout the experiment and

cumulative damage was estimated for each seedling at the end of the

experiment. Herbivore damage was generally light, except for weeks 2

through 5, when seven seedlings were defoliated and killed. Six of these

seedlings were in the medium and high fertilizer treatments. Defoliations

were due to late instar larvae of the yellowstriped armyworm (Spodoptera

ornithogalli Gn.). Other insect herbivore species were detected, but their

impacts were minor. Eleven seedlings received no detectable herbivore

damage during the experiment; 10 of these received zero and low fertilizer

treatments. Results suggest that the susceptibility of potted sweetgum

seedlings to insect herbivore damage was associated with fertilizer

concentration. This effect was not related to differences in seedling size

because major defoliation events occurred before significant differences

in seedling size among fertilizer treatments were detected. Yellowstriped

armyworm has potential as a pest of sweetgum seedlings in intensively

managed plantings.

Keywords: Herbivore-plant interactions, Liquidambar syraciflua,

nutrient levels, short-rotation plantings, yellowstriped armyworm.

Introduction

The anticipation of an increased demand for renewable

woody biomass is leading forestry into a new era of more

intensive silviculture; hardwoods seem to fit this need very

well (Farnum and others 1983). Large productivity gains

are already being realized in forest plantations through the

use of a combination of intensive management practices,

including selection of superior genotypes, intensive site

preparation, competition and pest control, irrigation, and

fertilization (Ranney and others 1987). One of the hardwoods widely used by forest industry in intensive culture

plantings is sweetgum (Liquidambar styraciflua L.).

Although sweetgum grows more slowly than such intensively managed species as American sycamore (Platanus

occidentalis L.) and cottonwood (Populus L. spp.), it

appears to have relatively few insect pest problems, in spite

of being host to a variety of insect herbivores such as fall

webworm (Hyphantrea cunea Drury), luna moth (Actias

luna L.), white-marked tussock moth (Orgyia leucostigma

J.E. Smith), forest tent caterpillar (Malacosoma disstria

Hubner), gypsy moth (Lymantria dispar L.), a variety of

tortricid and pyralid leaf-tiers/folders (Choristoneura spp.,

Platynota spp., Nephopterix spp., Pococera spp.), and

gracillariid leafminers (Phyllocnistis spp.).1

There is relatively little information on potential insect

problems in intensively cultured short-rotation sweetgum

plantings. It is reasonable to expect populations of certain

insect herbivores to reach damaging levels as acreage

planted to sweetgum increases. In addition, intensive

cultural practices, including irrigation and fertilization,

may affect the nutritional quality of host tissue, which

could lead to or intensify insect herbivore infestations.

Materials and Methods

Study Area and Experimental Setup

Sweetgum seedlings were obtained by germinating seeds in

plastic tubs of commercial potting medium (Pro-MixTM BX)

in April 1998. Seedlings were transferred after 6 weeks,

each to an 11.4-L black plastic pot containing the same

potting medium. After 6 weeks of acclimation, 14 potted

seedlings were randomly assigned to each of 4 treatments.

Treatments were application of water only (0) or the application of 20-20-20 soluble fertilizer (Peter’sTM; N, P, and K

derived from ammonium phosphate, potassium nitrate,

urea, and potassium sulfate) at 1.5 g/L (21.4 mM N; low),

3.0 g/L (42.8 mM N; medium) or 6.0 g/L (85.6 mM N;

high). Initial applications were made on June 28, 1998, in

the greenhouse. Solutions were applied so that the potting

medium was saturated to the point of runoff. Potted seedlings were transferred on July 2 to the Winfred Thomas

Agricultural Research Station (WTARS), Alabama A&M

University, located approximately 8 miles north of

Huntsville, AL. Subsequent applications of treatment

solutions were made weekly throughout the experiment,

which was terminated after 11 weeks. It was necessary

during parts of August and September to water the

seedlings between scheduled applications due to drought.

At WTARS, potted seedlings were placed in a pasture near

the margin of a small bottomland mixed hardwood forest

1 Ward, K. and Ward, R. 1997. Unpublished data. On file with: Alabama

A&M University, P.O. Box 1208, Normal, AL 35762.
2

that contained scattered mature sweetgum trees. The

location and time of year were considered optimum for

colonization by the most common sweetgum herbivores in

the area, primarily pyralid and tortricid leaf-tiers/folders,

gracillariid leafminers, white-marked tussock moth, and fall

webworm (see footnote 1). Treatments were replicated by

randomly assigning 1 seedling from each treatment group

to each of 14 blocks. Blocks of four potted seedlings each

were randomly aligned in a single row along the margin of

the woodland. Seedlings within blocks were spaced approximately 1 m apart; blocks were spaced approximately 3 m

apart. This arrangement placed all seedlings in a similar

light environment and at similar distances from sources of

herbivore colonization. Vegetation adjacent to pots was

clipped regularly and removed to avoid excess shading.

Plastic saucers were placed under pots to prevent root

escape into underlying soil.

Seedling and Herbivore Damage Measurements

Seedling condition was quantitatively estimated by measuring height and counting the number of leaves at 2- to 3-week

intervals. In addition, seedling condition was assessed qualitatively at 2- to 3-week intervals by evaluating relative leaf

chlorosis on a scale of 1 to 4: (1) poor (highly chlorotic),

(2) fair, (3) good, and (4) excellent (no visible chlorosis).

Every 2 to 3 weeks throughout the experiment, numbers

and types of insect herbivores found on seedlings were

recorded, and each seedling was assigned a damage rating

of 1 to 4: (1) no damage, (2) light damage (< 5 leaves

damaged), (3) moderate damage (between 6 and 20 leaves

damaged), and (4) heavy damage (> 20 leaves damaged).

Cumulative estimates of damage were made for each seedling at the end of the experiment.

Statistical Analysis

Seedling height, number of leaves, and seedling condition

as a function of fertilizer treatment were evaluated by a

repeated measures analysis of variance. Cumulative herbivore damage by the end of the experiment, as related to

fertilizer treatment, was assessed using a randomized

complete block analysis of variance with 14 blocks. Main

and interaction effects were considered significant at

probability P < 0.05. The Tukey test at P < 0.05 was used

to detect significant differences between means.

Table 1—Repeated measures analysis of variance for sweetgum seedling

height, leaf number, and condition as related to fertilizer treatment and

time since treatment initiation

Source df SS MS F-value P > F-value

Seedling height

Fertilizer (A) 3 2,031.0 677.0 12.86 < 0.001

Time (B) 4 23,299.9 5,825.0 110.63 < 0.001

A x B 12 2,447.9 204.0 3.87 < 0.001

Residual 235 12,373.4 52.7

Leaf number

Fertilizer (A) 3 4,657.9 1,552.6 11.4 < 0.001

Time (B) 3 55,933.3 18,644.4 136.7 < 0.001

A x B 9 11,056.6 1,228.5 9.0 < 0.001

Residual 191 26,047.2 136.4

Seedling condition

Fertilizer (A) 3 73.8 24.6 58.8 < 0.001

Time (B) 4 29.4 7.4 17.6 < 0.001

A x B 12 52.9 4.4 10.5 < 0.001

Residual 234 97.9 0.4

df = degrees of freedom; SS = sum of squares; MS = mean square; P > F-value = probability

of a greater F-value.
3

Results

Seedling Growth

Fertilizer treatment, as expected, had significant effects on

seedling height and leaf number by the end of the experiment (table 1); however, significant effects did not appear

before week 8, as seedlings receiving fertilizer treatments

attained significantly greater height and leaf numbers than

seedlings receiving no fertilizer treatment (figs. 1 and 2).

Interestingly, although not statistically significant, the

medium (3.0 g/L) fertilizer treatment elicited greater height

growth and added more leaves than the high (6.0 g/L)

fertilizer treatment, which was similar to the low (1.5 g/L)

0

5

10

15

20

25

30

35

40

45

Week 0 Week 2 Week 5 Week 8 Week 11

Time

Height (cm)

0

Low 

Medium

High

Figure 1—Sweetgum seedling height (cm) in response to fertilizer treatment over time since

treatment initiation. Fertilizer treatments are application of 0 (no fertilizer), low (1.5 g/L),

medium (3.0 g/L), or high (6.0 g/L) concentration of Peter’sTM 20-20-20 soluble fertilizer.

0

10

20

30

40

50

60

70

80

Week 0 Week 2 Week 5 Week 11

Time

Leaf number

0

Low

Medium

High

Figure 2—Sweetgum seedling leaf number in response to fertilizer treatment over time since

treatment initiation. Fertilizer treatments are application of 0 (no fertilizer), low (1.5 g/L),

medium (3.0 g/L), or high (6.0 g/L) concentration of Peter’sTM 20-20-20 soluble fertilizer.

Data are missing for week 8.
4

fertilizer treatment. This suggests the high fertilizer treatment may have actually been supraoptimal (Mengel and

Kirkby 1982). The zero fertilizer treatment produced seedlings with dramatically fewer leaves and less height growth

than any other treatment.

Seedling Condition

In general, seedling condition was also affected significantly

by fertilizer treatment over the course of the experiment

(table 1). Initially, seedlings in the low fertilizer treatment

group had a significantly lower condition rating, but

recovered to a similar level with other groups by week 2

(fig. 3). At week 5, seedling condition was significantly

lower for the seedlings receiving no fertilizer, while other

groups remained similar. By the end of the 11-week experiment, seedling condition was similar for the medium and

high treatments and the condition of these seedlings was

numerically, though not statistically, higher than the condition of seedlings receiving the low fertilizer treatment. The

condition of seedlings receiving the zero fertilizer treatment was significantly lower than for any other treatment

at week 11.

Herbivore Damage

Although herbivore pressure was generally light during

most of the 11-week experiment, two distinctive patterns

were evident. First, there was a burst of herbivore activity

during weeks 2 through 5, when seven seedlings were completely defoliated and killed (fig 4.). Of these seedlings, 33

and 57 percent received medium and high fertilizer treatments, respectively (table 2). Furthermore, defoliations

occurred before significant effects of fertilizer treatment on

seedling height, leaf number, and seedling condition were

detected (figs. 1 through 4). In all cases, defoliation was

caused by late instar larvae of the yellowstriped armyworm

(Spodoptera ornithogalli Gn.), which were usually found

on the potting medium surface of the defoliated seedlings.

We were unable to find a record of sweetgum as a host for

yellowstriped armyworm. However, several larvae of this

species were successfully reared to adults on sweetgum

foliage in the laboratory (see footnote 1). Secondly 11

seedlings received no detectable defoliation during the

experiment. Of these seedlings, 64 and 27 percent were

from the zero and low fertilizer treatments, respectively.

By the end of the experiment, seedlings receiving no

fertilizer sustained significantly less cumulative herbivore

damage than those receiving low, medium, or high fertilizer treatments (tables 2 and 3) (fig. 4). Although not

significant, a trend of increased cumulative herbivore

damage with higher rates of fertilization was also apparent.

In addition to yellowstriped armyworm, other insect herbivores encountered during the experiment included several

unidentified noctuid and geometrid species, several species

of pyralid or tortricid leaf-tiers/folders, white-marked

Figure 3—Sweetgum seedling condition in response to fertilizer treatment over time since

treatment initiation. Fertilizer treatments are application of 0 (no fertilizer), low (1.5 g/L),

medium (3.0 g/L), or high (6.0 g/L) concentration of Peter’sTM 20-20-20 soluble fertilizer.

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

Week 0 Week 2 Week 5 Week 8 Week 11

Time

Condition

0

Low

Medium

High
5

Figure 4—Sweetgum seedling herbivore damage rating in response to fertilizer treatment over time

since treatment initiation. Fertilizer treatments are application of 0 (no fertilizer), low (1.5 g/L),

medium (3.0 g/L), or high (6.0 g/L) concentration of Peter’sTM 20-20-20 soluble fertilizer.

0

0.5

1

1.5

2

2.5

3

Week 0 Week 2 Week 5 Week 8 Week 11

Time

Damage rating

0

Low

Medium

High

Table 2—Randomized complete block analysis of variance for sweetgum

seedling cumulative herbivore damage as related to fertilizer treatment

Source df SS MS F-value P > F-value

Block (A) 13 11.4 0.88 1.38 0.213

Fertilizer (B) 3 13.1 4.38 6.87 < 0.001

Residual 39 24.9 0.64

df = degrees of freedom; SS = sum of squares; MS = mean square; P > F-value = probability of

a greater F-value.

Table 3—Mean cumulative insect herbivore damage rating

(standard errors in parentheses), number of potted sweetgum

seedlings killed by defoliation, and number of seedlings with

no defoliation at week 11, as related to fertilizer treatmenta

Fertilizer Damage Seedlings killed Seedlings with

treatmentb ratingc by defoliation no defoliation

 - - - - - - - - - - - no. - - - - - - - - - - -

0 1.5 (0.17)a 0 7

Low 2.1 (0.22)b 1 3

Medium 2.5 (0.20)b 2 0

High 2.7 (0.28)b 4 1

a Cumulative insect herbivore damage ratings were 1 to 4 (1 = no foliar damage;

2 = light damage (< 5 leaves damaged); 3 = moderate damage (6 to 20 leaves

damaged); 4 = heavy damage (> 20 leaves damaged).

b Fertilizer treatments are application of 0 (no fertilizer), low (1.5 g/L), medium

(3.0 g/L), or high (6.0 g/L) concentration of Peter’sTM 20-20-20 soluble fertilizer. c Damage ratings followed by different letters are significantly different at

P < 0.05 by the Tukey test.
6

tussock moth, and several species of cicadellid leafhoppers.

The majority (75 percent) of observed insect herbivores

occurred on seedlings that received the medium and high

fertilizer treatments.

Discussion

The concentration of essential nutrients in plant tissue

influences interactions between insect herbivores and their

host plants. As with plants, nitrogen is most often the

limiting nutrient for insect herbivores (Mattson 1980).

However, insects typically have a much higher nitrogen

concentration in their tissues than plants (Southwood 1973).

Scarcity of nitrogen-rich plant tissue may directly limit

resource utilization by insect herbivores or their population

density or both (Mattson 1980, White 1974). In addition,

plant nitrogen may indirectly influence insect herbivore

performance or population levels through effects on production of plant secondary metabolites such as terpenoids

and tannins (Mattson 1980, Price 1997). Many secondary

metabolites protect plants by reducing the palatability or

nutritional quality of plant tissues, thus causing a decrease

in insect growth and survival (Mattson 1980, Price 1997).

How insect herbivores respond to the nutrient concentration of their host depends on a variety of factors, including

degree of feeding specialization, host plant species, and

feeding mode, i.e., tissue chewer vs. phloem feeder. Based

on fertilizer studies, a few patterns have emerged regarding

the response of tree-feeding insect herbivores to host

nutrient concentration. Fertilization apparently improves

the quality of deciduous host trees as food for insect herbivores by increasing nutritional quality and water content

which, in turn, results in enhanced larval growth and

survival (Kyto and others 1996). In addition, increases in

mineral nutrient availability have sometimes been associated with decreases in the foliar concentration of carbonbased defensive compounds, resulting in further increases

in insect performance (Kyto and others 1996). In conifers

the effect of fertilization is often more complicated; in

some cases simultaneously increasing tree host nutrient

content and defensive compound levels resulting in counteracting effects (Kyto and others 1996). Finally, a positive

impact of fertilization on growth and survival at the level

of individual insects does not necessarily translate into

enhanced population growth for insect herbivores. This is

attributed to impacts of fertilization on higher trophic levels

such as natural enemies, which, in turn, impact insect

herbivore populations (Kyto and others 1996).

Conclusions

Pressure from naturally occurring insect herbivores on

potted sweetgum seedlings observed in this study appeared

to be related to level of fertilization. Most observed insect

herbivores occurred on seedlings receiving the highest

fertilizer treatments. Major defoliation events occurred

primarily on seedlings receiving the highest fertilizer treatments and cumulative herbivore damage showed a positive

trend with increasing level of fertilizer. Surprisingly, the

major herbivore encountered was the yellowstriped armyworm, well known as a pest of various field crops and

herbaceous species, but not as a tree-feeding herbivore.

This insect may have potential as a pest of seedling stage

sweetgum in short-rotation plantings.

Summary

1. Herbivore pressure on potted sweetgum seedlings

receiving four fertilizer treatments was observed as

complete defoliation at weeks 2 through 5 and cumulative herbivore damage at week 11 which was the end of

the experiment.

2. Complete defoliation of seedlings by insect herbivores

at weeks 2 through 5 was not associated with seedling

size and was positively related to the application and

rate of fertilizer applied.

3. Insect herbivore pressure on potted sweetgum seedlings,

measured as cumulative damage to foliage at week 11,

was positively related to the application and rate of

fertilizer applied.

4. Late instar armyworm larvae represent a potential

insect pest in fertilized sweetgum plantations.

Acknowledgments

We thank Pamela Dempsey for her help in maintenance of

sweetgum seedlings, Rufina Ward for assistance in rearing

insect specimens, and Richard Brown for assistance in

identification of the insect herbivores encountered in this

study.
7

Literature Cited

Farnum, P.; Timmis, R.; Kulp, J.L. 1983. Biotechnology of forest yield.

Science. 219: 694-702.

Kyto, M.; Niemela, P.; Larsson, S. 1996. Insects on trees: population and

individual response to fertilization. Oikos. 75: 148-159.

Mattson, W.J. 1980. Herbivory in relation to plant nitrogen content.

Annual Review of Ecological Systems. 11: 119-161.

Mengel, K.; Kirkby, E.A. 1982. Principles of plant nutrition. Third ed.

Bern, Switzerland: International Potash Institute. 655 p.

Price, P.W. 1997. Insect ecology. Third ed. New York: John Wiley. 874 p.

Ranney, J.W.; Wright, L.L.; Layton, P.A. 1987. Hardwood energy crops:

the technology of the intensive culture. Journal of Forestry. 85: 17-26.

Southwood, T.R.E. 1973. The insect/plant relationship – an evolutionary

perspective. Symposium of the Royal Entomological Society. 6: 3-30.

White, T.C.R. 1974. A hypothesis to explain outbreaks of looper

caterpillars, with special reference to populations of Selidosema

suavis in a plantation of Pinus radiata in New Zealand. Oecologia.

16: 279-301.
8
Ward, Kenneth E.; Sword Sayer, Mary Anne. 2004. Susceptibility of potted sweetgum

seedlings to insect herbivore damage as influenced by fertilization. Res. Pap. SRS-33.

Asheville, NC: U.S. Department of Agriculture, Forest Service, Southern Research

Station. 7 p.

We report the influence of fertilization on the susceptibility of sweetgum (Liquidambar

styraciflua L.) seedlings to naturally occurring insect herbivores. Thirteen-week-old

potted sweetgum were placed in a pasture near the margin of a hardwood forest containing scattered sweetgum trees. Groups of 14 seedlings were treated weekly with either no

(0), low (1.5 g/L), medium (3.0 g/L), or high (6.0 g/L) concentrations of 20-20-20

soluble fertilizer. Seedling performance was assessed by measuring height and leaf

number and rating seedling condition throughout the 11-week experiment. Types and

numbers of insect herbivores were recorded at 2- to 3-week intervals for each seedling throughout the experiment and cumulative damage was estimated for each seedling

at the end of the experiment. Herbivore damage was generally light, except for weeks 2

through 5, when seven seedlings were defoliated and killed. Six of these seedlings were

in the medium and high fertilizer treatments. Defoliations were due to late instar larvae of

the yellowstriped armyworm (Spodoptera ornithogalli Gn.). Other insect herbivore

species were detected, but their impacts were minor. Eleven seedlings received no

detectable herbivore damage during the experiment; 10 of these received zero and low

fertilizer treatments. Results suggest that the susceptibility of potted sweetgum seedlings to

insect herbivore damage was associated with fertilizer concentration. This effect was not

related to differences in seedling size because major defoliation events occurred before

significant differences in seedling size among fertilizer treatments were detected.

Yellowstriped armyworm has potential as a pest of sweetgum seedlings in intensively

managed plantings.

Keywords: Herbivore-plant interactions, Liquidambar syraciflua, nutrient levels, shortrotation plantings, yellowstriped armyworm.
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