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Three models of the evolution of host-feeding behaviour in parasitoid wasps are developed. The first assumes that the wasp host feeds purely to obtain resources to mature eggs (limited resource model) while the second assumes that host feeding provides energy for maintenance (pro-ovigenic model). The third model assumes that host feeding provides resources for both maintenance and egg maturation (resource pool model). Two variants of the third model are examined: the first assumes that the risk of mortality is constant and state-independent, the second that resource-depleted individuals suffer a higher risk of mortality. The models are analysed using a combination of stochastic dynamic programming and analytical techniques. The models make different predictions about the relationships between the probability of host feeding and egg load and host density. The available experimental evidence best supports the resource pool model.

McGinley and Charnov (1988) propose that seasonal seed weight decline results from optimization of independently varying resource components: in particular, carbon and nitrogen. Canada thistle (Cirsium arvense) (L.) Scop, does not express seasonal seed weight reduction when the number of seeds competing for the plant's resources is reduced by low pollination success. Seeds sampled from thistles treated to high and low pollination regimes were analyzed here for relative investment of carbon and nitrogen. The ratio of these two elements remained constant over the season in both treatment groups. The seasonal decline in mean seed weight displayed by this plant under high pollination is therefore not explainable by McGinley and Charnov's multiple resource pool hypothesis.

We extend the classical Smith-Fretwell model for the optimal size of an offspring to the case of allocation of two or more fitness enhancing resources. Unlike the results of the single-resource model, the new model predicts that the optimal allocations will depend on the resource pool sizes. We apply this new model to the problem of carbon and nitrogen allocation to seeds and conclude (1) that the optimal seed size (carbon allocation) should be positively correlated with the ratio of the size of the carbon and nitrogen pools available for investment to offspring (C/N ratio) and (2) that there should be a negative correlation between seed size and absolute seed nitrogen content. These results may account for some of the within- and between-plant variation in resource allocation to seeds.