Could neonicotinoid stake dips interfere with biological control of the cassava mealybug (Phenacoccus manihoti) in Thailand?

The cassava mealybug, Phenacoccus manihoti is a recent invasive pest in Southeast Asia. First detected in eastern Thailand in late 2008, P. manihoti has rapidly spread to most of SE Asia’s prime cassava-growing regions and caused devastating impacts on cassava production. Capable of causing yield impacts up to 84%, this pest is affecting a staple, bio-energy and industrial crop that’s grown on 4 million ha, primarily by resource-poor smallholders. To address mealybug infestations, the following integrated pest management (IPM) tactics were promoted: releases of the (exotic) parasitoid Anagyrus lopezi and prophylactic treatments of cassava planting material (i.e., stem cuttings) with systemic neonicotinoids. In this study, we evaluate the effect of the insecticide thiamethoxam on P. manihoti development and survival, and on survival rates of its key parasitoid, A. lopezi. Cassava cuttings were soaked in a 0.2 g/l solution of thiamethoxam for 15 min, and subsequently planted in 5 l plastic pots. Mealybug survival was measured at subsequent 7-day intervals following planting. Also, newly-emerged A. lopezi wasps were allowed to feed on sugar-rich cassava petiole exudate (i.e., extra-floral nectar) of treated plants. Stake dips effectively prevented mealybug development up till 49 days after planting. However, both male and female A. lopezi quickly died upon consumption of petiole exudate of treated plants during the first 30 days, and exhibited sharply reduced longevity thereafter. Our work shows that neonicotinoid stake dips are a highly-effective management tactic for P. manihoti, but also bring about undesirable (lethal) impacts on locally-occurring natural enemies. This study lays the necessary groundwork for an in-depth exploration of environmentally-sound pest management options for SE Asian cassava crops, and the development of IPM packages that are fully compatible with biological control.