Baculovirus resistance in the noctuid Spodoptera exempta is phenotypically plastic and responds to population density

Abstract

Parasite resistance mechanisms can be costly to maintain. We would therefore predict that organisms should invest in resistance only when it is likely to be required. Insects that show density-dependent phase polyphenism, developing di¡erent phenotypes at high and low population densities, have the opportunity to match their levels of investment in resistance with the likelihood of exposure to pathogens. As high population densities often precipitate disease epidemics, the high-density form should be selected to invest relatively more in resistance. We tested this prediction in larvae of the noctuid Spodoptera exempta. Larvae reared at a high density were found to be considerably more resistant to a nuclear polyhedrosis virus than those reared in isolation. A conspicuous feature of the high-density phase of S. exempta and other phase-polyphenic Lepidoptera is cuticular melanization. As melanization is controlled by the phenoloxidase enzyme system, which is also involved in the immune response, this suggests a possible mechanism for increased resistance at high population densities. We demonstrated that melanized S. exempta larvae were more resistant than non-melanized forms, independent of rearing density. We also found that haemolymph phenoloxidase activity was correlated with cuticular melanization, providing further evidence for a link between melanization and immunity. These results suggest that pathogen resistance in S. exempta is phenotypically plastic, and that the melanized cuticles characteristic of the highdensity form may be indicative of a more active immune system.

Density–dependent prophylaxis in the mealworm beetleTenebrio molitorL. (Coleoptera: Tenebrionidae): cuticular melanization is an indicator of investment in immunity

“…Hence, this paper is the ¢rst demonstration, to our knowledge, that DDP occurs against a non-speci¢c pathogen in a host from a population not previously exposed to it. The similarity of the results presented here with those of Reeson et al (1998) (i.e. insects reared at high density were found to be more pathogen resistant) and the fact that the system used is non-speci¢c suggests that plasticity in immune investment is a common response to £uctuations in population density, at least in insects exhibiting other density-dependent polyphenisms.…”

Heritability of immune function in the caterpillar Spodoptera littoralis

“…For example, Reeson et al (1998) showed that larvae of the African armyworm, Spodoptera exempta, that had been reared at high densities had significantly higher haemolymph PO levels and higher NPV resistance than those reared solitarily (Reeson et al, 1998). However, as far as we are aware, a direct link between PO activity and intra-specific variation in parasite resistance has yet to be conclusively demonstrated.…”

“…Heredity (2002) 88, 229-234. DOI: 10.1038/sj/hdy/6800031 1998; Reeson et al, 1998). However, PO in other parts of the body may also play an important role in immunity.…”

Diversity of entomopathogenic fungi near leaf-cutting ant nests in a neotropical forest, with particular reference to Metarhizium anisopliae var. anisopliae

“…However, relatively few parasites are known from social insects, in spite of their abundance and ecological dominance (Schmid-Hempel, 1998). This may relate to a genuine paucity, possibly resulting from a reduction in intergroup interactions (Wilson et al, 2003), or from the effective defences group-living insects can show, such as density-dependent immune defence (Barnes and SivaJothy, 2000;Reeson et al, 1998;Wilson et al, 2002), and enhanced behavioural or antimicrobial defences of groups against diseases Rosengaus et al, 1998;Traniello et al, 2002). There are, however, very few published surveys of the natural prevalence of social insect parasites.…”