Crop yield and insect resistance: Effects of selection for increased fruit size on codling moth resistance in wild and domestic apples

Plant defense theory suggests that plants in natural systems must allocate limited resources among different physiological processes, including growth, reproduction, and the production of chemical or physical defenses.  However, it is unclear whether these same constraints apply in agricultural systems, which are generally characterized by high resource availability and reduced selective pressures from herbivores and disease.   Since the beginning of domestication ~10,000 years ago, most crops have been subject to strong selection for favorable traits, including an increased yield of edible biomass. If trade-offs between growth and defense apply in agricultural systems, then high-yielding crop varieties should also be characterized by reduced resistance to herbivores.  We examined the variation in fruit size and resistance to codling moth (Cydia pomonella) across 50 genotypes of wild apples (Malus sieversii) and 50 genotypes of domestic apples (Malus x domestica). Resistance was measured using a series of bioassays for both the oviposition preference of codling moth adults and the performance of codling moth larvae. Preliminary results suggest that as fruit size increases, resistance to codling moth decreases for both wild and domestic apples, providing evidence that growth/defense trade-offs exist even in managed orchard conditions and with a domesticated species that has an extensive history of artificial selection. Both fruit size and resistance are highly variable, particularly for wild apples; however the majority of commonly cultivated varieties are large fruited and low resistance types. These results provide an improved understanding of the mechanisms by which domestication, and in particular selection for increased yield, can influence the trajectory of plant-herbivore interactions in agroecosystems.