Abstract
The virulence and proteolytic activity of some entomopathogenic fungi isolates, viz., Metarhizium anisopliae, Beauveria bassiana, Verticillium lecanii, and Trichoderma harzianum, against the two-spotted spider mite, Tetranychus urticae Koch (Acari: Tetranychidae), were evaluated. Common maize plants (Zea mays L.) infested with females of T. urticae were treated in vivo by spraying with suspensions of 1 × 108 conidia ml−1 concentration of selected isolates. Lethal effects of fungal isolates were assessed as percentages of daily mortalities of mites, compared to the mortality in control. Virulence of the fungi isolates was estimated based on the LC50 values calculated by probit analysis for the individuals treated by 1 × 105 conidia ml−1 concentration. Proteolytic activity of isolates was assayed on casein substrate to reflect their virulence towards T. urticae. The mite mortality rates increased with increasing conidial concentrations as well as days after treatment. The mortality rate caused by M. anisopliae isolate varied from 18.75 to 85%, with LC50 value of 4.6 × 105 conidia/ml and LC90 value of 2.4 × 108 conidia/ml during 7 days, respectively. The isolate of B. bassiana caused 15 to 70% mortality, and its LC50 and LC90 values estimated 3.3 × 106 and 7.8 × 109 conidia/ml, respectively. However, V. lecanii isolate caused 11.25 to 72.50% mortality with LC50 of 5.2 × 106 conidia/ml, while T. harzianum was potentially less virulent than other isolates causing 8.75 to 63.75% mortality rate to T. urticae with LC50 of 9.4 × 106 conidia/ml. M. anisopliae showed the highest proteolytic activity at all concentrations, followed by B. bassiana in 3rd, 5th, and 7th day post treatment. These findings recommend the selection of virulent fungal isolates for use as natural and environmentally safe agents in biological control programs to combat mite pests.
Highlights
Spider mites (Acari: Prostigmata: Tetranychidae) are obligated predatory plant mites, which are unique in possessing highly elongated movable stylets used to puncture the individual cells of their host plants (Walter et al, 2009)
Culture conditions, purification, and identification The fungi, Metarhizium anisopliae, Beauveria bassiana, Verticillium lecanii, and Trichoderma harzianum were collected from dead cadavers of the two-spotted spider mite individuals on leaf surface of maize plants (Zea mays L.)
To isolate the fungi grown on the surface of the mite body, the mites were superficially sterilized with 70% ethanol for 1 min and 5% sodium hypochlorite for 3 min and rinsed three times with sterile distilled water; the fungi were transferred to Sabouraud’s dextrose agar plus yeast extract (SDAY) medium
Summary
Spider mites (Acari: Prostigmata: Tetranychidae) are obligated predatory plant mites, which are unique in possessing highly elongated movable stylets used to puncture the individual cells of their host plants (Walter et al, 2009). The conventional control of this pest includes acaricides that have been widely used for mite control in greenhouses, orchards, and many other cropping systems (Van Leeuwen et al, 2005), but it can cause undesirable problems like the death of natural enemies such as predators or development of pesticide-resistant races of mites and residue concerns (Draganova and Simova, 2010). Research and biological control strategies for spider mites have largely focused on preserving natural enemies and releasing predator mites (Zhang, 2003), and an additional spray of acaricides is needed. The entomopathogenic fungi (EPF), developed as mycoacaricides, can naturally manage mite populations and can be used in a useful control strategy as standalone to replace synthetic acaricides already used, or as an integrated component for mite management (Maniania et al, 2008). The successful use of EPF as agents for biological control of mites depends upon the extent of selected fungal strains (Maniania et al, 2008)
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