Natural regulation of Helicoverpa armigera larvae by hymenopteran parasitoids in northern Cameroon: Is there scope for application of conservation biological control in cotton agroecosystems?

International audience

English. Highlights: • Influence of wild or cultivated host plants of H. armigera on parasitism of larvae was investigated in northern Cameroon over 7 years. • The role of Corynandra (=Cleome) viscosa and Hyptis spp. on H. armigera population dynamics was confirmed. • Meteorus laphygmarum (Braconidae) was the main parasitoid species found among the 9 hymenopteran species. • The challenges of the conservation biological control approach and the need to enhance studies on the role and life cycles of wild host plants are addressed.Abstract: The conservation biological control (CBC) concept has been explored through research conducted in northern Cameroon on larval parasitism of Helicoverpa armigera, a major cotton pest, on different host plants year round, from early September 2008 to late December 2014. A total of 47,152 caterpillars were reared over the 6-year period. They were found regularly present on crops such as cotton and okra and on wild plants, including Corynandra viscosa and Hyptis spp. The overall percentages of nematode- and disease-infected caterpillars were very low (1.05% and 2.92%, respectively). Diptera species accounted for less than 0.1% of the parasitoids that had emerged from caterpillars. Total hymenopteran parasitism was 10.5%, and varied markedly depending on the host plants. Nine Hymenoptera species were identified, with Meteorus laphygmarum being the dominant one (almost 80% of all 2698 adult parasitoids obtained after rearing), followed by Schoenlandella variegata (16.42%) and Charops spinitarsis (<2%). Out of this total, 35%, 28% and 25% of the adults were obtained respectively on C. vicosa, cotton and Hyptis spp., i.e. < 5% on all other host plants. The analysis conducted on 237 collections showed that, after adjusting for year and month effects, a highly significant difference was found between the parasitism rates observed on the different host plant species. The plants most suitable for H. armigera parasitism were Acantospermum hispidum, C. viscosa and Hyptis spp. The proportion of M. laphygmarum among emerged hymenoptera was highest on C. viscosa, medium on cotton and Hyptis spp. and lowest on L. esculentum. Future studies that could be carried out are outlined in the discussion with the aim of facilitating the transfer of parasitoids from caterpillars present on one plant species to those infesting cotton crops.