Host quality does not matter to native or invasive cactus moth larvae: grave implications for North American prickly pears

Abstract

1. Relationships between prickly pear cactus (Cactaceae: Opuntia) and specialist moth larvae (Lepidoptera: Pyralidae) are exemplar of exclusive interactions between herbivorous insects and host plants. 2. We measured nutritional and defensive factors indicative of Opuntia quality to native and invasive host-specialised moth larvae. Our goals were to investigate whether host qualities varied among and within Opuntia species, use that information to test whether host quality axes predicted occupancy by larvae in two focal O. engelmannii populations, and draw conclusions about whether these herbivores were resource limited. 3. There were minimal differences in protein, carbohydrate, fatty acid, fibre, water, and cuticular thickness, among four Opuntia species distributed throughout Texas. Macronutrient content did not differ between most species. Mucilage exudation was an induced response to herbivory. Field experiments compared pairs of O. engelmannii in which one individual hosted native Melitara doddalis or invasive Cactoblastis cactorum larvae, and the other did not; host quality and nutritional space did not predict herbivore occupancy. Moreover, ordination of host quality traits did not reveal spatial aggregation of cacti with and without herbivores. 4. These results demonstrated that neither moth species was resource limited. An overall lack of variation among and within Texas Opuntia indicate that these species were suitable resources to invasive C. cactorum that recently established in Texas. 5. Outcomes of this study have serious implications for Opuntia, native moths, and associated food webs, that will soon interact with an expanding C. cactorum population as it spreads inland from the Texas Gulf Coast.