Two's a Crowd: Phenotypic Adjustments and Prophylaxis in Anticarsia gemmatalis Larvae Are Triggered by the Presence of Conspecifics

Abstract

Defence from parasites and pathogens involves a cost. Thus, it is expected that organisms use this only at high population densities, where the risk of pathogen transmission may be high, as proposed by the "density-dependent prophylaxis" (DDP) hypothesis. These predictions have been tested in a wide range of insects, both in comparative and experimental studies. We think it pertinent to consider a continuum between solitarious and gregarious living insects, wherein: (1) solitarious insects are those that are constitutively solitary and do not express any phenotypic plasticity, (2) the middle of the continuum is represented by insects that are subject to fluctuations in local density and show a range of facultative and plastic changes; and (3) constitutively gregarious forms live gregariously and show the gregarious phenotype even in the absence of crowding stimuli. We aimed to chart some of the intermediary continuum with an insect that presents solitarious aspects, but that is subject to fluctuations in density. Thus, Anticarsia gemmatalis (Lepidoptera: Noctuidae) larvae reared at higher densities showed changes in coloration, a greater degree of encapsulation, had higher hemocyte densities and were more resistant to Baculovirus anticarsia, but not to Bacillus thuringiensis. Meanwhile, with increased rearing density there was reduced capsule melanization. Hemocyte density was the only variable that did not vary according to larval phenotype. The observed responses were not a continuous function of larval density, but an all-or-nothing response to the presence of a conspecific. As A. gemmatalis is not known for gregarious living, yet shows these density-dependent changes, it thus seems that this plastic phenotypic adjustment may be a broader phenomenon than previously thought.

Density‐dependent prophylaxis in primary anti‐parasite barriers in the velvetbean caterpillar

“…The density‐dependent prophylaxis hypothesis proposes that organisms should up‐regulate their defences at moments of high population density when the likelihood of being infected by horizontally‐transmitted pathogens is higher (Wilson & Reeson, ; Reynolds et al ., ). In line with previous studies, we have found support for this hypothesis in the velvetbean caterpillar: when reared at a higher population density, these were darker and more resistant to an entomopathogenic virus than green ones (Silva et al ., ). However, to date, the underlying mechanisms of such resistance have not been fully elucidated.…”

“…To avoid pseudoreplication, only one caterpillar from a given pot was ever used. Caterpillars were kept at these population densities as a way to stimulate their colour phenotypic changes (Silva et al, 2013). Thus, lone-reared caterpillars usually keep the green phenotype, whereas changes in colour (like black) are more often in group-reared caterpillars.…”

“…Despite this, most work on the DDP hypothesis has focused on the plasticity of physiological immune responses of the host [e.g. capsule melanization -the level of pigmentation (or melanin deposition), encapsulation response -the area (or volume) of cell cover, lysozyme activity, haemocyte counts, haemolymph phenoloxidase activity and T-cell response] in response to a perceived increase in density (and risk of infection) (Møller et al, 2003;Cotter et al, 2004a;Bailey et al, 2008;Silva et al, 2013). To the best of our knowledge, a DDP effect on the induction capacity in primary barriers upon a pathogenic challenge has yet to be investigated.…”

“…For this species, a suite of traits change in response to the local density of conspecifics, such as larval development time (Fescemyer & Hammond, 1986), pupal weights (Anazonwu & Johnson, 1986), and adult wet, dry, lipid, and protein weights (Fescemyer & Hammond, 1988). One of these traits is the melanization of the cuticle, which is indicative of up-regulation of immune function, and consequently, pathogen resistance (Silva et al, 2013). The aim of the present study was to examine whether morphometry and structure of the midgut and peritrophic matrix change in accordance with colour transition in velvetbean caterpillars, and whether such changes may provide the caterpillars a more protective barrier against invasion by AgMNPV.…”

O genoma de um baculovírus isolado de cadáveres de larvas de Lonomia obliqua (Lepidoptera: Saturniidae) : uma lagarta de interesse médico

“…Por conseguinte, conforme Lemaire (2002), é geralmente observado o hábito gregário, caracterizado pela aglomeração de centenas de indivíduos, formando manchas escuras em troncos ou galhos de árvores frutíferas, em lagartas da família Saturniidae, como L. obliqua e P. cynthia, bem como em lagartas da família Noctuidae, como A. gemmatalis, que no entanto, são consideradas um estágio transitório da evolução de polifenismo de fase, uma vez que suas agregações variam de acordo com a densidade de suas populações (Silva et al 2013). …”