Environmental Entomology (1992) 21, 1035-1045

Juli R. Gould, Joseph S. Elkinton and Roy G. Van Driesche (1992)
Suitability of approaches for measuring parasitoid impact on Lymantria dispar (Lepidoptera: Lymantriidae) populations
Environmental Entomology 21 (5), 1035-1045
Abstract: This study was designed to evaluate biases associated with four methods of estimating stage-specific parasitism and one method of estimating time-specific parasitism for two parasitoids attacking gypsy moth, Lymantria dispar (L.), larvae. Estimates for all five methods were calculated using both observed death rates and marginal attack rates. Calculating percentage parasitism by pooling all hosts collected during the season resulted in an underestimation of stage-specific parasitism by both Cotesia melanoscela (Ratzeburg) and Parasetigena silvestris (Robineau-Desvoidy). Percentage parasitism by C. melanoscela peaked after completion of recruitment of hosts and parasitoids and before substantial losses (death or emergence) of either hosts or parasitoids. Consequently, peak sample percentage parasitism was an unbiased estimate of stage-specific parasitism by C. melanoscela. Oviposition and emergence of P. silvestris overlapped considerably with each other and also with losses of hosts not parasitized by P. silvestris. As a result, the peak value of percentage parasitism was not appropriate for use with this species. The release of laboratory-reared larvae (trap-hosts) to directly measure the number of parasitoids recruited into the susceptible stage of the host did not provide accurate estimates of percentage parasitism by either species, possibly because of different sizes and behaviors of trap-host larvae. The graphical method of estimating numbers entering a stage used by Southwood and Jepson in 1962 is unbiased only if minimal mortality occurs during sampling. This was true for C. melanoscela, but not for P. silvestris. Mortality of parasitized hosts can also affect estimates of time-specific parasitism of hosts collected in the field.. Because mortality of gypsy moths parasitized by P. silvestris was high, this method is not recommended for P. silvestris, and an alternative technique is suggested.
(The abstract is excluded from the Creative Commons licence and has been copied with permission by the publisher.)
Database assignments for author(s): Juli R. Gould, Roy G. Van Driesche

Research topic(s) for pests/diseases/weeds:
biocontrol - natural enemies
Research topic(s) for beneficials or antagonists:
surveys/distribution/isolation
population dynamics/epizootiology

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Pest and/or beneficial records:

Beneficial	Pest/Disease/Weed	Crop/Product	Country	Quarant.
	Lymantria dispar
Cotesia melanoscela (parasitoid)	Lymantria dispar
Parasetigena silvestris (parasitoid)	Lymantria dispar