Effect of weeds on insect pests of maize and their natural enemies in Southern Mexico

A pilot study performed on the Pacific coastal plain of Chiapas, Mexico, focused on the prevalence of maize crop infestation by insect pests, parasitism of pests and the abundance of insect predators in maize plots with weeds compared with plots under a regime of rigorous manual weed control. Sampling was conducted on four occasions at 20, 32, 44 and 56 days post-planting. Infestation of maize by fall armyworm larvae, Spodoptera frugiperda (Lepidoptera: Noctuidae), was more than twice as great in plots with strict weed control compared with weedy plots at 20 days post-planting, but declined thereafter in both treatments. The prevalence of aphid infestation and the abundance of nitidulid beetles were consistently greater in weed-controlled plots. In contrast, the density of beneficial predatory Coleoptera increased significantly in plots with weeds, and it is suggested that this probably explains the lower incidence of pests. S. frugiperda egg masses placed in experimental plots suffered a significantly higher incidence rate of parasitism by Chelonus insularis (Hymenoptera: Braconidae) in clean plots (42.0%) compared with those placed in weedy plots (3.75%); it is suspected that weeds may hinder the location of egg masses by parasitoids. Overall, the presence or absence of weeds had a marked influence on the arthropod community present in maize fields. The weeds did not affect maize plant height, the levels of plant damage or the yield of grain from plants under each type of weed regime, implying that competitive effects of weeds may be offset by greater numbers of beneficial insects in weedy plots. Our pilot study indicates that strict weed control in maize may be unnecessary.