Effects of a tachinid parasitoid, Ormia ochracea, on the behaviour and reproduction of its male and female field cricket hosts (Gryllus spp)

The larvae of the tachinid parasitoid fly, Ormia ochracea, develop within the body of their host, the field crickets Gryllus integer and Gryllus rubens. The fly will also develop within the cricket Gryllus bimaculatus, which is not the natural host. For the first 3 days after entering a cricket, the first instar larvae grew inside the large thoracic muscles, but did little damage to the muscles during this time. They then migrated into the abdomen where they moulted, attached themselves to the body wall and fed primarily on the host's muscle and fat body, sparing the digestive system and CNS. Usually the host's reproductive system was only partially damaged. After the larvae entered the abdominal cavity, the host's mating, egg-laying and fighting ability (males) declined. This decline probably occurred due to the tissue damage wrought by the parasitoid. Less energy-demanding behaviours, such as grooming and feeding, were not affected during the entire period of infestation. During the first instar (when the parasitoid is embedded in the host's muscle), male crickets showed an increased tendency to attack other males. This increased incidence of agonistic behaviour occurred before the parasitoid caused significant tissue damage to the host and must therefore have been due to some other effect of the parasitoid.