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Twig to root: egg-nest density and underground nymph distribution in a periodical cicada (Hemiptera: Magicicada septendecim (L.)).

Abstract.--The egg-laying behavior of female insects could directly benefit the mother, her offspring, or a combination of both. Periodical cicada (Magicicada septendecim (L.)) females oviposit in twigs in the forest canopy, and newly hatched nymphs fall to the ground, where they spend 17 years feeding on tree roots. If nymph dispersal from their mother's oviposition site is limited, then female oviposition site selection could influence offspring fitness and survival. Here, we show that there is no correlation between egg-nest density in the forest canopy and nymph density directly below. Although the extent of nymph post-hatching dispersal remains unknown, our findings cast doubt on direct benefits to offspring as an explanation for female oviposition site choice.

Key words: Egg-laying behavior, post-hatching dispersal, oviposition site choice.

INTRODUCTION AND BACKGROUND

Periodical cicadas (Magicicada spp.) exhibit some of the most extreme life histories of any insect, with large populations of adults appearing only once every 13 or 17 years. Mating occurs within dense, noisy choruses, and after mating, female Magicicada cut shallow, v-shaped egg-nests in twigs of 3-11 cm diameter and insert up to thirty eggs in each (White, 1980). Magicicada nymphs spend 13 or 17 years underground feeding on xylem from roots (White and Strehl, 1978), and are reported to be fairly sessile, moving only several centimeters during their development (White and Lloyd, 1975). An adult female's choice of oviposition sites could substantially affect nymph survival if her decisions determine the environment and resources available to her developing nymphs.

Several patterns in Magicicada oviposition behavior have emerged. Ovipositing females show some preference for certain tree species. Ovipositing M. septendecim (L.) females prefer sugar maples and oaks, while generally avoiding conifers (White, 1980). Oviposition density correlates positively with sunlight availability (Yang, 2006), and females tend to concentrate their oviposition activities along sunny forest edges (White, 1980). Females also appear to be attracted to areas already containing cicadas (Simon et al., 1981), either because they are all making the same site-choice decisions, or because they are actively aggregating. Explanations for these behaviors include direct benefits to females over their lifetime of reproduction (e.g., high sunlight levels are advantageous for ovipositing female cicadas, or high adult densities reduce predation risk), or direct benefits to offspring in a given oviposition site (e.g., trees with more access to sunlight can grow more vigorously and provide more nutrition to the root-feeding nymphs). Although some predation may occur as nymphs hatch and move below ground, these losses must be negligible or randomly distributed if females gain benefits by manipulating the underground distribution of their offspring via oviposition behavior. Thus, any hypothesis that females manipulate the underground distribution and density of their offspring through their oviposition activities requires at least a strong initial correlation between aboveground oviposition density and belowground patterns of nymph density. A lack of such correlation severely discounts the hypothesis that female behaviors can substantially influence the belowground distribution of nymphs. This study examines the question of whether the density of first instar nymphs reflects the density of egg-nests immediately above them.

MATERIALS AND METHODS

We sampled a forest edge near the offices of the American Physical Society on the west side of the

William Floyd Parkway, Suffolk County, New York, on September 21, 2008. This forest consisted of densely spaced, uniform trees (< 50 cm dbh, < 10 m height) in a narrow strip overhanging lawn on either side. Soils were thin and sandy. During the peak of the 2008 emergence of Magicicada Brood XIV, adult cicadas were concentrated mainly on the edges of this strip. By August, the area had notable "flagging" damage from female oviposition. Only Magicicada septendecim has been found on Long Island (unpublished data).

[FIGURE 1 OMITTED]

We demarcated the forest edge using a 0.25 m x 0.25 m sampling frame, placed so that the area enclosed within the frame showed 50% lawn and 50% leaf litter; we designated the centerline of the flame as the forest edge. Along this edge, we established a transect starting 34.3 m north of Research Road and extending northward. We dug holes along the transect every 2 m, starting at zero meters and ending at 80 m, for a total of 40 holes. Each hole had a diameter of 0.3 m and a depth of 0.08 m. We inspected and sifted the dirt from each hole, and we removed all periodical cicada nymphs and counted them. Because of their ivory color and movement, though small, they stood out in the fine-grained soil. All nymphs were first instar, and no nymphs of other cicada species were found.

We estimated egg-nest density above each hole by sampling the egg-nests above. Using a 'plumb bob', we identified the lowest woody twig or branch of approximately pencil diameter (preferred oviposition substrate; (Williams and Simon, 1995)) directly above each hole and within 2 m of the ground. The forest at our study site was characterized by densely spaced, thick branches near to forest edge and low to the ground, with only thinner branches above and behind the edge: nymphs falling from higher than 2 m in the canopy would face an obstructed path to the ground. After identifying the lowest branch overhanging each of our plots, we counted egg-nests on these branches within 10 cm of the starting location. If a plot had no suitable twigs above it lower than 2 m, we eliminated that plot from further consideration. We documented the height from the ground and species of each twig. We performed a series of regression analyses on our data set to determine whether nymph number was dependent on egg-nest number or the height of the egg-nests.

RESULTS

A regression analysis of egg-nest number and nymph number revealed no significant relationship between these two variables (see Table 1, Fig. 1A). After removing all the data points containing one or fewer nymphs and looking only at data points under oak trees, regression analysis revealed only a weak relationship between eggnest number and nymph number (p [less than or equal to] 0.054, see Table 1, Fig. 1B). A regression analysis of branch height and nymph number also showed no significant relationship between these two variables (p [less than or equal to] 0.32, see Table 2, Fig. 2A). Removing data points did not result in a relationship between nymph number and egg-nest height (see Fig. 2B). We found no egg-nests on pine branches, but we did find nymphs directly beneath the branches of pine trees.

DISCUSSION

We found no strong association between the densities of egg-nests and the nymphs under them. Our goal was to identify those branches that had the most direct and unimpeded access to the ground below; nymphs falling from higher branches would have hit the lower branches before hitting the ground, and thus egg-nest densities on upper branches would not be expected to correlate strongly with nymph density in our plots. We sampled only this thicker, dense subset, where we would expect the maximum correlation between egg-nest density in the branches and nymph density below ground. The lack of any such correlation would suggest that either cicada nymphs do not fall straight down from their egg-nests into the ground, or that once on the ground or in the soil, they move. Although it is possible that the initial distribution of nymphs did match egg-nest distribution and that subsequent mortality reduced variance in density, we find this explanation implausible; although such density effects may operate over the long term, the cicadas in our study had been underground for less than two months. Karban (1984) observed high nymph mortality (up to 98%), but over a two-year period; more research must be done to pinpoint the exact timescale of nymph mortality underground. The lack of any discernable patterns at our study site, as well as the presence of cicada nymphs underneath pine trees without obvious egg-nests in them, suggests that nymphs do not fall straight to the ground. Thus, females probably cannot control the exact underground distributions of their offspring. While female oviposition site choice may determine the broad area in which nymphs ultimately establish, for example on a forest edge rather than in a forest interior, at a smaller scale the female's decision may have little effect on nymph success. Thus, our results cast doubt on hypotheses that explain female cicada oviposition behavior as an adaptation for increasing nymph survival (Yang, 2006). Other studies have failed to demonstrate that female oviposition behavior optimizes the fitness of hatching offspring; Hessian fly (Mayetiola destructor (Say)) revealed that females fail to choose optimal plant hosts for oviposition and larval feeding despite a complete lack of larval dispersion (Harris et al., 2001), and parasitoid wasp females (Aphidius ervi Haliday) likewise choose sub-optimal aphid hosts for oviposition and larval feeding (Henry et al., 2005). Considering that oviposition site choice seems suboptimal in these examples, in which there is no offspring dispersal, our results with periodical cicadas, in which nymphs do appear to disperse, seem even more plausible. Thus, in periodical cicadas, the hypothesis that females choose oviposition sites to optimize their offspring distribution appears to have little support. Future studies should investigate how far, and by what means, first-instar nymphs travel to the ground, how long they remain aboveground after hatching, and if they travel either aboveground or belowground before feeding.

[FIGURE 2 OMITTED]

ACKNOWLEDGMENTS

This study was conducted as part of the E&EB 230a Methods in Field Ecology course at Yale University. The authors would like to thank the American Physical Society for facilitating this study. Mike Neckermann and Elias Bonaros provided assistance in the field.

Received 16 June 2009; accepted 27 January 2010

LITERATURE CITED

Harris, M. O., M. Sandanyaka and W. Griffin. 2001. Oviposition preferences of the Hessian fly and their consequences for the survival and reproductive potential of offspring. Ecological Entomology 26: 473-486.

Henry, L. M., D. R. Gillespie and B. D. Roitberg. 2005. Does mother really know best? Oviposition preference reduced reproductive performance in the generalist parasitoid Aphidius ervi. Entomologia Experimentalis et Applicata 116: 167-174.

Karban, R. 1984. Opposite density effects of nymphal and adult mortality for periodical cicadas. Ecology 65: 1656-1661.

Lloyd, M. and H. S. Dybas. 1966. The periodical cicada problem. I. Population Ecology. Evolution 20: 133-149.

Simon, C., R. Karban and M. Lloyd. 1981. Patchiness, density, and aggregative behavior in sympatric allochronic populations of 17-year cicadas. Ecology 62:1525-1535.

White, J. 1980. Resource partitioning by ovipositing cicadas. American Naturalist 115: 1-28.

White, J. and C. E. Strehl. 1978. Xylem feeding by periodical cicada nymphs on tree roots. Ecological Entomology 3:323-327.

White, J. A. and M. Lloyd. 1975. Growth rates of 17 and 13-year periodical cicadas. American Midland Naturalist 94: 127-143.

Williams, K. S. and C. Simon. 1995. The ecology, behavior, and evolution of periodical Cicadas. Annual Review of Entomology 40: 269-295.

Yang, L. H. 2006. Periodical cicadas use light for oviposition site selection. Proceedings of the Royal Society of London Series B-Biological Sciences 273: 2993-3000.

ADRIANNE SMITS (1), JOHN COOLEY AND ERICA WESTERMAN

Department of Ecology and Evolutionary Biology, Yale University, 165 Prospect Street, New Haven CT 06520

(1) E-mail address for correspondence: adriennesmits@ msn.com
Table 1. Results of regression analyses with eggnest number
as independent variable.

Nymph number regressed on eggnest    n    coefficient     p

all data                            32      0.03       0.817
nymphs > 1                          10      0.137      0.619
nymphs under oaks                   24      0.268      0.09
nymphs > 0, under oaks              14      0.365      0.103
nymphs > 1, under oaks               8      0.403      0.054

Table 2. Results of regression analyses with height of
eggnest-bearing branch as independent variable.

Nymph number regressed on   n    coefficient     p
height of eggnests

all data                    32     -0.036      0.32
nymphs > 1                  10     -0.118      0.08
nymphs under oaks           24     -0.028      0.517
nymphs > 0, under oaks      14     -0.096      0.103
nymphs > 1, under oaks       8     -0.036      0.604
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Author:Smits, Adrianne; Cooley, John; Westerman, Erica
Publication:Entomologica Americana
Article Type:Report
Geographic Code:1USA
Date:Jan 1, 2010
Words:1981
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