Management of white grubs (Coleoptera: Scarabeidae) with entomopathogenic nematodes in Rwanda

Abstract

Abstract White grubs are largely unsolved problems in vegetable and tuber production in East Africa. Novel Rwandan as well as international entomopathogenic nematodes (EPNs) were screened in nine laboratory bioassays and two small-scale field trials in 2014 and 2015. Soil-based laboratory bioassays revealed that all EPNs were able to infect Anomala graueri larvae (Coleoptera: Scarabeidae), although a relatively high number of infective juveniles were needed. Rwandan EPNs were as infectious as their corresponding international strains. At 100 infective juveniles per larva, the Heterorhabditis bacteriophora strains (Rwanda14-N-C4a and international H06) as well as the international Steinernema carpocapsae All caused 18 to 22% grub mortality within 7 days compared to the control. At 1000 infective juveniles, both H. bacteriophora strains as well as the Rwandan S. carpocapsae RW14-G-R3a-2 killed 34 up to 58%. The Rwandan Steinernema RW14-M-C2a-3 least performed in the bioassays (2 to 6%). In two Irish potato fields, the into-soil-applied 1 × 109 infective juvenile Steinernema RW14-M-C2b-1 per hectare reduced 29 ± 33% and 96 ± 3% of grubs within 30 and 60 days, respectively. About 1 up to 2.5 × 109 infective juvenile Steinernema longicaudum X7 per hectare reduced 77 up to 85% of grubs until day 30, respectively, and 82 up to 95% until day 60. Avermectin + Chlorpyrifos tuber coatings reduced 39% of grubs, and Fipronil + Chlorpyrifos reduced 27%, whereas handpicking did not help much. In conclusion, at least Steinernema RW14-M-C2b-1 and S. longicaudum X7 are promising for managing white grubs in tuber production, this is at a rate of at least 1.5 × 109 EPNs per hectare, but H. bacteriophora RW14-N-C4a needs further field research. Findings will support the biocontrol product development in Rwanda, including registration if any would be needed for native macrobial biocontrol agents.

Keywords

• Anomala graueri
• Augmentation biological control
• Entomoparasitic nematodes
• Rwanda