Potential predators and parasitoids for conservation biological control in smallholder bean farming tropical ecosystem

Abstract

Conservation biological control (CBC) is an attempt to protect the already existing natural enemies (predators, parasitoids or pathogens) of insect pests within the agricultural systems by manipulating the environment and farming practices to provide the required resources for their survival. This study assessed the major arthropod predators and parasitoids of common bean (Phaseolus vulgaris) insect pests in smallholder bean farming tropical ecosystem and their contribution in pest management, in three elevation zones during 2016 and 2017. The farmers’ knowledge about natural enemies, insect pest and pesticide use was investigated followed by field surveys and experiments to determine the contribution of major predators and parasitoids to pest management in smallholder bean fields. The importance of field margin vegetation to the population of predators and parasitoids was also examined. The study identified a severe lack of knowledge about natural enemies among the smallholder farmers. However, the field survey revealed the existence of a rich community of natural enemies, where a total of 5003 natural enemies were identified out of 13 961 insects collected. The natural enemy abundance differed along the elevation gradient where the high zone was leading with 50.3%, while mid and low zones had 31.7% and 18% respectively. Majority of the natural enemies were sampled along the margin vegetation compared with the bean fields for low (61.1% in margin vs 38.9% in field) and mid (52.1% in margin vs 47.9% in field) zones, but in the high zone they were more abundant within the bean fields (44.6% in margin vs 55.4% field). A dye experiment to monitor their movement revealed high levels of spatial flux (71%) between the two locations. Aphids (Aphis fabae) mortality rates measured by predation and parasitism of sentinel aphids did not significantly differ between the field edges and field centre in all the three elevation zones, indicating the centre of the fields still receive comparable pest control service as the field edge. Parasitoid wasps were the most abundant natural enemy while A. fabae were the most damaging insect pests in the smallholder bean fields. Molecular identification of A. fabae parasitoids revealed 85% primary parasitoids (Aphidius colemani) and two species of secondary parasitoids (Pachyneuron sp., 7% and Charipinae sp., 1%) which may have significant effects in biological pest control. The study revealed the potential predators and parasitoids important for pest control within the smallholder bean fields which can be enhanced through CBC.