The size-number trade-off in clonal broods of a parasitic wasp: Responses to the amount and timing of resource availability

Life-history traits that shift in response to environmental conditions are well studied; however, allocation patterns of traits in the context of a trade-off shifting with the environment are hardly examined. Using a polyembryonic parasitoid wasp system, we revealed features of the plasticity in allocation by observing the trade-off outcome between individual body mass and the number of offspring in a wasp brood in response to different host conditions. We manipulated food availability to the host caterpillar at two different phases of development: early host instars during wasp embryo division and late host instars after completion of wasp embryo division. The food manipulation allowed us to shift the rate of development and the total mass of the wasp brood both separately and together through altered host development time and final size. With greater host mass, the trade-off relationship between wasp body mass and brood size shifted, to the same extent for both sexes, towards higher wasp brood size and individual body mass. On the other hand, with faster host development, the trait combinations shifted towards greater wasp body mass but smaller brood size along the same trade-off relationship. These shifts indicate a reduction of wasp brood size when resource level is reduced late in host development. This adjustment yields wasp brood size and body mass in rough proportion to each other and positively related to resource availability late in development. Our demonstration of simultaneous shifts in traits under the trade-off relationship indicates that wasp broods respond differently to the different environmental conditions by altering body mass and the number of wasp individuals, suggesting that the plastic response in the trade-off allocation pattern may be adaptive.