Natural control of Helicoverpa armigera in sunflower: assessment of the role of predation
In three field experiments in Kenya, the seasonal population trend of Helicoverpa armigera (Hubner) (= Heliothis armigera) in sunflower was followed in plots from which predators were excluded and in plots from which predators were not excluded. In experiment 1, complete exclusion of crawling predators (predominantly Pheidole spp. of ants occurring at densities of 25 per plant) resulted in H. armigera densities 3 - 5 times higher than in plots where ants were not excluded. Pheidole sp. had more impact on young H. armigera larvae (instars 2 - 3) than on older larvae (instars 4 - 6). The results of experiment 2 were less striking, because H. armigera infestation was low, and ant densities were moderate. Here, Myrmicaria spp. and Camponotus spp. were the predominant ants. Exclusion of ants resulted in a 1. 8 - fold increase in densities of large H. armigera instars. In experiment 3, the impact of predators on H. armigera was studied under three conditions: exclusion of crawling predators, exclusion of both crawling and flying predators, and the control where no predators were excluded. To evaluate the role of predation in total mortality, the recruitment of H. armigera larvae was determined with Southwood and Jepson' s graphical method, and recruitment of newly laid eggs was directly measured on trap plants. Because it was difficult to detect older eggs due to colour change, the direct measurement of egg recruitment was superior to the graphical method. Ants and Anthocoridae were the dominant predators. Mortality from egg to older larvae (instars 4 - 6) was 73 - 78%. Exclusion of ants and anthocorids did not affect the densities of H. armigera larvae. Anthocorid predators increased only after the main oviposition peak of H. armigera and, therefore, their exclusion had little impact on the pest. Ant density was considerably lower than in experiment 1, and did not significantly suppress H. armigera.
Van den Berg, H.; Cock, M.J.W.; Oduor, G.I. Natural control of Helicoverpa armigera in sunflower: assessment of the role of predation. Biocontrol Science and Technology (1997) 7 (4) 613-630. [DOI: 10.1080/09583159730668]