Citation: Dubois, T.; Onsongo, S.K.; Omuse, E.R.; Odhiambo, J.A.; Akutse, K.S.; Mohamed, S.A. Efficacy of Metarhizium anisopliae against the Greater Pumpkin Fly Dacus bivitattus. Sustainability 2023, 15, 13185. https://doi.org/10.3390/ su151713185 Academic Editor: Marko Vincekovi´ c Received: 1 July 2023 Revised: 9 August 2023 Accepted: 21 August 2023 Published: 1 September 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). sustainability Article Efficacy of Metarhizium anisopliae against the Greater Pumpkin Fly Dacus bivitattus Thomas Dubois * , Susan K. Onsongo, Evanson R. Omuse , Joseph A. Odhiambo, Komivi S. Akutse and Samira A. Mohamed International Centre of Insect Physiology and Ecology (icipe), Nairobi 00100, Kenya; onsongosusan@gmail.com (S.K.O.); eomuse@icipe.org (E.R.O.); jodhiambo@icipe.org (J.A.O.); kakutse@icipe.org (K.S.A.); sfaris@icipe.org (S.A.M.) * Correspondence: tdubois@icipe.org; Tel.: +254-(0)-762925796 Abstract: The greater pumpkin fly Dacus bivittatus (Bigot) is a fruit fly indigenous to Africa, which causes extensive damage to Cucurbitaceae. To control this pest, farmers rely on synthetic chemi- cals, often organophosphates, which have negative effects on human health and the environment. However, the sustainable management of D. bivittatus may be obtained through integrated pest management (IPM) practices, with the use of biopesticides as a key component. In this study, the effect of nine isolates of the entomopathogenic fungus Metarhizium anisopliae (Metschnikoff) Sorokin (ICIPE 18, ICIPE 20, ICIPE 30, ICIPE 48, ICIPE 62, ICIPE 69, ICIPE 84, ICIPE 91 and ICIPE 94) was directly evaluated on adult D. bivittatus mortality. Adult flies were allowed to walk for 5 min on 0.3 g of dry conidia of each isolate and monitored daily for 10 days. We also evaluated the effect of sand inoculated with M. anisopliae on larval and pupal mortality and adult eclosion and mortality in three replicated experiments. Larvae were exposed to the same isolates at a concentration of 1 × 10 7 conidia/mL in sterile sand, and adult eclosion and mortality were monitored for 15 days. The median lethal time (LT 50 ) of adults after direct exposure was shortest for ICIPE 18, ICIPE 20, ICIPE 30 and ICIPE 69 (3.11–3.52 days). In infested sand, larval mortality was highest for ICIPE 18 and ICIPE 20 (≥42.50%), while pupal mortality was highest for ICIPE 30 (≥41.25%). The lowest eclosion was observed for ICIPE 18, ICIPE 20, ICIPE 30 and ICIPE 69 (≤40.00%). The LT 50 of adults eclosed from infested sand was shortest for ICIPE 18, ICIPE 20 and ICIPE 30 (4.48–6.95 days). ICIPE 18, ICIPE 20, ICIPE 30 and ICIPE 69 are, therefore, potential isolates for subsequent field testing on D. bivittatus populations. Keywords: cucurbit; Dacus bivittatus; entomopathogenic fungus; fruit fly; Metarhizium anisopliae; mortality 1. Introduction Cucurbits are an important dietary source of vitamins and minerals [1]. In sub- Saharan Africa, cucurbits are equally valued for their medicinal properties and their potential as a source of income for smallholder farmers [2]. In Kenya, the major species of cucurbits commonly grown by smallholder famers are butternut Cucurbita moschata Duchesne, pumpkin Cucurbita maxima Lamarck, cucumber Cucumis sativus L., courgette Cucurbita pepo L. and watermelon Citrullus lanatus (Thunberg) Matsumura & Nakai. These crops are primarily grown in Kajiado, Machakos, Makueni, Isiolo, Tharaka Nithi and Embu counties [3]. However, cucurbit production in the country is threatened by abiotic and biotic factors [3]. Tephritid fruit flies are the most important biotic constraints and include both alien species, such as the melon fly Zeugodacus cucurbitae (Coquillett), and native African species, such as the lesser pumpkin fly Dacus ciliatus Loew and the greater pumpkin fly Dacus bivittatus (Bigot) [4–7]. For example, in Kenya, based on a field study in the coastal region, 67% of the losses of bitter gourd Momordica charantia L. were largely attributed to infestation by a complex of these fruit fly species [8]. In Africa, members Sustainability 2023, 15, 13185. https://doi.org/10.3390/su151713185 https://www.mdpi.com/journal/sustainability