Influence of the Health of the Food Plant of the Host on Host-finding by Tachinid Parasites

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Abstract
In the laboratory, the parasitic tachinids Drino bohemica Mesn. and Bessa harveyi (Tns.) made two to four times as many attacks on tenthredinid sawfly larvae exposed on unhealthy as on those on healthy food trees. Olfactory stimuli from the unhealthy trees were preferred by the parasites to those from healthy trees. The possibility of greater masking of stimuli from hosts on healthy foliage than from those on unhealthy foliage was apparently of little importance. In the field a moderate to severe attack by sawfly larvae on a tree will cause the tree to become unhealthy and this increases its attractiveness to the parasites. Thus, the host larvae indirectly assist the parasites to find them. Stimuli that are directly influenced by the health of the food plant of the host may interact with those from other environmental factors in determining the apparent degree of preference by tachinid parasites for different hosts.