Male-biased susceptibility to parasites is common in dioecous plants. However, why males have higher parasite loads than females is unclear. Unlike males, females have to subsidize post-fertilization costs of reproduction (e.g. seed and fruit development). As a result, females may have smaller pools of resources potentially available to parasites, thus leading to lower parasite loads. We tested this prediction in New Zealand’s largest native moth (Aenetus virescens: Lepidoptera), whose larvae parasitize Aristotelia serrata (Elaeocarpaceae), an endemic species of dioecous tree. We measured parasite loads in male and female trees, as well as annual seed set in females. We then derived a technique to equate the energetic cost of seed set in females to an equivalent number of parasitic larvae. Our results showed evidence for male-biased parasitism; male trees harboured more larval parasites than female trees. However, when parasite loads in males were compared to parasite loads in females, plus the energetic cost of seed production calculated as an equivalent number of parasitic larvae, differences in parasitism between the sexes disappeared. We conclude that male-biased parasitism in plants could arise from parasite-offspring competition for female resources.