Development of Mycopesticide for the Management of Sorghum Chafer, Pachnoda Interrupta (Olivier) (Coleoptera: Scarabaeidae) in Ethiopia

Abstract

Pachnoda interrupta (Olivier) (Coleoptera: Scarabaeidae) is a damaging pest of sorghum,other cereals and horticultural crops such as maize, wheat, barley, bananas, guava etc. in Africa which at times results in complete crop losses. Current control strategies heavily rely on chemical pesticides. Controlling adult beetles through application of insecticides,in addition to being detrimental to the environment and the ecology, does not provide long lasting control.Thus, efficient biological control agents that can control the pest in the breeding sites need to be developed.There is evidence that microbial pesticides can effectively control P. interrupta. Entomopathogenic fungi isolated from soils and insects in Ethiopia were evaluated under laboratory and field conditions with the objective of developing a myco-pesticide against P. interrupta. Evaluations were conducted in three phases. In the first phase 116 Metarhizium spp. and Beauveria spp. were evaluated based on high viability as measured by percentage germination on SDA media. Fourty-seven isolates with greater than 70% viability were selected and further evaluated using conidial suspensions applied to Galleria mellonella (Fabricius) (Lepidoptera, Pyralidae) larvae at a standard concentration of 1x108 conidia/ml. Four Beauveria spp. and three Metarhizium spp. isolates which caused over 60% mortality were selected for final virulence assay on P. interrupta adults at a dose of 1mg dry conidia/10 beetles. Highest mortality of beetles (82%) was recorded from the Metarhizium spp. isolates PPRC51 followed by PPRC2 (80%) and the Beauveria sp. MP3POST (80%) which were not significantly different. The three isolates were selected as candidates for additional tests to develop mycopesticide against P. interrupta. Molecular identification of the seven entomopathogenic fungal isolates used in the study was carried out using PCR amplification of the ITS regions of Ribosomal DNA and chitinase gene primers (chi1 and chi4) and resulted in effective identification of three isolates as M. anisopliae and four isolates as B. bassiana. In-vitro characterization of the selected isolates using four artificial solid media at five distinct temperatures showed that two M. anisopliae isolates (PPRC51 and PPRC2) exhibited better radial growth, germination, and sporulation at temperatures of 20o C, 25o C and 30°C while these characteristics were limited at 15o C and 35°C. These two isolates were selected as best candidates for further tests.Field studies on fungal auto-dissemination device development from locally available materials conducted over three feeding and two mating seasons resulted in two efficient traps baited with a five compounds blend lure which were not significantly different in catch performance with the standard Japanese beetle trap. The two selected virulent isolates of M. anisopliae (PPRC51 and PPRC2) were tested for field efficacy using these two designs of locally affordable auto-dissemination traps loaded with 1gm of dry conidia. PPRC51 induced 41% field mortality on P. interrupta under high temperature and low relative humidity conditions. Field viability of the two isolates was 36 % and 40 % in five days for PPRC51 and PPRC2, respectively. Based on the efficacy data, PPRC51 is a promising candidate for development of a myco-insecticide against P. interrupta at commercial level in the form of augmentation biological control in the pest‟s natural habitat as a component of integrated pest management against P. interrupta.Three liquid phase media were evaluated against the selected M. anisopliae and B.bassiana isolates in order to develop mass production and preservation protocols using the diphasic fermentation method and to evaluate their effects on virulence to P.interrupta using rice as sold substrate. PPRC2 produced highest number of spores (4.57x1010 spores/g and 3.85x1010 spores/g) when using LB and MYE as liquid phase media respectively which were not significantly different. The highest spore yield per kg of rice substrate (119.72 g/kg) was obtained from the commercialized isolate IC69 which was mass produced by using MYE as a liquid phase medium followed by PPRC2 (43 g/kg) on LB and PPRC51 (48 g/kg) on MYE liquid media. Spore production was significantly affected by isolate, but not by liquid media.Farmers in the study area have low perception on use of biological control as a component of IPM for P. interrupta and more than 70% of the interviewed farmers are inclined towards using chemical pesticides against the pest.