Use of entomopathogenic fungus and nematodes indigenous to Thailand for controlling of fall armyworm (Spodoptera frugiperda (J.E. Smith))

Abstract

The Fall armyworm, Spodoptera frugiperda is an insect pest indigenous to tropical and subtropical regions of the Americas. It is a polyphagous pest reported to affect more than 353 plant species belonging to 76 plant families. This pest was never reported outside America until 2016 when it was reported for the first time in Africa, since its invasion, the pest is causing huge economic losses in Africa. In December 2018, this pest was reported for the first time in Thailand. The invasions of this alien species have threatened maize growers in Thailand. Currently, pesticides are used to control and minimize the distribution of Fall armyworm in the maize field. The use of chemical pesticides is not sustainable in long run. Moreover, synthetic chemical pesticides are not good for human health and the environment. Therefore, research is being conducted to identify and develop alternatives to synthetic chemical pesticides that are more sustainable and safer for the environment. Entomopathogens like viruses, bacteria, nematodes, fungi, etc., are reported as the best alternatives. This study was conducted to evaluate the efficacy of entomopathogenic fungus and nematodes indigenous to Thailand for controlling of Fall armyworm under laboratory and greenhouse conditions. Two isolates of entomopathogenic fungus Beauveria bassiana (TBRC 2781 and TBRC4755) and each isolate of indigenous entomopathogenic nematode Heterorhabditis indica isolate AUT 13.2 and Steinernema siamkayai isolate APL 12.3, were selected for this study. Two different larval stages of Fall armyworm were selected for the test, the second instar representing the younger stage and the fifth instar representing an older stage of the pest. The susceptibility of these larval stages was compared, and the most susceptible stage was selected for the greenhouse experiments.

The result from our study showed that all isolates of entomopathogens used were effective in both tested conditions. In the laboratory, the isolates of entomopathogenic fungus Beauveria bassiana TBRC 2781 and TBRC 4755 caused mortality of 73% and 64% respectively and similarly, the highest mortality of fifth instar larvae 35% and 25% were obtained with TBRC 2781 and TBRC 4755. In the greenhouse, these isolates caused mortality of 35% and 33%. The entomopathogenic nematodes H. indica isolate AUT 13.2 and S. siamkayai isolate APL 12.3 caused mortality of 83% and 68% of second instar larvae and 45% and 33% of fifth instar larvae. In the greenhouse, the highest tested dosage gave mortality of 58% and 45% for H. indica isolate AUT 13.2 and S. siamkayai isolate APL 12.3, respectively. From the result, it is confirmed that mortality was positively correlated to the dosage and younger larvae of Fall armyworm were more susceptible to these isolates.

The results from this study confirm that all these isolates have the potential to be used as a biocontrol agent to control Fall armyworm. However, the host stage, dosage, and environmental conditions need to be considered for effective results. Our results are based on the experiments conducted in the laboratory and the greenhouse conditions and it is not known how these isolates would perform in the field conditions. Therefore, future work would focus on evaluating the efficacy of these isolates in the field and develop a commercial form of these isolates, and test their efficacy. Furthermore, the effectiveness of these isolates can be increased by integrating them with other biocontrol agents and this can be explored in future works.

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