Effects of aphid-induced semiochemicals from cover plants on Harmonia axyridis (Coleoptera: Coccinellidae)

Aphid-Induced Volatiles and Subsequent Attraction of Natural Enemies Varies among Sorghum Cultivars

The production of herbivore-induced plant volatiles (HIPVs) is a type of indirect defense used by plants to attract natural enemies and reduce herbivory by insect pests. In many crops little is known about genotypic variation in HIPV production or how this may affect natural enemy attraction. In this study, we identified and quantified HIPVs produced by 10 sorghum (Sorghum bicolor) cultivars infested with a prominent aphid pest, the sorghum aphid (Melanaphis sorghi Theobald). Volatiles were collected using dynamic headspace sampling techniques and identified and quantified using GC-MS. The total amounts of volatiles induced by the aphids did not differ among the 10 cultivars, but overall blends of volatiles differed significantly in composition. Most notably, aphid herbivory induced higher levels of methyl salicylate (MeSA) emission in two cultivars, whereas in four cultivars, the volatile emissions did not change in response to aphid infestation. Dual-choice olfactometer assays were used to determine preference of the aphid parasitoid, Aphelinus nigritus, and predator, Chrysoperla rufilabris, between plants of the same cultivar that were un-infested or infested with aphids. Two aphid-infested cultivars were preferred by natural enemies, while four other cultivars were more attractive to natural enemies when they were free of aphids. The remaining four cultivars elicited no response from parasitoids. Our work suggests that genetic variation in HIPV emissions greatly affects parasitoid and predator attraction to aphid-infested sorghum and that screening crop cultivars for specific predator and parasitoid attractants has the potential to improve the efficacy of biological control.

Behavioral mechanism of transfer and dispersal of Propylaea japonica in cotton adjacent to sorghum fields

Increasing crop biodiversity, such as by adjacent managed crops, is recognized as an effective biological control measure. However, few studies have focused on the mechanisms involved in how adjacent managed crops increase natural enemy populations, leading to reduced pest numbers. This study investigated the hypothesis that cotton grown adjacent to sorghum would positively influence the feeding and oviposition preferences of the ladybug Propylaea japonica, which predates cotton aphids, leading to enhanced pest control. The populations of Aphis gossypii were significantly lower and those of P. japonica were significantly higher in cotton grown adjacent to sorghum compared with monoculture cotton fields. Regardless of diet on which the larva of P. japonica were reared (Melanaphis sacchari, A. gossypii, and 50% M. sacchari + 50% A. gossypii), the adults always consumed significantly more M. sacchari compared with A. gossypii. P. japonica also showed significantly higher feeding and oviposition preferences for host plants bearing aphids to only host plants. P. japonica fed M. sacchari preferred to lay eggs on cotton, whereas those fed A. gossypii preferred to lay eggs on sorghum. These results suggest that the habitat of natural enemies can be expanded by influencing their feeding and oviposition preferences to achieve pest control in adjacent cropping systems. This research, which incorporates field and laboratory studies, suggests an approach for the successful conservation and biological control of cotton aphids using adjacent managed cotton and sorghum crops.