establishment of early instar heliothis zea on soybeans

Entomol. exp. appl. 51: 233-240, 1989. �9 t989 Kluwer Academic Publishers. Printed in Belgium. 233

Establishment of early instar Heliothis zea on soybeans

Irene Terry 1, j. R. Bradley, Jr. & J. W. Van Duyn2 Department of Entomology, North Carolina State University, Raleigh, NC 27695, USA. Present address: 1 University of Arizona, Department of Entomology, Tucson, A Z 85721, USA ; 2 Tidewater Research Station, Rt. 2, Box 141, Plymouth, NC 29762, USA

Accepted: February 13, 1989

Key words: larval establishment, corn earworm, Glycine max, spin-down behavior

Abstract

Early instar Heliothis zea (Boddie) (Lepidoptera: Noctuidae) established on all stages of soybean (Glycine max [L.] Merrill) tested when larvae were confined to plants, although establishment decreased after podset stage and as seeds matured. In tests where larvae were allowed to disperse freely on plants, establishment also decreased as maturity increased, but was much lower in each stage than when confined to plants. Neonate larvae settled more quickly on terminals and expanding trifoliates than on mature trifoliates based on initiation of feeding and movement away from initial sites of placement. The ability of neonate larvae placed a short distance from soybean plants to reach a host was affected by high soil and ambient temperature. The results of these experiments suggest that larval host establishment and spin-down behavior play a major role in the late instar population distribution among soybean fields and that maturity of the host strongly influences that behavior.

Introduction

The corn earworm, Heliothis zea (Boddie), is a noctuid insect which infests many crops and wild hosts including corn, peanuts, tomato, soybeans and cotton. In eastern North Carolina (USA), corn serves as the primary host for the first two larval generations (F1 and F2). Soybean (Glycine max [L.] Merrill) is a dominant host of the F 3

larval population after corn matures and is no longer attractive to ovipositing F 2 females. Soybean fields are patchily distributed among an array of acceptable cultivated and wild host species for H. zea. Areas of cultivated host fields range from 2-4 ha (Bradley et al., 1986). Even within the soybean host system, plants from field to field vary in maturity and growth characters

depending primarily on the planting date and cultivar. Adjacent soybean fields have markedly different population densities (Deitz etal., 1976). Bradley & Van Duyn (1979) suggested that only certain phenological stages and growth conditions are suitable for oviposition by adult moths or for larval nutrition. There appears to be a window of susceptibility for soybeans to H. zea, where plants of a particular stage are more likely to sustain damaging larval populations. Impor- tant questions are 1) how adults and larvae select soybeans at various phenological stages and 2) how host quality affects survival.

With H. zea, a highly polyphagous, lepidop- terous insect, cues for acceptance must be satis- factory for both ovipositing females and neonate larvae. The cues and response thresholds for such

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a wide acceptance of host species may be very complex with possibly a graded response rather than an 'all or nothing' acceptance of plant species or plant stages within species. Terry et al. (1987a) present evidence from field studies that ovipositing H. zea females preferred younger prebloom to podset (R0 to R3) phenological stage over older more mature plants (R4 to R5). Addi- tionally, survival from egg to third instar was higher on less mature, late planted soybeans. Predator differences were insufficient to account for the differences in survival, which implies that either differential larval acceptance or suitability of soybeans at various stages may play a role in the population structure found among soybean fields. A plant may be accepted by adults and larvae but yet be unsuited for larval development due to nutritional aspects of antibiosis. Labora- tory studies with greenhouse grown plants indicated that prebloom, bloom and podfill plants are equally suitable nutritionally for growth and development once larvae establish (Terry et aL, 1987c).

Early instar H. zea larvae exhibit a behavior of spin-down (or silking) from plants, which is presumably a dispersal mechanism as found in other lepidopterans, e.g. Ectropis excursia (Ge- ometridae) (Ramachandran, 1987) and Lymantria dispar (Lymantriidae) (Lance, 1983 and Capinera & Barbosa, 1976) or perhaps an escape from predators. After spin-down, a larva can spend considerable energy in locating a host. Pitre & Hillhouse (1981) in a greenhouse study reported that many first and second instar corn earworm larvae dropped from soybean plants when moving from leaf to flower bud. Perhaps certain morpho- logical structures, especially at particular phenological stages, mechanically interfere with movement or deter larvae from feeding and thus increase the incidence of spin-down behavior. To determine whether lack of larval establishment contributes to loss of early instars, we initiated studies to compare establishment (i.e., reach at least second instar) on various stages of soybean under two conditions: with larvae confined to the plant and with larvae free to disperse. Specific behaviors that might contribute to the ability of

neonate larvae to establish on soybeans were also examined: 1)neonate larval settlement and spin- down from various plant parts; and 2) host location ability of early instars after spin-down.

Materials and methods

Two types of experiments were designed to determine the effects of soybean growth stage on establishment by young instar H. zea, one allowing and another preventing dispersal. Tests were initiated at least four times during each of the 1980-1982 soybean growing seasons on plantings of'Forrest' (Group V maturity) and 'Ransom' (Group VII) at various phenological stages: prebloom, early bloom to peak bloom, pod set, pod elongation, and pod maturing stages (V or R0, R1-R2, R3, R4, and R5-R7 respectively, Fehr & Caviness, 1977). Additionally, two other cultivars 'Bragg' (Group VII) and 'Coker 156' (Group VI) were tested at several maturity stages. All of the above cultivars are known to be susceptible to H. zea. Fields of early, mid, and late planting dates (before 20 May, 20 May - mid June, and after mid June, respectively) were used for Ransom in 1980, for 'Forrest', 'Bragg', and 'Ransom' in 1981, and for 'Coker 156', 'Forrest' and 'Ransom' in 1982. Row spacing on all plantings was 0.92 m.

Tests preventing larval dispersal. Fifty, 60, and 100 plants per planting were used in 1980, 1981, and 1982, respectively, with randomly chosen groups of 10 plants each in a 5 m section of row forming one experimental unit. One week prior to test initiation, plants within and surrounding the test area were sprayed with methyl parathion (1980, 1981) and methomyl and benomyl (1982) to prevent predators and the pathogen. Nomuraea rileyi (Farlow) Sampson, from affecting larvae. Additionally, predators, predator eggs, and field H. zea larvae and eggs were removed from the selected plants at test initiation. Larvae used in tests were obtained from a 7-10th generation North Carolina State University (NCSU) lab culture. One neonate larva was placed on each plant and was confined to the plant by a cage.

This cage consisted of a styrofoam (946 ml) drink cup frame which had most of the mid portion removed and covered with organdy to allow air and transpiration flow. The cup was attached to the plant by the cup lid which was fitted around the mainstem of the plant. Three to four trifoliates from top nodes (including terminals and expanding trifoliates) would fit inside the cage. Every three to four days status of each larva was recorded.

Establishment for each larva was acknowl- edged when it reached at least second instar. Proportion of larva that established from the average of seven and ten day observations (or until larvae molted or died) were used in the analyses. Proportion (P) data were transformed to arcsine (P)~/: before analysis. First, cultivar differences were tested across each phenological stage by analysis of variance (ANOVA). If the test of cultivar effects indicated no difference, then an ANOVA would include all cultivars to compare phenological stages. Otherwise, an analysis was completed for each cultivar across stages with years as replicates.

Tests allowing larval dispersal Three fine mesh saran field cages (1.86 • 1.86 • 1.86 m), each covering two rows • 1.86 m, were placed in each planting during each test period. Each experiment was initiated within several days of initiation of a test confining larvae to plants on plantings of 'Forrest' or 'Ransom' during 1981 and 'Ransom' during 1982. 'Forrest' was used in 1981 because of the failure of an early planted 'Ransom' field. Plants inside cages were sprayed with the same pesticides as in the previously described tests confining larvae. I-I. zea eggs were laid onto paper towels by adult female moths from the NCSU lab culture. Pieces of paper towel were cut that had four fertile eggs each. These pieces were pinned to younger expanding trifoliates of the test plants at a rate of 20 per 0.31 m row (10-20 plants) when eggs were at the 'black head' stage (within 24 h of hatch). The pieces of paper towel were collected 48 h later and eggs were evaluated for hatch. At seven and ten days after infestation of plants, six randomly located 0.31 m sections of the two rows

235

were sampled. Plants were clipped at ground level over a ground cloth, placed into plastic bags and were searched for larvae in the lab. The cloth and ground surrounding the plants were also searched at the time plants were removed. Similar tests were repeated in 1982 except that neonate larvae (rather than eggs) from NCSU lab culture were placed randomly among test plants (20 per 0.31 m row).

Pooled averages of the seven and ten day observations of proportion established (transformed by the arcsine (P)~/~) for various phenological stages were examined by ANOVA. Differences among years and phenological stages as well as 'year xphenological stage' interaction were tested. Treatments consisted of various phenological stages of soybean plantings throughout the season, which ranged from prebloom through reproductive stages to the R6 stage.

Comparison of dispersal and no dispersal tests. Since differences in test designs preclude strict statistical comparison between the two types of tests, first observational comparisons were made by a graphical representation of the results. Unpaired t-tests compared establishment values of each test type where there were matching cultivars and phenologies during 1981 and 1982.

Larval spin-down. Neonate larvae (from 1980 NCSU laboratory culture) were placed on terminals, expanding trifoliates, and mature leaves (one larva per plant) ofprebloom plant 'Ransom' cultivar field grown soybeans. Each larva on each plant part was observed for three hours. This was repeated on each of three days to evaluate the influence of the plant part on movement, spin- down behaviour and initial feeding. Larvae were checked at least every 5 min for their location and for evidence of feeding. ANOVA procedures in a randomized complete block (RCB) design compared all three sites for the variables measured with day of test as block (n = 3) and an individual larva (n = 10) for each treatment as a replicate within days.

Host location. Tests of host location ability of first instar larvae were completed initially in the field.

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Neonate larvae, after feeding on a soybean plant for 5 min, were placed approximately 3-5 cm from the base of the plant to determine whether a larva could locate a host after spin-down to the ground. Individuals were observed for a maxi- mum of 2 h or until they either 1)located a host or 2) died before finding a host.

A second set of tests compared host location in a greenhouse with either sun or shade conditions (39-44 ~ or 29-31 ~ soil surface temperature, respectively). Plants were grown to the fourth node stage (V4) in the greenhouse in 25 cm diameter pots. Plant spacing was approximately 10 cm. Initially instars one through four were tested to determine if individuals of any instar were able to locate a host under shade conditions. Subsequent tests were completed on neonate larvae only. Paired t-tests compared host location ability of neonate larvae in sun versus shade in six replicates with each replicate containing 15 individuals. Individuals were observed for a maxi- mum of 90 min.

Results

Tests not allowing dispersal Larvae established on all stages of soybean plants when they were confined to the plant, although establishment decreased as plant stage matured beyond bloom

1 O0-.

O0 >_ 9 0 - 0 u3 8O z 0 70 F-- Z 6O Ld

5= 5 0 - U3

4 0 - m

3 0 - b3 L,J 2 0 - <- ~. 1 0 -

~ o

UNCONFINED :D

RO R2 R3 R4 R5 R6

"v~ATURITY STACE OF SOYBEANS

1

R 7

Fig. 1. Percent e s t ab l i shmen t (+_ SE bars ) o f first ins tar

Heliothis zea larvae on soybeans at var ious matur i ty

s tages.

stage (Fig. 1, confined). Since larvae were some- times missing, establishment was calculated assuming that missing larvae were dead. It as difficult to locate dead first instars on plants, especially plants with blooms and terminals present. Because all cultivars were not tested at all phenological stages, the first set of ANOVA's analyzed the cultivar effect at each maturity stage separately. There were significant cultivar effects at only certain stages. Early phenological stages (R0-R2) indicated no differences among cultivars while later stages indicated differences (Table 1). Cultivar differences, however, were not consistent

Table 1. Effects o f cult ivar on successful H. zea hos t e s t ab l i shmen t ( reach second ins tar ) on the indicated soybean phenological

s tage under confined condi t ions

Cult ivar % es tab l i shed Phenological s tage

R0 R2 R3 R4 R5 R6 R7

' R a n s o m ' 88.1 a a 78 a 66 b 59.8 b 64.1 a 65.8 a -

'Bragg' 97.1 a - 86 ab 49.7 b - - - 'For res t ' 80.0 a 87 a 83 a 75.2 a 63.9 a 41.8 b 43

'Coker 156' 91.7 a 95 a - - 57.7 a 60.0 a -

F 1.99 1.05 5.7 4.9 0.37 6.7 P 0.13 0.37 0.007 0.014 0.70 0.004

df 3,58 2,28 2,38 2,35 2,33 2,30

�9 i a Analys is comple ted on a rcsme (P)~ data , bu t p resen ted as reverse t ransformed�9 M e a n s in the s ame co lumn followed by the

s a m e letter are not significantly different (~ = 0.05, D u n c a n s Mult iple Range Test) .

over time in these later stages. Separate analyses for 'Ransom' and 'Forrest' both indicated that earlier stages (R0 and R2 for Ransom and R0-R3 for Forrest) had higher levels of establishment (F= 28.8 and 13.9, d f = 5,127 and 5,53, P < 0.001 respectively) than later stages. No year effect was indicated ( F < 1.5 and P > 0.23 for both cultivars) nor an interaction among years and phenological stage ( F < 1.9, P>0.15) . A similar trend was indicated with 'Bragg' and 'Coker 156' where larvae established better on soybeans up to podset stage than older soybeans.

Tests allowing dispersal. In tests where larvae were free to disperse, there was year to year varia- bility indicated by a significant year effect (F -- 10.9df = 5,11P = 0.003)in the ANOVA. In 1981 establishment was relatively high in both the R0 and R2 stages, with 26.2 ~o and 19.9 ~o, respectively. Establishment on all later stages was quite low, all less than 10~o. However, in 1982, establishment was less than 10~ in all treatments except the peakbloom stage (10.8~o). In 1982, methomyl and benomyl were used to kill predators and the N. rileyi pathogen, respectively, while in 1981 only methyl parathion was used. Cooler moist weather conditions which promoted an outbreak of N. rileyi in field populations necessi- tated use of a fungicide in 1982. Numbers of predators were low in each treatment (P < 0.05

237

per 0.31 m row). N. riley# killed larvae were not found in these tests. Also, eggs were used instead of larvae in 1981. The effect of plant maturity was significant ( F = 14.9, d f = 5,11 P = 0.0001) (see Fig. 1) with higher establishment at R0 and R2 stages rather than remaining stages (17 avg. both years versus 4.9). The interaction effect was significant due to the year effect on the early stages (F = 6.6, df = 5,11 P = 0.0006). Later stages indicated no difference among years.

An average of 71.3~o versus 8.5~o establishment across all stages was observed in confined versus unconfined tests (Fig. 1). Specific stages as shown by the individual histograms indicated differences between test types at each stage. Addi- tionally, t-tests of matching phenologies, years and cultivars indicated that establishment of soybeans was greater under confined conditions compared with unconfined test procedures for each stage (t-- 3.3-17.7, df-- 10, P < 0.007 for 1981 comparisons and t = 5.0-9.3, d f = 13, P < 0.002 for 1982).

Spin-down and host relocation. When larvae were placed on terminals, expanding trifoliates or mature leaves, the least larval movement from any plant part was on the expanding trifoliates (Table 2). Movement and spin-down were highest when larvae were placed on mature trifoliates. Sample sizes were low in these tests (total

Table 2. Neonate H. zea larval behavior on various 'Ransom' cv. soybean (preblooming) plant parts a

Effect of ANOVA Original location location on:

F P Terminals Expanding Mature trifoliate trifoliates

To that moved 4.9 Time to move (rain) 3.2 % spin-down 4.2 % initially feed elsewhere 4.8 % feed at initial site 2.9 % mortality 7.0 Time to initiate feeding (min) 1.0

0.08 43.3 ab 23.0 b 66.7 a 0.14 60.7 a 31.7 ab 21.3 b 0.094 6.7 b 6.7 b 30.0 a 0.09 18.0 ab 6.7 b 26.7 a 0.16 70.0 ab 93.3 a 43.3 b 0.05 10.0 ab 0 b 30.0 a 0.44 24.7 a 56.0 a 46.0 a

Analysis performed on arcsine (P)�89 and significantly different (Duncans Multiple were observed for 3 h.

square root of count data. Means across rows followed by the same letter are not Range, z = 0.05); presented reverse transformed; df = 2,4. Individuals (total n = 30)

238

Table 3. Success of neonate H. zea larvae at locating a soybean host when placed 5 cm from plant, field conditions

Time of day Ambient T Soil surface T ~

n ~o find Time to Time to host find host death a

Shade Sun min (s.c.) min (s.c.)

08 : 30-10:30 7 26.5-30.5 26-28 33-43 57 36 (5.8) 36.7 (9.7) 11 : 05-14 : 30 10 32.0-35.5 32-34 40-46 30 15 (3.6) 15.6 (1.3)

a Of those that died before finding a host from both time periods, 70~o died in areas exposed to the sun.

n = 30), but indication is that feeding begins readily and dispersal is less frequent when larvae are on younger tissue. All instars tested could locate a host (with 100~o ofinstars two-four finding hosts and 75~o ofinstar one) from 7 cm when greenhouse temperature was 28 ~ In the field most first instar larvae died before finding the host due to exposure to the sun or by crawling into small (< 2 cm diameter and depth) holes in the soil from which they could not crawl out (Table 3). Greater distances from a host would probably be much harder for neonate or early instar larvae to travel, especially when canopies are open and soil surface temperatures are high (up to 46 ~ in these tests). Under greenhouse conditions, few neonates survived or located hosts when soil temperatures were > 39 ~ and most died in < 30 min (Table 4).

Discussion

H. zea larvae established on all soybean stages tested whether under confined or unconfined test conditions. However, establishment levels were much higher under the confined situation.

Because our establishment study does not exclude post-feeding mortality, our investigation here does not strictly test for larval acceptance of soybeans at various stages. The larval spin-down test offers evidence that older post-blooming plants that lack terminals and expanding trifoliates may not be as acceptable. Possibly, larvae act in a 'graded' (Rausher 1983) rather than an 'all or nothing' fashion, where the threshold for acceptance lowers to a point where a larva would remain on a less desirable plant and feed. Few larvae that spin-down would have a chance to test a lower threshold due to the difficulty in finding a host after dispersing. When larvae are confined in these tests, they still may spin-down, but relocation is easier and a lower threshold would allow a greater acceptance. Further behavioral studies would have to confirm this phenomenon.

Although young instars may not establish as well on more mature soybeans as readily as on younger prebloom and bloom stage plants, later stage plants are nutritionally suitable for development, especially for later instars as beans within pods are preferred feeding sites (Terry etal. , 1987b). Although trends among each cultivar were similar (i.e., larvae established better on

Table4. Success ofH. zea ne~nate ~arvae at ~cating a s~ybean h~st when p~aced 5 cm fr~m the p~ant~ greenh~use c~nditi~ns

Soil T Reps b ~o find host Time to ~o mortality Time to ~ find host (in 120 min) death r

min (se) (se) min (se)

29~176 6 41 72.8 (35.8) 21 (8.1) 65 (10.4) 390-44 ~ 6 5 10.5 (3.5) 95 (2.3) 16 (3.2)

a Potted plants with sandy soil. If larvae crawled outside the pot, they were placed inside again. b Each replicate consisted of 15 1st instar larvae (< 8 h after hatching) each. c Time to death of those that did not find a host.

younger than on more mature plants), cultivars differentially affected larval establishment at later stages. All cultivars used in these tests are of determinate growth and are known to be susceptible to H. zea. Establishment was approximately equal among prebloom and bloom stages, but larvae established better on some cultivars at R3 through R7 stages than other cultivars. This may be due to differential branching among cultivars/ plantings which would provide younger vegetative tissue (such as terminals and expanding trifoliates) for a longer period after bloom. Also, the texture of trifoliates of different cultivars at R5-R8 stages varies which may influence young instar behavior.

In tests where we evaluated the potential ability for larval host location after spin-down, most first instars did not f'md a host; perhaps, there is a high probability of mortality associated with spin- down behavior. This may account in large part for population density differences observed among fields at different stages, in addition to predator and ovipositional differences (Terry et al., 1987a). Once larvae spin-down, unless they land on another host, chances of locating another host are slim, even at short distances, while crawling over uneven or hot soil surface. Spin-down behavior, if a dispersal mechanism, may not be adaptive in a crop system, where dispersal from a less- desirable plant in a monoculture will most likely lead to locating a similar quality plant. However, dispersal by spin-down may at best lead to a younger leaf or a different structure which may be more desirable.

Our results indicated that within-plant dispersal even within acceptable prebloom plants, depended upon the plant part. Mature trifoliates induced greater dispersal as well as spin-down from the host. Since more eggs are laid on mature trifoliates than other plant parts and mature trifoliates are a greater proportion of plant area (Terry et al., 1987c) (especially in more mature plants of determinate soybeans), the probability of movement or spin-down may be higher on older than on younger plants. Ramachandran (1987) found that host plant species as well as age and quality of the foliage within species influenced

239

dispersal of first instar E. excursia. Further, the acceptability of a plant was correlated with its suitability as host, as larval survival after establishment was high. This is, in part, similar to findings in the present and previously reported studies of H. zea on soybeans. Capinera & Barbosa (1976) found that both suitability of the host, larval population quality and the interaction of these factors were responsible for the frequency of dispersal in gypsy moth first instar larvae.

Several phases of the host selection process are important in contributing to the differences observed in the densities of late instar H. zea

larvae among plantings of soybean. Results of field studies on incipient populations (Terry et al.,

1987a) have indicated that the greatest H. zea mortality on soybeans occurs during the egg to early instar period, particularly on the more mature plants. The present study substantiates results of field tests and indicates that early instar establishment on the host is important in H. zea

host selection. The more influential factors in selection of soybeans are 1) ovipositional preference either due to differential adult female host recognition or acceptance and 2)early instar mortality from differential larval establishment, with preference for less mature prebloom to podset stage plants.

R6sum6

L 'installation sur soja des chenilles de premier stade d' Heliothis zea

Des exprriences ont 6t6 rralisres dans deux types de conditions (avec ou sans possibilit6 de dispersion) pour drterminer si le taux d'instaUation des chenilles de premier stade de H. zea Bod. drpen- dait d'une mortalit6 lire au stade drveloppement de Glycine m a x L. L'installation 6tait considrrre comme rrussie quand les chenilles avaient atteint le second stade. Bien qu'il fflt possible de dis- tinguer les varirtrs de soja tant par le tanx d'installation des chenilles que par les drg~ts provo- qurs, les plantes les plus jeunes 6taient plus colo- nisres par des chenilles que les trmoins plus ftgrs.

240

Toutes les vari6t6s de soja fi n'importe quel stade 6taient accept6es lorsque la dispersion des chenilles 6tait impossible. Avec possibilit6 de dispersion, les chenilles ont aussi accept6 tousles stades ph6nologiques, mais en moins grands nombres que lorsque la dispersion 6tait impossible. Le comportement de dispersion 6tait beaucoup plus fr6quent sur les plantes fi port ind6termin6 que sur celles fi port d6termin6. Les possibilit6s de r6- installation apr6s dispersion des chenilles de premier stade ont 6t6 limitges.

Acknowledgements

The authors would like to thank the technical assistance of Elizabeth Huckaba, Nancy Leidy, Donna Baker and Angela Baker. This is a paper No. 11689 of the Journal Series of the North Carolina State Agricultural Research Service, Raleigh, N.C.

References

Bradley, J. R., Jr. & J.W. Van Duyn, 1979. Insect pest management in North Carolina soybeans, pp. 343-354 In: F. T. Corbin (ed.), World Soybean Research Conference II: Proceedings. Westview Press, Boulder, Colorado.

Bradley, J. R., Jr., G. A. Herzog, S. H. Roach, R. E. Stinner & L.I. Terry, 1986. Cultural control in southeastern United States cropping systems, pp. 22-28. In: Theory

and tactics o f Heliothis population management. I. Cultural and Biological Control. So. Coop. Series Bull. No. 316.

Capinera, J. L. & P. Barbosa, 1976. Dispersal of first instar larvae in relation to population quality. Oecologia 26: 53-60.

Deitz, L. L., J. W. Van Duyn, J. R. Bradley, Jr., R. L. Rabb, W.M. Brooks & R.E. Stinner, 1976. A guide to the identification and biology of soybean arthropods in North Carolina. N.C. Agric. Exp. Stn. Tech. Bull. No. 238.264 pp.

Fehr, W. R. & C. E. Caviness, 1977. Stages of soybean development. Iowa Coop. Ext. Serv. Spec. Rep. 80. 11 pp.

Lance, D. R., 1983. Host-seeking behavior of the gypsy moth: The influence of polyphagy and highly apparent host plants, pp. 201-224 In: S. Ahmad (ed.), Herbivorous Insects: Host-seeking behavior and mechanisms. Aca- demic Press, New York.

Pitre, H. N. & T. L. Hillhouse, 198l. Establishment of infestation and behavior of Heliothis zea on soybeans in the greenhouse. J. Ga. Entomol. Soc. 16: 28-34.

Ramachandran, R., 1987. Influence of host-plants on the wind dispersal and the survival of an Australian geometrid caterpillar. Entomol. exp. appl. 44: 289-294.

Rausher, M. D., 1983. Ecology of host-selection behavior in phytophagous insects, pp. 223-257. In: Denno, R. F. and M. S. McClure (ed.), Variable plants and herbivores in natural and managed systems. Academic Press, Inc., New York, NY.

Terry, I., J. R. Bradley, Jr. & J. W. Van Duyn, 1987a. Popula- tion dynamics ofHeliothis zea (Lepidoptera: Noctuidae) as influenced by selected soybean cultural practices. Environ. Entomol. 16: 237-245.

Terry, I., J. R. Bradley, Jr. & J. W. Van Duyn, 1987b. Survival and development ofHeliothis zea (Lepidoptera: Noctuidae) larvae on selected soybean growth stages. Environ. Entomol. 16: 441-445.

Terry, I., J. R. Bradley, Jr. & J. W. Van Duyn, 1987c. Within- plant distribution of Heliothis zea (Boddie) (Lepidoptera: Noctuidae) eggs on soybean. Environ. Entomol. 16: 625-629.

establishment of early instar heliothis zea on soybeans

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