The nature of resistance to Dectes texanus (Col., Cerambycidae) in wild sunflower,Helianthus annuus

The longhorned beetle Dectes texanus LeConte is a serious pest of cultivated sunflowers, Helianthus annuus L. and soybeans, Glycine max (L.), throughout much of the American midwest. Significant yield losses arise from the pre‐harvest lodging of plants when mature larvae girdle the base of stalks in preparation for overwintering. We investigated possible reasons why infestation of wild H. annuus by D. texanus is remarkably rare, whereas 80–90% of plants may be infested in fields of cultivated varieties. When caged on wild sunflower plants in the field, laboratory‐reared, gravid females fed less and made significantly fewer ovipositional punctures than they did when subsequently caged on a cultivated plant. Furthermore, only 18.4% of ovipunctures in wild plants resulted in an egg being laid, compared with 44.9% in cultivated plants, suggesting that additional resistance factors reduce the probability of eggs being laid during ovipositor insertion. Female behaviour was not affected by adult diet prior to testing (wild vs. cultivated H. annuus petioles), nor was there any difference in response between females obtained as larvae from cultivated sunflower or from soybean. Petioles of wild H. annuus exuded more than four times the weight of resinous material when severed as did those of cultivated plants, were significantly less succulent as determined by water content, and required significantly more force to penetrate the epidermis in a standardized puncture test. We conclude that a combination of physical and chemical properties of wild H. annuus confers substantial resistance to oviposition by D. texanus and that this natural antixenosis has been inadvertently diminished in the course of breeding cultivated varieties.