Influence of Diet and Density on Laboratory Cannibalism Behaviors in Gypsy Moth Larvae (Lymantria dispar L.)

Transcription

1 Influence of Diet and Density on Laboratory Cannibalism Behaviors in Gypsy Moth Larvae (Lymantria dispar L.) Charles J. Mason, Zachary Cannizzo & Kenneth F. Raffa Journal of Insect Behavior ISSN Volume 27 Number 6 J Insect Behav (2014) 27: DOI /s

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3 J Insect Behav (2014) 27: DOI /s Influence of Diet and Density on Laboratory Cannibalism Behaviors in Gypsy Moth Larvae (Lymantria dispar L.) Charles J. Mason & Zachary Cannizzo & Kenneth F. Raffa Revised: 13 June 2014 /Accepted: 25 June 2014 / Published online: 3 July 2014 # Springer Science+Business Media New York 2014 Abstract The gypsy moth (Lymantria dispar) is an insect folivore that feeds on a broad range of hosts, and undergoes intermittent outbreaks that cause extensive tree mortality. Like many other herbivorous insects, gypsy moth larvae consume a substrate that is low in nitrogen. Gypsy moth larvae have been known to cannibalize under crowded conditions in the laboratory. In this study, we assessed the influence of nitrogen and density on cannibalism behavior in gypsy moth larvae. Cannibalism rates increased with decreased nitrogen and increased density. There was no interaction between these two parameters. Developmental experiments confirmed that low dietary nitrogen is detrimental, in agreement with previous studies. In a second experiment, we assessed the influence of previous cannibalism experiences on subsequent cannibalism behavior. Gypsy moth larvae that had previously cannibalized other larvae subsequently exhibited higher cannibalism rates than those larvae that had not cannibalized. In conclusion, low nitrogen, high larval density, and previous cannibalism experience are important factors contributing to gypsy moth larval cannibalism. Future studies are needed to estimate benefits to larvae, and to more closely approximate field conditions. Keywords Gypsy moth. cannibalism. nitrogen. density. plant-insect interactions Introduction Plants properties that adversely affect insect herbivores include both chemical and physical defenses (Carmona et al. 2011; Mithöfer and Boland 2012), and low concentrations of nutrients that are required for insect growth and reproduction. Nitrogen, among the most critical nutrients for insect development, typically occurs in lower quantities in most plant species and tissues than herbivores require (Mattson 1980). C. J. Mason: Z. Cannizzo : K. F. Raffa (*) Department of Entomology, University of Wisconsin-Madison, 1630 Linden Dr., Madison, WI 53706, USA raffa@entomology.wisc.edu

4 694 J Insect Behav (2014) 27: Insect herbivores employ several physiological and behavioral mechanisms to cope with limited nitrogen contents, including nutritional symbioses (Douglas 2009), omnivory (Coll and Guershon 2002), compensatory feeding (for ex. Lindroth et al. 1997), and differential substrate feeding (Moreau et al. 2003). While many of these mechanisms are under strong genetic control, there are also more variable behaviors, such as cannibalism, that may supplement nitrogen and caloric intake. Cannibalism occurs among many animal species, including a diversity of arthropods. Cannibalism is not simply confined to stressed populations, but rather is a common response to changing environmental conditions (Fox 1975). Cannibalism in non-carnivorous insects is taxonomically diverse, occurring within many orders, and is frequently observed in Lepidoptera larvae (Richardson et al. 2010). Cannibalism occurs in many lepidopteran families, and can be among the largest sources of mortality in natural populations (Stinner et al. 1977). In addition to density, food quality is considered an important driver of cannibalism events (Kakimoto et al. 2003). The influence of cannibalism on Lepidoptera larval performance appears to be beneficial, as several studies have reported gains in development (Joyner and Gould 1985), diet processing (Raffa 1987), and survival (Chilcutt 2006) when compared to suboptimal diets. In addition to the direct benefits of cannibalism on insect fitness, cannibalism of natural populations may have indirect effects. For example, Chapman et al. (2000) found that cannibalism in Spodoptera frugiperda (Smith) can reduce the risk of predators responding to herbivory. Collectively, cannibalism potentially has important, yet poorly understood, consequences on larval fitness and population dynamics in a diversity of insect species. The gypsy moth (Lymantria dispar L.) is a foliage-feeding polyphagous herbivore that undergoes intermittent population outbreaks, in which they cause severe landscape-scale defoliation (Elkinton and Liebhold 1990; Davidsonetal. 1999). The gypsy moth has an extensive host range (Liebhold et al. 1995), encompassing a breadth of physical and chemical defenses, and nutritional qualities. Nitrogen availability is a major limitation in the performance of gypsy moth larvae (Hough and Pimentel 1978; Barbosa and Greenblatt 1979; Lindroth et al. 1997). The incorporation of nitrogen into eggs is a integral component of gypsy moth reproduction (Montgomery 1982), where larger eggs have higher quantities of vitellogenin proteins (Capinera et al. 1977). Egg size has been suggested to influence individual interactions, that may scale to population level-processes (Leonard 1970; CapineraandBarbosa1976). Like other herbivorous Lepidoptera, gypsy moth larvae partially contend with nitrogen limitations through compensatory feeding (Lindroth et al. 1997). Gypsy moth larvae cannibalize siblings in the laboratory (eg., Leonard and Doane 1966), although the factors influencing this behavior are not well understood. We assessed three factors that potentially contribute to gypsy moth cannibalism under laboratory conditions. First, we tested the effects of experimentally controlled larval densities and nitrogen concentrations on cannibalism rates. We then evaluated how previous cannibalism experiences influenced subsequent larval behaviors and performance. We used artificial diets modified to emulate nitrogen concentrations found in foliage, and densities mimicking field populations.

5 J Insect Behav (2014) 27: Methods Artificial Diet Preparation and Insect Rearing Wheat germ-based artificial diets were made following the methods described by Stockhoff (1993). We modified the casein and α-cellulose content to correspond to 1.25, 2.25, 3.0, and 4.75 % nitrogen weight to volume. These nitrogen concentrations represent contents found in foliar tissues in tree species among gypsy moths hosts over the growing season (Hough and Pimentel 1978). All other components of the artificial diet were identical among the various nitrogen treatments. Gypsy moth eggs were obtained from the USDA APHIS rearing facility in Otis, MA. The egg masses were hatched in 8.5 cm-diameter plastic petri dishes in a growth chamber under a 16 h light : 8 h dark photoperiod at 25 C. Preliminary experiments indicated newly hatched larvae suffered extensive mortality under low N diet concentrations, so all newly hatched larvae were first reared with 3.0 % N diet until molting into the 2nd stadium when the treatments were administered. Diet was replaced as needed, every two to three days. Effects of Diet and Larval Density on Cannibalism and Development Upon molting into the second star, larvae were randomly assigned a diet and density treatment. Larvae were randomly selected, and reared in 8.5 cm-diameter plastic petri dish arenas at densities of 5, 10, or 20 larvae per dish. Each arena was assigned one of four nitrogen treatments. Each group of insects was treated as an individual replicate, and each diet by density combination had four replicate arenas. Total cannibalism and the development of larvae were assessed three times per week. After four weeks, four, or all of the remaining larvae, were randomly selected and reared individually until pupation. The total cannibalism, days until pupation, and pupal masses were recorded for each experiment. Effect of Previous Cannibalism Experience on Subsequent Behaviors We assessed the effects of previous cannibalism experiences on subsequent cannibalism rates using two cohorts of gypsy moth larvae. Gypsy moths were hatched from two different egg masses 48 h apart and reared on 3.0 % nitrogen diet. When larvae from the first cohort (C1) molted into the 2nd stadium, they were reared in 24 well plates paired with a single larvae from the younger cohort (C2). In addition to the treatments to the C1 larvae, 80 C2 larvae were reared individually. All larvae were reared on 1.25 % nitrogen diet upon separation. Cannibalism by the C1 larvae was recorded every 12 h. Upon molting into 3rd stadium, C1 larvae that either had or had not engaged in cannibalism were placed into a separate 8.5 cm diameter arenas containing four C2 larvae. Larvae were fed 1.25 % nitrogen diet for two weeks, and diet was replaced every two to three days. The amount of cannibalism per arena was determined after two weeks.

6 696 J Insect Behav (2014) 27: Statistical Analyses All data were analyzed in the statistical package R v (R Core Team 2013). The influences of dietary nitrogen content and larval density on gypsy moth cannibalism behavior were analyzed using a two-way analysis of variance in which both variables were treated as continuous. The influences of diet, density, and cannibalism rate (Note: for any treatment, this overall rate does not distinguish between which larvae did or did not cannibalize, so potential benefits from this behavior cannot be quantified) on the time to pupation and pupal mass were analyzed by fitting linear models to the data. We then analyzed the effects of density and cannibalism under each individual nitrogen treatment. The influence of previous cannibalism events on the proportion of subsequent larval cannibalism was analyzed using a two-sample t-test. Results General Observations of Cannibalism Behavior among Gypsy Moth Larvae Gypsy moth larvae cannibalized sibling larvae, but not eggs containing unhatched siblings. Cannibalism did not occur among first instar larvae, but rather began in the second instar. Oftentimes, the cannibalized individual was either initiating ecdysis or immediately following the molt. Older instars more readily cannibalized larvae that were not preparing to molt than did younger instars. Diet and Density Influence Cannibalism Rates Cannibalism rates by gypsy moth larvae increased under both low nitrogen (F 1,44 = 52.12, P<0.001) and high density treatments (F 1,44 =19.18, P<0.001; Fig. 1). There was no interaction effect (F 1,44 =0.31, P=0.58). Larval cannibalism was observed in at least one sample under all nitrogen and density combinations. At low larval densities, the level of cannibalism was similar among all but the lowest nitrogen treatment. At Fig. 1 Mean proportion of cannibalism (±1 standard error) by gypsy moth larvae under different nitrogen concentrations and larval densities. Nitrogen and density both had significant effect on larval cannibalism (P<0.001). The interaction had no effect (P=0.58)

7 J Insect Behav (2014) 27: higher densities, a distinction in cannibalism amounts between the other nitrogen treatments was observed, where the highest (4.75 %) had the least amount of cannibalism and increased as a factor of nitrogen concentration in the diet. The developmental studies confirmed that low dietary nitrogen decreases the size of resulting pupae (Table 1). As expected, increasing nitrogen content increased pupal mass and trended toward decreased time to pupation, but the latter effect was not statistically significant. Density showed an unexpected inverse relationship with development time, but this appears to be anomalous because this effect was always lost when analyzed within nitrogen categories. Similarly, cannibalism rate within a treatment showed a putative direct relationship with development time, but again cautious interpretation is needed, both because we cannot distinguish which larvae had cannibalized and this correlation was always lost within nitrogen categories. Previous Cannibalism Experience Increases Subsequent Cannibalism Behaviors Among larvae reared in unequally aged pairs, approximately half of the older, 2nd instar larvae exhibited cannibalism by the time they molted into the 3rd instar. Gypsy moth larvae that had cannibalized previously consumed more larvae than those larvae that had not engaged in a previous cannibalism event (Fig. 2). This amounted to an 85.7 % increase in cannibalism compared to the control insects over the same time (t df=6 =4.24, P=0.005). Discussion The increase in cannibalism rates with decreasing nitrogen content and increasing density may relate to the natural conditions under which this polyphagous, eruptive folivore feeds. Individual gypsy moth larvae commonly feed on many leaves, often of multiple species, having a broad range of nutritional qualities (Stoyenoff et al. 1994). Also, larvae from different population phases consume foliage of variable qualities, Table 1 Results of linear model fitting effects of diet, density, sex, and cannibalism on gypsy moth development time and pupal mass Time to pupation (days) Pupal Mass (g) Estimate t-value P-value Estimate t-value P-value All nitrogen treatments intercept < < nitrogen density < nitrogen:density < sex (male) < < cannibalism a < a Cannibalism corresponds to the amount of cannibalism in the treatments rather than whether the insect, itself, had cannibalized

8 698 J Insect Behav (2014) 27: Fig. 2 Influence of previous cannibalism behavior on subsequent cannibalism rates. Larvae either had no cannibalism experience (black) or a single, previous cannibalism experience (gray). Asterisk indicates significant difference between treatments at P=0.005 various forest community structures provide larvae with different host species and age compositions, and seasonal fluctuations yield different degrees of phenological synchrony and foliar suitability (Elkinton and Liebhold 1990; Rossiter 1991; Hunter and Lechowicz 1992; Kleinerl and Montgomery 1994). This variability, specifically in nitrogen content and density, can influence the rate of cannibalism, at least under controlled laboratory conditions (Fig. 1). Increased cannibalism in response to low dietary nitrogen may be common in Lepidoptera (Al-Zubaidi and Capinera 1983). Additional factors that contributed to gypsy moth cannibalism were larval age and experience. We did not observe cannibalism in early first instar larvae. This is not unique to gypsy moth (Goodbrod and Goff 1990), but contrasts with some other insects (Pienkowskp 1964; Agarwala and Dixon 1992). Size differential is probably an important factor (Hopper et al. 1996), and the large quantities of urticating hairs covering gypsy moth larvae may pose a physical barrier against early but not later stadia. We observed that cannibalism began when insects were undergoing ecdysis early in the second instar, and continued throughout development. We found that larvae that had previously cannibalized had a significantly higher rate of cannibalism (Fig. 2). Our observations noted that treatments in which larval cannibalism was high tended to include more individuals that were much larger than others within comparable nitrogen availabilities and densities. However, our experimental design did not allow for comparative measurements between larvae that did or did not cannibalize, so future experiments are needed to test if this behavior represents a form of compensatory feeding. It may be that cannibalism provides an immediate benefit, enabling a larva to obtain enough nitrogen and calories to successfully molt into the next instar. In our study we attempted to emulate natural conditions by providing diet with realistic nitrogen concentrations and population along a continuum. However, two factors in nature for which we could not account were variation in foliar quality over the insect s lifetime and the opportunity for prey to escape cannibals. Also, higher-scale and tritrophic effects of cannibalism can include population regulation (Stinner et al. 1977), through reduced risk of predation (Chapman et al. 2000), culling of sick

9 J Insect Behav (2014) 27: individuals (Rosengaus and Traniello 2001), and altered sex ratios (Hurd et al. 1994). Future experiments, in which the performances of individual larvae are tracked, and more heterogeneous arenas facilitate prey escape, are needed to better understand how cannibalism behavior influences overall fitness, establishment and persistence in suboptimal habitats, and density-dependent feedback in gypsy moth populations. Acknowledgments We thank the anonymous reviewer for a critical review that had improved this manuscript. This work was funded by a University of Wisconsin-Madison Hilldale Undergraduate Research Fellowship awarded to ZC, USDA Hatch #WIS01598 awarded to KFR, and the University of Wisconsin-Madison College of Agriculture and Life Sciences. References Agarwala BK, Dixon AFG (1992) Laboratory study of cannibalism and interspecific predation in ladybirds. Ecol Entomol 17: Al-Zubaidi FS, Capinera JL (1983) Application of different nitrogen levels to the host plant and cannibalistic behavior of beet armyworm, Spodoptera exigua (Hubner) (Lepidoptera: Noctuidae). Environ Entomol 12: Barbosa P, Greenblatt J (1979) Suitability, digestibility and assimilation of various host plants of the gypsy goth Lymantria dispar L. Oecologia 43: Capinera JL, Barbosa P (1976) Dispersal of first-instar gypsy moth larvae in relation to population quality. Oecologia 26:53 64 Capinera JL, Barbosa P, Hagedorn HH (1977) Yolk and yolk depletion of gypsy moth eggs: implications for population quality. Ann Entomol Soc Am 70:40 42 Carmona D, Lajeunesse MJ, Johnson MTJ (2011) Plant traits that predict resistance to herbivores. Funct Ecol 25: Chapman JW, Williams T, Martínez AM et al (2000) Does cannibalism in Spodoptera frugiperda (Lepidoptera: Noctuidae) reduce the risk of predation? Behav Ecol Sociobiol 48: Chilcutt CF (2006) Cannibalism of Helicoverpa zea (Lepidoptera: Noctuidae) from Bacillus thuringiensis (Bt) transgenic corn versus non-bt corn. J Econ Entomol 99: Coll M, Guershon M (2002) Omnivory in terrestrial arthropods: mixing plant and prey diets. Annu Rev Entomol 47: R Core Team (2013) R: A Language and Environment for Statistical Computing. Davidson CB, Gottschalk KW, Johnson JE (1999) Tree mortality following defoliation by the European gypsy moth (Lymantria dispar L.) in the United States: a review. For Sci 45:74 84 Douglas AE (2009) The microbial dimension in insect nutritional ecology. Funct Ecol 23:38 47 Elkinton JS, Liebhold AM (1990) Population dynamics of gypsy moth in North America. Annu Rev Entomol 35: Fox LR (1975) Cannibalism in natural populations. Annu Rev Ecol Syst 6: Goodbrod JR, Goff ML (1990) Effects of larval population density on rates of development and interactions between two species of Chrysomya (Diptera: Calliphoridae) in laboratory culture. J Med Entomol 27: Hopper KR, Crowley PH, Kielman D (1996) Density dependence, hatching synchrony, and within-cohort cannibalism on young dragonfly larvae. Ecology 77: Hough JA, Pimentel D (1978) Influence of host foliage on development, survival, and fecundity of the gypsy moth. Environ Entomol 7: Hunter AF, Lechowicz MJ (1992) Foliage quality changes during canopy development of some northern hardwood trees. Oecologia 89: Hurd LE, Eisenberg RM, Fagan W et al (1994) Cannibalism reverses male-biased sex ratio in adult mantids: female strategy against food limitation? Oikos 69: Joyner K, Gould F (1985) Developmental consequences of cannibalism in Heliothis zea (Lepidoptera: Noctuidae). Ann Entomol Soc Am 78:24 28 Kakimoto T, Fujisaki K, Miyatake T (2003) Egg laying preference, larval dispersion, and cannibalism in Helicoverpa armigera (Lepidoptera: Noctuidae). Ann Entomol Soc Am 96:

10 700 J Insect Behav (2014) 27: Kleinerl KW, Montgomery E (1994) Forest stand susceptibility to the gypsy moth (Lepidoptera: Lymantriidae): species and site effects on foliage quality to larvae. Environ Entomol 23: Leonard DE (1970) Intrinsic factors causing qualitative changes in populations of Porthetria dispar (Lepidoptera: Lymantriidae). Can Entomol 102: Leonard DE, Doane CC (1966) An artificial diet for the gypsy moth, Porthelria dispar (Lepidoptera: Lymantriidae). Ann Entomol Soc Am 59: Liebhold AM, Gottschalk KW, Muzika RM et al (1995) Suitability of North American tree species to the gypsy moth: a summary of field and laboratory tests. U.S. Department of Agriculture Forest Service NE Forest Experimental Station General Technical Bulletin NE-211. U.S. Department of Agriculture, Washington, D.C Lindroth RL, Klein KA, Hemming JDC, Feuker AM (1997) Variation in temperature and dietary nitrogen affect performance of the gypsy moth (Lymantria dispar L.). Physiol Entomol 22:55 64 Mattson WJ (1980) Herbivory in relation to plant nitrogen content. Annu Rev Ecol Syst 11: Mithöfer A, Boland W (2012) Plant defense against herbivores: chemical aspects. Annu Rev Plant Biol 63: Montgomery ME (1982) Life-cycle nitrogen budget for the gypsy moth, Lymantria dispar, reared on artificial diet. J Insect Physiol 28: Moreau G, Quiring DT, Eveleigh ES, Bauce E (2003) Advantages of a mixed diet: feeding on several foliar age classes increases the performance of a specialist insect herbivore. Oecologia 135: Pienkowskp RL (1964) The incidence and effect of egg cannibalism in first-instar Coleomegilla maculata lengi (Coleoptera: Coccinellidae). Ann Entomol Soc Am 58: Raffa KF (1987) Effect of host plant on cannibalism rates by fall armyworm (Lepidoptera: Noctuidae) larvae. Environ Entomol 16: Richardson ML, Mitchell RF, Reagel PF, Hanks LM (2010) Causes and consequences of cannibalism in noncarnivorous insects. Annu Rev Entomol 55:39 53, 4 Rosengaus R, Traniello J (2001) Disease susceptibility and the adaptive nature of colony demography in the dampwood termite Zootermopsis angusticollis. Behav Ecol Sociobiol 50: Rossiter MC (1991) Maternal effects generate variation in life history: consequences of egg weight plasticity in the gypsy moth. Funct Ecol 5: Stinner RE, Jones JW, Tuttle C, Caron RE (1977) Population mortality and cyclicity as affected by intraspecific competition. Can Entomol 109: Stockhoff BA (1993) Diet heterogeneity: implications for growth of a generalst herbivore, the gypsy moth. Ecology 74: Stoyenoff AJL, Witter JA, Montgomery ME et al (1994) Effects of host switching on gypsy moth (Lymantria dispar (L.)) under field conditions. Oecologia 97: