Oecologia (Berl.) 36, 203-209 (1978) Oecologia �9 by Springer-Verlag 1978

Population Quality, Dispersal and Numerical Change in the Gypsy Moth, Lymantria dispar (L.) *

Pedro Barbosa and John L. Capinera** Department of Entomology, University of Massachusetts, Amherst, MA 01003, USA

Summary. First instars from small and large gypsy moth eggs differ signifi- cantly in their head capsule width, weight, hatching time and the length of thoracic setae. Pupal weight and the developmental period of immature stages of the gypsy moth originating from small or large eggs do not differ significantly. The mean number of eggs per mass produced by females origi- nating from small eggs is greater than that of females from large eggs al- though not statistically significant. Highly significant differences in mean egg size of egg masses of each type of female were also observed. The relationship between egg size and dispersal strategies are discussed.

The gypsy moth is one of the most successful invaders and colonizers of New England forests. The spread of this species throughout and beyond New England is a paradigm of effective dispersal and outbreaks. Turner (1963) characterized the pattern of growth as extended periods of scarcity, rapid increases to epidemic levels and sudden declines. Although various studies have dealt with the impact of external factors, including parasitoids, predators, pathogens and weather (see Campbell, 1967, 1976; Campbell and Sloan, 1977a, b; Leonard, 1974; Bess, 1961), the intrinsic regulatory mechanisms affecting the size and rate of increase of gypsy moth populations have received relatively little study. Poly- morphism in the dispersal behavior of the gypsy moth may have a strong influence on numerical change (Capinera and Barbosa, 1976).

Adult gypsy moth females are flightless and dispersal is accomplished primar- ily by ballooning first instars. Investigations of gypsy moth dispersal (Burgess, 1913; Collins, 1915, 1934; Leonard, 1967, 1971; McManus, 1973)have resulted in an incomplete understanding of this process. Very little effort has been devoted

* Paper No. 2229 Massachusetts Agricultural Experiment Station. University of Massachusetts at Amherst. This research supported (in part) from Experiment Station Project No. 355 ** P r e s e n t address." Department of Zoology and Entomology, Colorado State University, Fort Collins, CO 80523, USA

0029-8549/78/0036/0203/$01.40

204 P. Barbosa and J.L. Capinera

~ M o M o ~ Unfavorable

re/-.~ ~ / [ t dispers~t ~ Small

1 Favorable } \ no. eggs / f o o d ] \ Small /

/ k a ~ e no. eggs Large/ e g g s " " More CroWding ~.J"2'la[vae

i ggs Viru~ ~ Small

Small no. eggs larvae Large /

pupae Less Extra j dispersal

~--instars -""

Fig. 1. Dispersal of first instar gypsy moths from large and small eggs in the presence of acceptable food. Based on data from Capinera and Barbosa (1976). "Large larvae" and "Small larvae" denote larvae which hatched from large (> 1.20 mm) and small (< 1.17 mm) eggs, respectively

to the s tudy of the significance o f pheno typ ie var iab i l i ty in d ispersa l capac i ty and the effect o f p o l y m o r p h i s m on numer ica l change in the popu la t ion . This va r i a t ion has been viewed as an annoyance to be averaged or expla ined away.

F e m a l e gypsy moths depos i t egg masses wi th some large and small eggs within each mass. A l t h o u g h la rvae f rom bo th egg types disperse, la rvae f rom larger eggs exhibi t a greater t endency to disperse (Fig. 1) (Cap ine ra and Barbosa , 1976). Thus, la rvae d i sp lay a behav io ra l or func t iona l p o l y m o r p h i s m reflected in egg size, and based pe rhaps on yo lk con ten t (Cap ine ra et al. , 1977). Larva l p o l y m o r p h i s m has been based on egg size differences and the behav io ra l differ- ences o f assoc ia ted larvae. This s tudy was des igned to fur ther charac te r ize the differences be tween la rvae f rom small and large eggs.

The egress a n d / o r ingress o f d ispers ing la rvae m a y have a s ignif icant influence on the survival and numer ica l change of local popu la t ions . F o r example , in our observa t ions and in those o f o ther inves t iga tors (Campbe l l , 1967), p o p u l a t i o n densi ty was found to be higher or lower than expected (based on egg mass counts) depend ing on the differences be tween the n u m b e r o f first ins tars dispers- ing away f rom or into an area. A work ing mode l can be p r o p o s e d to descr ibe

the re la t ionship be tween va r ia t ion in dispersal , hos t species, and reproduc t ive phys io logy, the re la t ionsh ip be tween deve lopmen ta l phys io logy and egg size and the her i tab i l i ty of egg size. This s tudy examines some of these re la t ionships and with o ther ava i lab le da ta p roposes a p re l imina ry mode l in tegra t ing some intr insic and extr insic fac tors affect ing gypsy m o t h popu la t ions .

Materials and Methods

One hundred and fifty-eight larvae from small eggs (< 1.17 mm) and 229 larvae from large eggs (> 1.20 ram) were reared individually in 8-oz unwaxed paper cups and fed on artificial diet (Leonard and Doane, 1966). In environmental growth chambers, set at a 16-h photophase, 80 ~ F, and 70% R.H. Observations were made on development time, pupal weight, sex, number of eggs per mass and mean egg size. As adults emerged from pupae of larvae from small eggs they were mated with the first available adult of the same type. The same procedure was used for adults raised from large eggs. Size measurements were made with an ocular micrometer.

Another cohort of small and large eggs were selected for measurements of morphological and weight differences. Sixty-five large eggs and 67 small eggs hatched over a 4-day period and

Population Quality in Gypsy Moth 205

for each larva, on the first day of hatch, measurements were made of head capsule width, body length, weight and the length of three setae. Measurements were made of the longest setae arising from the left lateral tubercle of the 1st and 3rd thoracic segments and the 2nd abdominal segment, respectively.

Evaluations of developmental and reproductive differences were made by the use of analysis of variance, Duncan's multiple range test and chi square tests. Morphometrie and 1st day, 1st instar weights of larval phenotypes, were analysized using t-tests.

Results and Conclusions

The polymorphism suggested by the differences in dispersal behavior of larvae from small and large eggs is confirmed by morphometric and weight differences of 1st instars (Table 1). All parameters evaluated were significantly different. In addition, small eggs hatched later than large eggs, as one would expect, since the smallest eggs are laid last.

The mean pupal weight of individuals originating from small eggs was greater than that of individuals from large eggs. Females originating from small eggs produced a higher mean number of eggs than females originating from large eggs. However, neither of these differences were statistically significant. The developmental period from hatch to adult emergence also did not differ among larvae of the two types (Table 2). However, mean egg size in masses produced by females originating from large eggs was very significantly larger (P<0.01) than in those produced by females from sinai! eggs (Table 2). The evaluation of the same data by a Chi square test to determine the proportion of large egg masses (mean egg size > 1.20 mm) to small egg masses (mean egg size < 1.17 ram) produced by each type of female, similarly indicated that females from large eggs produced a significantly higher proportion of egg masses of large mean egg size compared with those produced by females from small eggs (Table 2). In general, females from small eggs may produce a greater proportion of resident (non-dispersant) larvae and females from large eggs may produce a greater proportion of dispersants (Table 3). Thus, since egg size is heritable, variation in egg size and subsequent larval dispersal behavior is expressive of genome potentialities, influenced by the environment.

Table 1. Variation in characterics of first instars from small and large eggs of the gypsy moth

Egg size Mean Head Seta A Seta B Seta C Body Body of emer- capsule length weight origin gence width

(day)" (ram) (mm) (mm) (ram) (ram) (mg)

Small 3.3 0.58 1.I0 t.88 1.39 2.84 0.503 (_+o.73) (-+0.01) (_+0.10) (_+0.14) (_0.11) (_+0.10 (_+0.l~7)

Large 2.9 0.63 1.16 2.06 1.48 3.12 0.725 (_+1.2) (-+0.02) (-+0.06) (+0.10) (___0.06) (-+0.11) (_+0.075)

Day of mean emergence of all larvae of each group

206 P. Barbosa and J.L. Capinera

Table 2. Developmental and reproductive differences in individuals from small and large eggs

Sex Egg size N Mean SD of origin

Pupal weight (g) LG 110 1.70 0.51 SM 71 1.82 0.59

Development time (days) LG 110 50 4.0 SM 71 50 4.0

Number of eggs LG 77 861 282 SM 58 893 292

Egg size (mm) LG 77 1.18a 0.02 SM 58 1.17b 0.03

Pupal weight (g) LG 111 0.49 0.09 SM 79 0.47 0.10

Development time (days) LG 111 47 5.0 SM 79 46 3.0

N=sample number; SD=standard deviation * Values in column followed by different letter are significantly different at P < 0.01

Table 3. Percent egg masses of small, medium, and large mean egg size produced by females from small and large eggs

Egg size of origin No. of egg masses Mean egg size produced a

Small Medium Large (< 1.17mm) (1.17-1.20 mm) (> 1.20mm)

Small 59 49. i 37.2 12.5 Large 78 21.7 47.4 30.7

a Z2= 13.078"* with d.f.

Discussion

Female larvae from any given egg mass, which feed on favorable host species, become adults which deposit egg masses with a higher mean egg size than those raised on less favorable species (Capinera and Barbosa, 1977). These large egg masses, have a higher proportion of large eggs, i.e., more dispersant- type larvae. Leonard (1968, 1970) reported that larvae from small eggs generally exhibited a greater proportion of extra-instars, formed larger pupae, and produced more eggs as adults. No mention was made of subsequent mean egg size. Finally, the data in this study indicates that the mean egg size of egg masses produced by females originating from large or small eggs have an accordingly large or small mean egg size. Although the slightly larger number of eggs produced by females from small eggs is not statistically significant,

Population Quality in Gypsy Moth 207

Fig. 2. Schematic model of alternate pathways in the numerical change and distribution of the gypsy moth

~30

~_~o- 20

lo ~ l l z a _ _

1 2 3 4 5 6 Day

these differences coutd be important over several generations. The higher propor- tion of resident larvae produced by females from small eggs may remain as a core population and produce subsequent populations with high proportions of residents, possibly leading to population buildup and local outbreaks. Gypsy moth habitats are a mosaic of host plant species with a gradient of favorable to unfavorable hosts. Feeding on less favorable hosts, which produces small egg masses with small mean egg sizes would reinforce the production of resident types. Concurrently, dispersants would colonize new favorable sites. Of course, this suggested sequence of events can be influenced by many other environmental factors. Even under favorable food conditions, egg mass size may be reduced by larval crowding, and possibly by disease (nuclear polyhedrosis virus) (Ca- pinera and Barbosa, in manuscript). These hypotheses are illustrated in Figure 2.

Experiments by Wellington and Maelzer (1967) suggest a nutritional basis for behavioral types. Capinera et al. (1976) have demonstrated that yolk reserves could be depleted by abnormally high environmental temperatures, and suggested that larvae from large eggs, which normally have large yolk reserves and tend to disperse more frequently, could be converted to the equivalent of larvae from small eggs (indicated by the broken line in Fig. 2). This would result in retention of potential dispersants and enhance the potential for local population increases.

Gadgil (1971) and others have stated that there must be overriding advantages favoring the tendency to disperse, due to the high risks of mortality of disper- sants. Den Boer (1968) has proposed that phenotypic variation (e.g., dispersal behavior) in natural populations may result in "spreading the risk" of extinction. Gypsy moth populations seem to conform to this strategy. The risk of dispersal may be minimized by a greater production of residents, generation after gener- ation. Van Valen (1971) suggested that new populations tend to be founded by dispersers but in each population the frequency of dispersers declines over time. The greater dispersal tendency of 1st instar gypsy moths from large eggs may be only a transient event since dispersants would occur in smaller propor- tions as a greater proportion of resident types occur in successive generation. An alternative strategy would be greater dispersal only when the population becomes overcrowded, but a dependence on greater dispersal then would be counter-productive since (a) the number of eggs produced by females decreases with increasing crowding and (b) dispersal by 1st instars undoubtedly results

208 P. Barbosa and J.L. Capinera

in high mortali ty. With such mortal i ty stresses on 1st instar, greater dispersal success would be expected only if females had higher fecundity with overcrowd- ing, i.e., more dispersants not less.

Obviously, since the developmental period o f larvae f rom large and small eggs does not differ significantly it would be possible for adults f rom large eggs to mate with adults f rom small eggs. However , most larvae f rom large eggs are likely to have been dispersed away f rom the popula t ion foci and mat ing by adults f rom small eggs would mos t likely occur with other f rom small eggs. Fur ther study will be devoted to the mat ing of adults f rom small and large eggs and to the dispersal range of adult males, since the relative p ropor t ion of resident and dispersant progeny may determine the rates o f numer- ical change. Another impor tan t factor requiring further e laborat ion is the possi- bility that larger larvae can undergo a behavioral shift (i.e., become resident types rather than dispersants) due to environmental stresses such as prolonged cold, starvation, etc. (see Capinera et al., 1977).

The nature o f popula t ion quality in the gypsy moth provides flexibility in a highly varied forest ecosystem. While exploiting a relatively suitable habitat which may ultimately be the center of a popula t ion outbreak and collapse, dispersal provides for radiat ion into surrounding, potentially suitable areas. Qualitative variat ion has been shown to influence the popula t ion dynamics of the western tent caterpillar (Wellington, 1957, 1960, 1965, 1977). As indicated by our model, we believe that popula t ion quality similarly affects gypsy moth popula t ion levels.

Acknowledgements. The authors express their thanks to W. Cranshaw, S. Kirouac, W. Winnie, E.A. Harrington, Jane Greenblatt and Drs. D.A. Damon, W. Calvert, J. Myers and W.G. Wellington for their thoughts and efforts on various drafts of this manuscript.

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Population Quality in Gypsy Moth 209

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Received February 4, 1978