Impact of Metarhizium Anisopliae and Beauveria Bassiana on Bee Pollinators (Hymenoptera: Apidae), and Modelling Their Performance in Bee Colonies

Abstract

Insect pollination sustains the biodiversity of 90% of wild plants, and 75% of crop species for food and nutritional security. Chemical pesticides used to manage arthropod pests constitute a key driver to the unprecedented declines of insect pollinators worldwide. Hence, biopesticides based on entomopathogenic fungi (EPF) are being promoted as safer alternatives. The effects of EPF on insect pollinators have not been investigated in detail for the application in pollinator-resourced crop systems. Thus, this study screened EPF isolates of Metarhizium anisopliae (ICIPE 7, ICIPE 20, ICIPE 62, ICIPE 69 and ICIPE 78), and Beauveria bassiana (ICIPE 284) for their effect on the Western honey bee (Apis mellifera) and African stingless bee (Meliponula ferruginea). The study was undertaken at the international centre of Insect Physiology and Ecology (icipe), Nairobi, Kenya, from November 2019 through February 2021. In the first part of the study, groups of 25–30 bees/cage were exposed to surfaces sprayed separately with six isolates (108 conidial/mL) or sterile water (control) and incubated for 10 days. The exposure assay was replicated four times and repeated twice for each bee species, and conidial acquisition was evaluated on five bees/cage. Apis mellifera acquired more conidia (2.8 × 104–1.3 × 105 colony-forming units [CFU]/bee) than M. ferruginea (1.1 × 104–2.3 × 104 CFU/bee) based on the analysis of variance. Except for ICIPE 7, ICIPE 20 and ICIPE 69 which caused significant A. mellifera mortality (25.8–40.4%) in the first experiment, none of the isolates had a significant effect on either of the bee species according to survival analysis. The isolates are harmless and/or slightly harmful to bees according to the International Organization of Biological Control classification. Bee colonies inherently thermoregulate their hives and, thus, the second part of the study evaluated the performance of six isolates in bee colonies using eight predictive models describing thermal requirements; (minimum [Tmin], optimal [Topt] and maximum [Tmax] thresholds; and maximal performance [Pmax]). The isolates were incubated at 12, 16, 20, 24, 28, 32 and 36°C, and conidial germination and mycelial growth were measured and fitted to the models. Models were compared numerically (the Akaike information criterion [AIC], adjusted R2) and statistically (likelihood ratio test). The best models were the cardinal temperature model with inflection (CTMI) and Ratkowsky 3 for germination; and CTMI, Ratkowsky 2 and Lactin 1 for growth. Temperature nonlinearly affected the isolates’ performance and the isolates had different thermal requirements. Germination had Tmin, Topt, Tmax and Pmax of 13.2–14.2°C, 26.2–28.9°C, 35.7–36.3°C and 95.4–100.0%; while growth had 7.0–13.2°C, 25.9–28.4°C, 34.5–37.9°C and 1.36–2.28 mm/day, respectively. The low Topt indicate that the isolates are unlikely to operate in bee colonies. Best-fitting models can be routinely used in the selection and re-evaluation of EPF candidates. The third part of the study involved the application of M. anisopliae ICIPE 69 in two greenhouses. Greenhouses were partitioned into plots and planted with cucumber (Cucumis sativus) following good agricultural practices. Each plot was installed with a colony of M. ferruginea at blooming inception and the crops were sprayed with either ICIPE 69 or sterile water (control). The trials were repeated three times in a completely randomized block design. Colony survival, pollination behaviour, fruit set and yield, and persistence on crops were recorded within 9 days before until 18 days after treatment application. Collected data were analysed using generalized linear models. ICIPE 69 isolate did not result in a significant effect on these parameters while conidial acquisition by foragers and persistence on crops declined periodically. These tiered studies establish that EPF developed in Africa can be safely used in integrated pest and pollinator management (IPPM) programmes.