Three isolates of the entomopathogen Beauveria bassiana along with one strain of Metarhizium anisopliae were cultured on seven media with different carbon/nitrogen (C/N) ratios. The effect of nutrition on virulence of the isolates was evaluated via measurement of colony growth, spore yield, germination speed, conidial C/N ratio and Pr1 (a serine protease) activity. 'Osmotic stress' medium produced the lowest colony growth with low numbers of conidia in all isolates. However, these conidia showed a high germination rate and virulence. However, conidial Pr1 activity was low in some isolates. In most but not in all cases conidia from 1% yeast extract, 2% peptone and low (10 : 1) C/N medium had higher Pr1 activity compared with conidia from other media. However, in some instances we could not conclude that there was a relationship among germination rate, conidial Pr1 activity and virulence. C/N ratio of conidia was statistically different among various media and fungal isolates. Conidia with lower C/N ratio generally produced lower LT(50) (lowest median lethal time) values (more virulent). Insect-passaged conidia from different media had lower C/N ratio compared with similar conidia from artificial cultures. Therefore, they should be more virulent than in vitro produced conidia. As germination rate, conidial Pr1 activity and C/N ratio are independent of host, it seems that host-related determinants such as insect cuticle and physiology and environmental conditions may influence host susceptibility and therefore fungal isolate virulence towards host insects.

No abstract

Astigmatid mites have potential as supplementary prey items to support generalist predator populations in crops. However, applying living prey mites has some disadvantages; if not predated they have the potential to cause crop damage and allergies. In this study, we evaluated various diets based on the astigmatid mite Tyrophagous putrescentiae (Schrank) as a supplemental food source for the predatory mite Amblyseius swirskii Ahias-Henriot. Eggs and larvae of T. putrescentiae were reared on a diet of dog food (rich in proteins and fat) or bran (rich in carbohydrate); they were offered either frozen or alive, and either with or without cattail pollen (Typha angustifolia L.). Oviposition rate of A. swirskii fed with frozen mite larvae reared on dog food was similar to the rate observed when they were fed with cattail pollen or living prey mites, but developmental time of A. swirskii was longer on this frozen diet than on a diet of living prey mites or pollen. Both living and frozen prey mites were, in contrast with cattail pollen, not suitable for oviposition by western flower thrips, Frankliniella occidentalis Pergande. In a greenhouse study, the use of frozen prey mite stages as supplemental food on chrysanthemum plants allowed populations of A. swirskii to establish, but not increase; in contrast, provision of living prey mites and pollen increased A. swirskii populations on plants. Hence, our study shows that living prey mites, but not frozen prey mites, had the greatest potential as a supplemental food source for A. swirskii.