Abstract
Sitophilus granarius is a serious stored-grain insect that can destroy the whole grain of cereal crops. The present research demonstrated that two species of entomopathogenic fungi (EPF), Beauveria bassiana and Metarhizium anisoplidae, killed the treated adults of S. granarius when applied via oral ingestion. Mortality of S. granarius was shown by growth and development of the ingested conidia in the insect gut causing death of infected insects. During bioassays, the fungus conidia were mixed with wheat flour then provided to the insects for feeding on the mixture for 24 h. The insects were surface-treated with a contact fungicide (Merpan®) to kill the external conidia that could penetrate the cuticle and develop to kill the infected insects. Results indicated that the most effective ratio of mixing fungus conidia with the wheat flour was 0.1:2.0 (g/g) where this ratio should contain 2.75 × 108 conidia/g of M. anisopliae mixture or 2.6 × 108 conidia/g of B. bassiana mixture. Ingested conidia of each fungus caused a first mortality of S. granarius adults after 10 and 12 days of B. bassiana and M. anisopliae treatments, respectively. They also caused 50% mortality after 2.787 and 3.047 weeks of treatments for the same fungi, respectively. The novelty of this research is in adjusting and applying a technique of disinfection of insect cuticle with a fungicide and then verifying the presence of applied fungi in the infected insects. The practical application of this method was discussed.
Highlights
Entomopathogenic fungi (EPF) are potential biocontrol agents of many insect pest species (Batta 2007; Gabarty et al 2014 and Han et al 2014)
Isolation of ingested EPF from S. granarius adults treated superficially with Merpan® Isolation of ingested conidia or its developing mycelium from S. granarius adults fed on mixtures of EPF conidia + wheat flour and treated with Merpan® was positive, if the fungus mycelium grows on the surface of culture plates (Fig. 1d, e)
For the mixtures of M. anisopliae conidia + wheat flour, positive isolations were obtained starting from the third day, following the feeding time for treatments T1, T2, and T3, until the first death of treated adults on 12th day for T1 and 13th day for T2 and T3 (Table 1)
Summary
Entomopathogenic fungi (EPF) are potential biocontrol agents of many insect pest species (Batta 2007; Gabarty et al 2014 and Han et al 2014). One of the main groups of agricultural insect pests that can be effectively controlled by EPF is the stored-grain insects (Batta 2016). The mode of action of these fungi is through germination of conidia that adheres to the outer surface of target insect cuticle producing germ tube that can penetrate the cuticle towards the insect hemocoel. On the cadavers of dead insects due to infection with EPF, the internal mycelium penetrates the cuticle to the outside grows on the cuticle surface producing large quantity of conidia at favorable conditions of temperature and relative humidity (Fig. 1a, b).
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