Evaluation of entomopathogenic fungi and nematodes for the biological control of Macchiademus diplopterus

Abstract

Macchiademus diplopterus (Distant) (Hemiptera: Blissidae), commonly known as the grain chinch bug (GCB), is a pest of grasses and grains in South Africa, with aestivating adults posing a phytosanitary concern on export deciduous fruit. Effective pre-harvest management measures for nymphs and aestivating adults are needed to reduce export rejections. Laboratory bioassays evaluated four entomopathogenic nematodes (EPNs) and fungi (EPF) isolates as potential biological control agents against nymph and aestivating adult GCBs. EPNs caused high infection-induced mortality in both life stages, with the local Steinernema yirgalemense (157-C) achieving consistently high infection in nymphs and adults. The commercial Heterorhabditis bacteriophora (Field bugs) also showed potential against nymphs. In contrast, EPF isolates were largely ineffective against adults, causing less than 12% mortality. For nymphs, high mortality but low infection in control treatments prevented conclusions regarding EPF pathogenicity. However, Metarhizium robertsii and M. anisopliae showed the highest colonisation in cadavers. No significant differences were observed between in vitro and in vivo produced S. yirgalemense and H. bacteriophora for adults, while efficacy varied depending on isolate and production method for nymphs. Lethal concentration (LC) and effective concentration (EC) estimates confirmed the superior performance of S. yirgalemense in adults, with infection positively correlated with nematode concentration. This study is the first to report entomopathogen pathogenicity against nymph and aestivating adult GCBs of this species. Given the economic benefits of managing GCB in orchards rather than in grain crops, field evaluation of S. yirgalemense is recommended on deciduous fruit, where it could provide the first pre-harvest biological control option for this pest.