COS 35-5 - Host quality and does not matter to native or invasive cactus moth larvae: Grave implications for North American prickly pears and trophic webs they support

Background/Question/Methods

Prickly pear cactus (Cactaceae: Opuntia) is a foundational plant genus in arid and semi-arid biomes of North America. Relationships between prickly pear and moth larvae that feed internally on cactus pads (Lepidoptera: Pyralidae) are exemplar of exclusive interactions between herbivorous insects and their host plants. As of late 2017, the South American cactus moth (Cactoblastis cactorum) that was successfully leveraged in biocontrol of introduced prickly pear around the planet has established a large population on the Texas Gulf Coast and is expanding its range quickly. This invasion may marginalize prickly pear cactus of western North America to a fraction of their foundational role in arid biomes. To begin addressing this situation, we asked if host quality factors of Texas Opuntia serve as biotic filters to host usage by invasive C. cactorum, or native cactus moths. We quantified nutritional and defensive factors indicative of prickly pear quality to host specialized pyralid moth larvae. Our objectives were to investigate whether host qualities varied among and within four Opuntia species distributed throughout Texas and use that information to test whether host quality axes predicted larval occupancy in two populations of the abundant and widespread species O. engelmannii.

Results/Conclusions

There were minimal differences in protein, carbohydrate, fatty acid, fiber, water, and cuticular thickness, among Opuntia species distributed throughout Texas. Macronutrient content did not differ significantly 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 C. 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. These results demonstrated that neither native nor invasive moth species were resource limited at our focal sites. Moreover, an overall lack of host quality variation among and within Texas Opuntia indicate that all four of these species are suitable resources to invasive C. cactorum. Findings of this study have serious implications for Opuntia of Mexico and the western United States, native moth species that specialize on eating them, and their associated food webs, that will soon interact with an expanding C. cactorum population as it spreads south and inland from the Texas Gulf Coast. Management recommendations and ongoing research in this system will be discussed.