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

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 × 10 9 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 × 10 9 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 × 10 9 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.

Potential of entomopathogenic nematode isolates from Rwanda to control the tomato leaf miner, Tuta absoluta (Meyrick) (Lepidoptera: Gelechiidae)

“…For instance, Youssef (2015) observed that with 3 dosages of 250, 500, and 1000 IJs/ml for S. carpocapsae, mortality rates of T. absoluta larvae reached to 80, 100, and 100%, respectively. A similar trend was obtained by Batalla-Carrera et al (2010), Mutegi et al (2017), Yuksel et al (2018), and Kajuga et al (2018) on various pests. This could be due to the fact that high number of EPNs would result in high number of symbiotic bacteria released in the host's body and thus enhanced killing speed owing to increase digestion of host tissues by toxins and hydrolytic enzymes secreted by these bacteria (Van Damme et al 2016).…”

“…Kamali et al (2018). The local EPN isolates were able to kill T. absoluta and had even been found to be effective against white grubs in Rwanda (Kajuga et al 2018), while it was not easy to find EPNs which can kill them (Laznik and Trdan, 2015). Although EPNs live naturally in soil, different researchers found that they can be used on above-ground parts of the plant to control effectively the pests living in cryptic habitats like in leaf galleries (Batalla-Carrera et al In the present study, the third instar larvae were used; other studies revealed that EPNs were able to find and kill all the four larval instars inside or outside the leaf galleries (Batalla-Carrera et al 2010 andVan Damme et al 2016).…”

“…RW14-M-C2b-1, S. carpocapsae RW14-G-R3a-2, and Heterorhabditis bacteriophora RW14-N-C4a, which were isolated from semi-natural and small-holder farming habitats of Rwanda in the year 2014 (Yan et al 2016). The two exotic EPN species, S. carpocapsae All and H. bacteriophora H06, were obtained from Lvbenyan Biotech Ltd., Guangdong Institute of Applied Biological Resources (GIABR) in China (Kajuga et al 2018). The exotic species were used as standard checks since they are among the most used for control of foliar and soil insect pests (Lacey and Georgis, 2012).…”

“…Before using the EPNs, they were allowed to acclimatize at room temperature for 1 h, and their viability was checked under a stereomicroscope with × 60 magnification, where live IJs were moving actively (Garcia-Del-Pino et al 2013). These EPNs were used in bioassay once more than 90% of IJs were viable (Kajuga et al 2018).…”

“…Dead larvae were recognized by being unable to respond to stereoscopic light or to probing by camel's hair brush tip (Van Damme et al 2016). Dead and alive larvae at the end of bioassay were dissected under the stereomicroscope to determine the presence or absence of nematodes in their body (Kajuga et al 2018). The number of dead larvae at each observation period was used to compute the observed mortality (%), which was obtained by dividing the number of dead larvae per treatment by the total number of larvae per treatment and multiplying the result by hundred.…”