Control of the sweet potato weevil (cylas puncticollis) with entomopathogenic nematodes.

Abstract

Sweet potato (Ipomoea batatas L) is one of the important crops worldwide because of its high calorific value. Sweet potato weevils (SPW) are the major pest affecting sweet potato, leading to a massive yield loss annually. An initial goal was to evaluate the diversity, incidence and damage severity caused to sweet potato by SPW in fields and in storage in three provinces, and the knowledge of small scale farmers about SPW. A survey was conducted in the Gauteng, North West and Limpopo provinces. Most of the farmers (96%) planted sweet potato for income generation, 3% farm for home consumption and 1% farm for employment. About 64% farmers knew about SPW, 28% had no knowledge and 8% were not sure. The crop was worst affected by SPW (79%), with rats, porcupines and other pests posing less of a problem (12%, 8% and 1%, respectively). Farmers were affected by SPW throughout the year. Farmers agreed that SPW had a negative impact on their production, with 34% of them expressing this sentiment, 28 were not sure and 14% did not think that SPW affected their crop yields. About 67% lost a quarter of their yield annually, 27% of the farmers lost a third of their yield and 6% lost half of their yield. All the three provinces are affected by SPW. Despite the majority of the farmers being aware of SPW damage, they had little knowledge of how to control the pest. Entomopathogenic nematodes (EPNs) have potential as biocontrol agents of economically important pests. This study was conducted to explore the potential of EPNs for the biological control of the SPW, Cylas puncticollis (Boheman) 1883, in an artificially infested field using larvae-infested sweet potato pieces placed in perforated containers. A total of six treatments were applied in the field, namely A - control, B - chemical insecticide, C - Steinernema tophus Cimen isolate ROOI 352 Formulation 1 with 2% Barricade® gel, D - S. tophus Formulation 2 (cadavers) E - Heterorhabditis bacteriophora Poinar, isolate SGI 245 Formulation 1 with 2% Barricade® gel, F - H. bacteriophora Formulation 2 (cadavers). After two weeks, larvae, pupae and adult SPW numbers were counted to determine the SPW populations. Heterorhabditis bacteriophora (Treatment F, cadaver formulation) was the most effective treatment with 0% live larvae and 36% dead larvae. Larvae, pupue and adult weevils were evaluated, Treatment F obtained 0% for live LPW and 48% for dead LPW. This short-term field trial demonstrated a successful reduction of all life stages of SPW by H. bacteriophoraSGI 245. Further trials of this EPN for the control of SPW are needed throughout the crop production cycle.

Entomopathogenic nematodes (EPNs) are of current research interest because of their ability to kill insect pests as biocontrol agents. However, EPNs are sensitive to ultraviolet (UV) light, high temperatures and desiccation. The aim of one study was to analyse the survival and efficacy of two strains of EPNs, produced either in vivo or in vitro, and formulated in either Barricade® gel or Potassium polyacrylate hydrogel (PPH) formulations. EPNs were produced in vivo using Galleria mellonella (Fabricius) 1798 (the greater wax moth), and in vitro using an artificial medium containing ground, desiccated larvae of Musca domestica (Linnaeus), the housefly. IJs of the three selected strains of EPNs were suspended in 2% gel formulations of Barricade® gel or PPH by mixing 98 ml of water containing infective juveniles (1000 IJs / ml) with 2 g of Barricade® gel or PPH. The formulations were stored in 2 ml Eppendorf tubes held at 15˚C. The survival of the IJs was then tested at two week intervals. The efficacy of formulated IJs was tested by infecting 10 mealworms with IJs (1000 IJs / ml), incubated for 72 hours at 25oC, and counting the number of dead mealworms at two weeks intervals. On Day One, the survival of the EPNs of both isolates, in both formulations, and the controls were 100%. After two weeks, the controls had zero IJs survival for all the isolates, produced either in vivo or in vitro. The three isolates in both the Barricade® gel and PPH formulations had a zero IJs survival after eight weeks. On Day One, the mortality of mealworms was 100% for all the formulations. Both formulations of the three isolates caused zero mortality of mealworms after eight weeks. The use of Barricade® gel and PPH in formulating three isolates of EPNs enhanced their survival for up to six weeks. The in vitro-produced EPNs had a higher level of survival than the in vivo produced EPNs. However, the in vivo production resulted in EPNs that were more effective at killing mealworms than the in vitro produced EPNs. However, these differences were marginal. Differences between the three strains of EPNs were also marginal. Entomopathogenic nematodes (EPNs) in the families Steinernematidae and Heterorhabditidae, coupled with their symbiotic bacteria, are utilised for the biological control of a wide range of agricultural insect pests. Persistence of EPNs in the field is affected by a number of abiotic factors such as UV light, fluctuation in temperature and desiccation. The aim of this study was to assess the persistence of EPNs under field conditions, applied in two different formulations (cadaver formulation and Barricade® gel formulation). Persistence was assessed after one and two months post treatment application by baiting soil with the greater wax moth larvae (Galleria mellonella) and observing the insect mortality. Heterorhabditis bacteriophora (SGI 245) cadaver formulation showed better survival in all the five sites, followed by H. bacteriophora Barricade® gel formulation. Persistence declined drastically two months post

treatment from a maximum 100% to 0%. The findings suggest that EPNs post application survival in the field can be enhanced by both cadaver and Barricade® gel formulations. Sweet potato weevils is one of the most important insect pests of sweet potato. The control of SPW is difficult due to the cryptic nature of the larvae, and night activity of the adults. In Africa, control of SPW relies primarily on the use of synthetic insecticides. However, the insect has developed some level of resistance against a wide range of insecticides. Entomopathogenic nematodes (EPN), which are cosmopolitan soil-borne entomopathogens, have gained interest as potential biological control agents of various economically important insect pests. The main aim of this study was to evaluate the potential of EPN isolates from the families Heterorhabditidae and Steinernematidae as biological control agents of SPW and their effect on different sweet potato cultivars, under field conditions. Plant resistance has also been viewed as an efficient, cost-effective and environmentally-safe form of pest control methods of SPW under field conditions. Four different cultivars were planted and six biocontrol treatment were applied after a month. Overall, both the cadaver and Barricade® gel formulations of H. bacteriophora SGI 245 were more effective in reducing the plant damage caused by SPW than the Barricade® gel formulation of S. tophus ROOI 352. Overall, the cultivar Monate suffered less insect damage (0.103) than the cultivars Blesbok and Bophelo, and it was the best yielding cultivar. A combination of Treatment F of H.bacteriophora and the sweet potato cultivar Monate can be recommended for further field testing.