The ability to understand the effect of abiotic variables on the infectivity of entomopathogenicfungi, and to use this information to predict efficacy in the field or glasshouse, is essential to the further development of these organisms as microbial control agents.

Temperature is a key variable affecting entomopathogenic fungi, however there have been few attempts to model the interaction and hence describe it in a meaningful way. The relationship between temperature and fungal development takes the form of an asymmetric bell-curve which may have an approximately linear region below the optimum. This relationship is typical of poikilothermic organisms and can be described mathematically using nonlinear regression models.

We have investigated in detail the thermal biology of isolates of entomopathogenicfungi that are being examined as potential microbial control agents of the varroa mite, Varroa destructor, a major parasite of honey bees. We first measured colony extension rates at three temperatures (20, 30 and 35°C) for 41 isolates of fungi from 6 taxa (Beauveria, Metarhizium, Hirsutella, Paecilomyces (= Isaria), Lecanicillium). Twenty two isolates were then selected and their colony extension rates measured at ten temperatures (12.5 – 35°C). The data were then fitted to the Schoolfield et al. (1981) model of poikilotherm development. The model consists of a thermodynamic description of a rate-limiting enzymic reaction combined with terms to describe the inhibitory effects of high and low temperatures. Overall, this model accounted for 87.6 – 93.9% of the data variance.

At a practical level, we're able to use the model to make predictions about fungi used against varroa. In general terms, the thermal requirements of the isolates we have examined against varroa are well matched to the temperatures in the broodless areas of honey bee colonies (around 25°C ) and a proportion of isolates will also be able to function within the drone brood areas (32 – 33°C).

References

Davidson, G., Phelps, K., Sunderland, K.D., Pell, J.K., Ball, B.V., Shaw, K.E. & Chandler, D. (2003). Study of temperature-growth interactions of entomopathogenic fungi with potential for control of Varroa destructor (Acari: Mesostigmata) using a nonlinear model of poikilotherm development. Journal of Applied Microbiology, 94, 816-825.

Schoolfield, R. M., Sharpe, P. J. H. and Magnuson, C. E. (1981) Non linear regression of biological temperature-dependent rate models based on absolute reaction-rate theory. Journal of Theoretical Biology 88, 719 – 731.