by Jay F. Brunner, originally published 1993
Ascogaster quadridentata is an efficient parasitoid of codling moth. It is a strong flier, good searcher, and its life cycle is synchronized with that of codling moth. This parasitoid is native to Europe and was evidently accidentally introduced into North America, probably on shipments of fruit. In 1919, A. quadridentata was introduced to the West Coast from the eastern United States. Insecticides used in commercial orchards eliminate A. quadridentata.
A. quadridentata is a braconid wasp, an important group of insect parasitoids. There are more than 1,700 species in North America. They are generally smaller than the ichneumonids (ca. 3 mm long). Although hosts vary from one species to the next, most prey on larvae of Lepidoptera. Most braconid larvae developinside the host’s body (endoparasitoids), but when the hosts are concealed (e.g., leafrollers or wood-boring beetles), th wasp larva may feed externally.
The minute egg is laid inside the host egg.
The first instar larva, which is 1/12 inch (2 mm) long, remains inside the host larva until it is nearly mature. It does not begin to develop until the host is in the fourth instar. Then, the parasitoid larva grows, consuming the entire host larva. The almost mature larva exits the body of the host larva (but still within its cocoon) and soon spins a glossy white cocoon about 1/8 inch (3 mm) long.
The pupa is inside the cocoon, which is within the cocoon of the host.
The adult is a small wasp, 1/8 to 1/5 inch (3 to 5 mm) long, with a black to dark brown robust body and long antennae.
A. quadridentata overwinters as a first instar larva inside a host larva that has spun a hibernaculum beneath bark of apple or pear. In spring, when unparasitized codling moth larvae continue development and change to the pupal stage, the development of parasitized larvae is blocked. Instead, the parasitoid larva begins to grow and consumes the contents of its host.
When the parasitoid larva is mature it pupates, and its emergence as an adult coincides with the emergence of codling moth adults. Adult A. quadridentata search out codling moth eggs, or other host eggs, and deposit their egg inside. When the codling moth egg hatches, the first instar A. quadridentata larva is inside. The host’s larva develops normally through the first three instars, but development is blocked at the fourth instar. The parasitized codling moth larva leaves the fruit and seeks a cocoon site. A. quadridentata then develops inside the host, leaving when mature and ready to pupate. There are at least two generations of A. quadridentata in the Northwest.
Parasitized larvae are smaller than unparasitized ones, and this can be used to an advantage in monitoring and managing populations of A. quadridentata. Banding with openings less than 1/20 inch (1.3 mm) will selectively collect parasitized codling moth larvae.
Healthy codling moth larvae can be collected in banding with larger diameter openings. The banding can be destroyed. A second cultural practice that can be used to favor parasitized larvae is to place infested fruit in special containers or bins with openings that are of a size that will allow parasitized larvae to escape while confining healthy ones. Healthy larvae can then be destroyed, and parasitized ones returned to the orchard.
A. quadridentata can be reared easily in the laboratory and can be forced into diapause by regulating the photoperiod. Dormant hosts can be accumulated during winter and held at 40°F for several months. When they are transferred to long days (16 hours light) and warm temperatures (80°F) dormancy ends. The breaking of dormancy can be programmed so that large numbers of parasitoids are available for mass release in the spring. In unsprayed trees, releases of A. quadridentata have resulted in parasitism levels of 35 to 40%. It is possible to place parasitized codling moth eggs on wild apple trees. When these eggs hatch, the codling moth larvae would attack fruit but all would eventually produce adult A. quadridentata that would be available to attack codling moth of following generations. This approach could also be used on crab apple pollinizers in commercial orchards to increase the level of parasitoids naturally without mass releases of adults. The use of insecticides in orchards for control of codling moth has eliminated populations of A. quadridentata. It can, however, be an important component in a pest management program where mating disruption and soft insecticides are used. It can also be important in reducing codling moth populations in wild or backyard trees, thus reducing the number of immigrating moths from these sources.