Resistance of Apanteles eggs to the haemocytic encapsulation by their habitual host, Pieris

Abstract

The calyx fluid in the lateral oviduct of a gregarious parasitoid, Apanteles glomeratus contained ellipsoid particles of ca. 130 × 200 nm. These calyx fluid particles did not appear to be embedded in a fibrous outer layer on the surface of eggs in the lateral oviduct. They were not observed on the surfaces of the eggs 3 to 4 hr after being deposited into the host haemocoele. Oviposition experiments indicated that the occurrence of haemocytic defence reactions of the late 2nd instar larvae of the Pieris rapae crucivora against 1 st instar larvae of the parasitoid increased with a decreasing number of the parasitoid eggs introduced into a host, and that more than 5 to 9 parasitoid eggs were needed for suppressing the ability of the host to encapsulate its parasitoid larvae immediately after hatching. When eggs with calyx fluid obtained from egg reservoir were injected into the host, they were found to be encapsulated 1 to 2 days after the injection. They could not start their embryonic development. When calyx fluid-free 3-hr-old eggs were injected in a number of more than 5 eggs into a 5th instar larva of Pieris, 58% of 31 eggs injected had normally hatched without evoking encapsulation reactions by the host. Both electron microscopic observations of parasitoid eggs in the host haemocoele and the experimental results suggested that calyx fluid or calyx fluid particles of the parasitoid might not be involved in the encapsulation-inhibiting activity of the parasitoid eggs. Rather it was anticipated that a substance (or substances) might be secreted by the parasitoid eggs into the haemocoele of the host, which suppressed defence reactions of the host.