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 different 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 …

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 …