Coping with crowds: density-dependent disease resistance in desert locusts

Abstract

Parasite transmission generally exhibits some form of positive density dependence. Thus, as population density increases, so too does the per capita risk of becoming infected. Under such circumstances, natural selection should favor individuals that use cues associated with population density to determine the optimal allocation of resources to disease resistance mechanisms. As a consequence, individuals experiencing crowded conditions are predicted to be more resistant to parasites and pathogens than those experiencing low-density conditions. This phenomenon (termed "density-dependent prophylaxis") [Wilson, K. & Reeson, A. F. (1998) Ecol. Entomol. 23, 100-101] is predicted to be particularly prevalent in outbreak pest species and in species exhibiting density-dependent phase polyphenism, such as the desert locust, Schistocerca gregaria. Here we show that, as predicted, desert locusts reared under crowded conditions are significantly more resistant than solitary locusts to the entomopathogenic fungus, Metarhizium anisopliae var. acridum, a key natural disease of acridids and an important agent in locust and grasshopper biocontrol. Moreover, enhanced pathogen resistance in crowded locusts is associated with elevated antimicrobial activity, but not with any difference in thermal preferences or behavioral fever response. These results have implications for understanding the development and biocontrol of locust plagues.

Figure 1

Response of desert locusts to infection by the fungus M. anisopliae var. acridum. (a) Log-survival curves for solitaria and gregaria phase locusts infected with the fungus. The two bold lines show the fitted values from the Cox's Proportional Hazard Model and the narrow lines and shading represent the 95% confidence intervals [calculated by using the S-Plus statistical package (62)]. The model includes mean body weight as a covariant in the model (see main text for details). None of the 84 locusts in the control group succumbed to fungal infection during the assessment period, compared with >90% positive mycosis for the treated insects. Infected gregaria locusts survived significantly longer than solitaria locusts (P < 0.0001). (b) Frequency distribution of temperatures selected on a thermal gradient by control (gray bars) and infected (black bars) solitaria adults. Infected solitaria locusts exhibited behavioral fever (P < 0.001).