Larval crowding leads to unusual reaction norms for size and time at maturity in a geometrid moth (Lepidoptera: Geometridae)

Abstract

Abstract. The theory of life history evolution generally predicts a negative across-environment correlation between development time and size at maturity in response to variations in environmental quality. Deviations from this pattern occur under specific circumstances. In particular, organisms may mature both early and at a small size when (1) some ultimate change (e.g. time constraint, resource exhaustion) in the environment precludes further growth, or (2) when there are predictable among-environment differences in mortality rates. The first scenario is frequently documented in insects but evidence for the second possibility is scarce. Here we report a crowding-induced plastic response resulting in a clear positive across-environment correlation between final weight and development time in a geometrid moth. The response was apparent during the entire larval period and in the last larval instar. Crowding also led to increased growth rates. As outbreaks have not been reported for this species it is unlikely that early pupation is a response to anticipated food shortage. Instead, we suggest that crowded larvae may perceive a higher risk of predation, perhaps because they are unable to distinguish conspecifics from potential predators. A possibility for a plastic increase in growth rate implies that the uncrowded larvae grow at submaximal rates, which indicates a cost of high growth rate.

Age and size at maturity: A quantitative review of diet‐induced reaction norms in insects

“…Studies in which variation in food amount or quality had been caused by crowding of the larvae were not considered in the present analysis. This was because crowding per se appears to often induce specific responses in developmental variables (Peters and Barbosa ; Tammaru et al ; Vellau and Tammaru ), not of direct relevance in the present context. Similarly, we did not include studies in which larval development had been modified in response to the cues of predator presence (Higginson and Ruxton ), or a seasonal cue such as photoperiod (Gotthard ; Teder et al ).…”

Temperature-modified density effects in the black soldier fly: low larval density leads to large size, short development time and high fat content

“…In particular, in several insect species, crowded larvae tend to mature earlier and at a smaller size (e.g. Esperk et al, 2007;Fantinou et al, 2008;Tammaru et al, 2000;Than et al, 2020;Vellau and Tammaru, 2012), which contradicts the general principle of a negative environmental correlation between size and age at maturity (Teder et al, 2014). Larvae grown at high densities are more likely to experience food shortages than those reared at low densities.…”

“…High larval densities in the BSF are previously shown to result in lower prepupal and pupal masses (Barragan-Fonseca et al, 2018;Jones and Tomberlin, 2019). Larval developmental time has been demonstrated to decrease in response to high conspecific densities in several insect species (Bauerfeind et al, 2005;Esperk et al, 2007;Lyimo et al, 1992;Vellau and Tammaru, 2012), but is reported to show the opposite pattern (longer at high densities) in the BSF (Jones and Tomberlin, 2019). In addition, previous larval density studies on the BSF suffer from methodological limitations that may hamper the interpretation of their results.…”

Ontogeny of sexual size dimorphism revisited: Females grow for a longer time and also faster

“…The species is univoltine in northern latitudes (including the study area), with the pupa as the overwintering stage [41]. Larval development of both sexes invariably consists of five instars ([41, 42, 43]; this study). The highly polyphagous larvae are external solitary feeders on leaves of their host plants.…”