Larval cannibalism, time constraints, and adult fitness in caddisflies that inhabit temporary wetlands

Abstract

The fitness of non-feeding adult insects depends on energy accumulated during the larval stage. Larvae of the caddisfly Asynarchus nigriculus primarily feed on plant detritus, but supplement their diet with animal material obtained through cannibalism. Habitat drying constrains development in many populations of this species, and we hypothesized that cannibalism should accelerate development to facilitate timely metamorphosis. We manipulated larval diets in a field experiment by supplementing detritus with animal material, and in a laboratory experiment by varying animal material and detritus quality (conditioned vs unconditioned). We measured the effects of dietary manipulation on larval and pupal growth and development, the timing of metamorphosis, and adult fitness correlates. The results of the laboratory experiment suggest that this species can metamorphose with a detritus-only diet, but development is extremely protracted. In the field experiment, individuals with animal material in their diet had higher larval survival, shorter larval and pupal development times, and earlier emergence dates (7-10 days), than those without a supplement. This delay in emergence should have important effects on survival in natural populations where the difference between desiccation and successful emergence can be only a few days. Dietary supplementation also affected adult body mass (30-40% increase), female fecundity (30% more eggs), and proportional allocation to different adult body parts. Our results are consistent with recent growth-development models that predict coupled (earlier emergence and larger adults) rather than tradeoff responses (earlier emergence and smaller adults) to pre-threshold manipulation of larval diets. Many detritivorous aquatic insects supplement their diets with animal material, and our data provide evidence that this supplementation can have strong effects on fitness. This type of dietary supplementation should be especially important for taxa that do not feed as adults, and in temporary habitats that impose time constraints on larval development.

Key themes in the study of seasonal adaptations in insects II. Life-cycle patterns

“…For example, abundant food can disable other trade-offs. Both earlier emergence and larger adults are possible when food is of high quality (Wissinger et al, 2004). Larger larvae of the moth Lobesia botrana, produced from larger eggs, perform better only in more difficult food conditions (Torres-Vila and Rodríguez-Molina, 2002).…”

Tufa Deposition in Karst Streams Can Enhance the Food Supply of the Grazing CaddisflyMelampophylax mucoreus (Limnephilidae)

“…Tufa cover was assumed to provide a higher food supply compared to the stones without crusts. However, females in the two treatments emerged in similarly periods which might display a sex specific response to different food supply (WISSINGER et al, 2004). Earlier emergence may favour males in competition for female mating partners and may additionally reduce the risk of predation during the larval stage due to shorter developmental time needed (ZONNEVELD, 1996).…”

“…However, food limitation can lead to a prolonged larval development and smaller adults (KLINGENBERG and SPENCE, 1997;TAMMARU, 1998;ROLFF et al, 2004). The outcome of food limitation affecting adult time of emergence and size at emergence depends on the life history plasticity of the particular species (BLANCKENHORN, 1999) and also on the type of metamorphosis with some differences between holo-and hemimetabolous insects (WISSINGER et al, 2004).…”

Biotic interactions enhance survival and fitness in the caddisfly Micropterna sequax (Trichoptera: Limnephilidae)

“…This resource addition mimics a natural situation in which Q. robur leaves, a very slow decomposing litter type (Swift et al, 1979), are occasionally distributed across the stream bottom in forested areas throughout the year (Minshall, 1967). Furthermore, additional food in the form of one cube (2 g) of frozen Tubifex oligochaetes (Dutch Select Food Tubifex; crude protein 6.2%, crude fat 2.6%, crude fibre 0.6%, ash 0.5%) was supplied every 2 weeks in each compartment to prevent cannibalism or predation in species which supplement their diet with (dead) invertebrates (Wissinger et al, 2004). These extra food sources were added to ensure that all potential components of the caddisflies diet were available and, therefore, would not limit survival and growth during the experiment.…”