Submitted to: Journal of Entomological Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 17, 2001
Publication Date: N/A
Interpretive Summary: A primary means of increasing populations of biological control agents is by periodic release of organisms that have been reared in the laboratory or greenhouse. Such releases can be effective only if adequate numbers of inexpensive organisms of good quality are available. Research is currently underway to identify efficient techniques for mass rearing the wasp Cotesia amarginiventris, an important parasite of lepidopteran caterpillars that feed on crop plants. Usually a single parasite egg is laid inside the body of the caterpillar host. In artificial conditions of mass rearing a female parasitoid may lay additional eggs into the same host. Experiments determined whether or not more than one parasitoid immature could successfully complete its development in a shared host. A rearing experiment revealed that 2 parasitoid progeny developed from a shared host nearly 6% of the time when the initial host density was 5 host larvae to 1 adult parasitoid female per arena (0.05 liter) and exposure time was 24 hr. A dissection experiment indicated that nearly 8 parasite eggs were laid inside parasitized hosts at this low host density (5 hosts to 1 parasitoid). Occasionally 2 immatures successfully developed and exited their shared host and constructed individual silken cocoons. These results should be important to government and university scientists concerned with the efficient mass production of biological control agents.
Technical Abstract: Experiments were designed to estimate the frequency of gregarious development of Cotesia marginiventris (Cresson) in beet armyworms, Spodoptera exigua (Hubner). Host density (5, 10, 20 larvae) had no effect on the percentage of parasitized S.exigua yielding 2 C.marginiventris cocoons. But, nearly 6% of the hosts of the 5 density/24hr and the 10 density/48hr treatment groups yeilded 2 cocoons. The adult emergence rate was 64.1% and the sex ratio was male-biased. Host density and day after parasitoid oviposition had a significant effect on the number of C. marginiventris eggs and 1st instars present inside parasitized hosts. Host density had no effect on the presence of 2nd instars, but slightly more than 1 was found in each host on days 4 and 5 post-oviposition. Apparently, 1st instars are sometimes able to avoid sibling cannibalism. When this occurs, a host may provide adequate resources (nutrients, oxygen) for 2 parasitoid 2nd instars to develop into 3rd instars, which egress from the host and spin separate cocoons.