Time-lagged intraspecific competition in temporally separated cohorts of a generalist insect

Abstract

Competition can have far-reaching consequences for insect fitness and dispersion. Time-lagged interspecific competition is known to negatively affect fitness, yet time-lagged intraspecific competition is rarely studied outside of outbreak conditions. We tested the impact of competition between larval cohorts of the western tent caterpillar (Malacosoma californicum) feeding on chokecherry (Prunus virginiana). We reared larvae on host plants that either had or did not have feeding damage from tent caterpillars the previous season to test the bottom-up fitness effects of intraspecific competition. We measured host-plant quality to test potential mechanisms for bottom-up effects and conducted field oviposition surveys to determine if female adult tent caterpillars avoided host plants with evidence of prior tent caterpillar presence. We found that time-lagged intraspecific competition impacted tent caterpillar fitness by reducing female pupal mass, which is a predictor of lifetime fitness. We found that plants that had been fed upon by tent caterpillars the previous season had leaves that were significantly tougher than plants that had not been fed upon by tent caterpillars, which may explain why female tent caterpillars suffered reduced fitness on these plants. Finally, we found that there were fewer tent caterpillar egg masses on plants that had tent caterpillars earlier in the season than plants without tent caterpillars, which suggests that adult females avoid these plants for oviposition. Our results confirm that intraspecific competition occurs among tent caterpillars and suggests that time-lagged intraspecific competition has been overlooked as an important component of insect fitness.

Host-mediated, cross-generational intraspecific competition in a herbivore species

“…(Wise & Weinberg, 2002;Poelman et al, 2008;Stam et al, 2014;Abdala-Roberts et al, 2019; but see Visakorpi et al, 2019). For instance, Barnes and Murphy (2018) recently observed that the number of eggs laid by western tent caterpillar (Malacosoma californicum) females was much lower on chokecherry trees (Prunus virginiana) that have been defoliated by conspecific caterpillars earlier in the season, as a likely result of induced increase in leaf toughness. In our case, it is also possible that induced defense reactions were delayed in box trees, or that they were already relaxed when we released BTM moths three weeks after the herbivory treatment (Karban, 2011), which remains to be evaluated.…”

“…This explanation is in line with studies that have documented in several plant species reduced herbivore performance and changes in plant-associated herbivore communities linked to induced defenses after prior herbivory (Nykänen & Koricheva, 2004; Karban, 2011; Stam et al, 2014). In the case of multivoltine species, negative relationship between prior herbivory and subsequent larva growth rate could indicate intraspecific plant-mediated cross-generation competition between cohorts of herbivores separated in time (Barnes & Murphy, 2018), which could influence herbivore population dynamics and distribution across host individuals. However, BTM is thought to have broad tolerance to variability in host traits, as suggested by previous observations that BTM larva growth rate did not differ significantly among box-tree varieties (Leuthardt et al, 2013).…”

“…A common mechanism is that herbivory induces changes in plant quality and defenses that generally reduce the performance of late coming herbivores (Wratten et al, 1988;Agrawal, 1999;Abdala-Roberts et al, 2019). For instance, prior damage by the western tent caterpillar Malacosoma californicum Packard (Lepidoptera: Lasiocampidae) induces the regrowth of tougher leaves acting as physical defenses and reducing the fitness of the next tent caterpillars generation (Barnes & Murphy, 2018). However, several authors reported a mismatch between prior herbivory effects on female oviposition preference vs larval growth, consumption or survival of their offspring (Wise & Weinberg, 2002;Bergamini & Almeida-Neto, 2015;Martinez et al, 2017).…”

Geographic variation in performance of a widespread generalist insect herbivore

“…Furthermore, it is known that other herbivores feeding on the same host can decrease its quality for other competing herbivores (Kaplan & Denno, ). For example, early‐season herbivory by western tent caterpillars ( Malacosoma californicum , Lasiocampidae) on choke cherry in Colorado makes the remaining and sprouting leaves tougher (Barnes & Murphy, ), which may negatively affect FWs later in the season. Thus, the factors leading to geographic variation in performance on different populations of the same plant species could be a combination of genetic and environmental (abiotic and biotic) differences across sites.…”

Forest Fire Severity Affects Host Plant Quality and Insect Herbivore Damage

“…Plant quality can influence herbivores through three common measures of quality: toughness, water content and ratio of carbon to nitrogen (C:N) in leaves. For instance, a generalist herbivore was found to have lower pupal mass when reared on tougher leaves (Barnes and Murphy, 2018), and tougher leaves usually have fewer digestible components, which can inhibit larval feeding (Feeny, 1970). Herbivory seems to be positively related to water content (Faeth, 1985), possibly because low water content can make leaves less succulent and attractive to insect herbivores (Feeny, 1970).…”