Lydella thompsoni is a solitary internal parasite of European corn borer larvae. This tachinid fly was one of 24 species of parasitoids introduced from Europe and the Orient as part of a USDA importation program from 1920-1938. It was one of six species that became established in the United States. Of these, only Lydella and the wasps Eriborus terebrans, and Macrocentrus grandii became widespread and effective biological control agents. L. thompsoni spread to many areas where it was never released.
The adult fly resembles a large, very bristly house fly. The females retain their eggs inside their body until each larva is ready to hatch. Each female is capable of producing up to 1,000 eggs, although far fewer are deposited or ever find a host borer. After the eggs have incubated for about 5 days, the female fly finds a potential host. She runs hurriedly along a stalk and appears to be searching from side to side. She is attracted to volatiles produced from host frass pushed out of the borer tunnel, and may "taste" the frass when it is encountered. Hatching usually takes place just as the female is depositing her offspring. Living larvae are deposited at the entrance to the host tunnel. The female stands over the burrow, bends her abdomen under until the ovipositor is pointing downwards, and then a larva wriggles out or is brushed off with a quick movement of the tip of the abdomen. This first instar must then move into the tunnel and find its way to the borer. It prefers to attack fourth instar borers. The maggot has sharp mouthparts with a sawtooth edge to penetrate the host body. The borer may squirm or wiggle, or bite at the area being attacked in an attempt to prevent parasitization, but rarely succeeds. Once in the host, the maggot feeds first on the body fluids, then on the fatty tissues and internal organs. Upon the death of the borer, the maggot forces an opening in the skin, but continues to feed until its development is complete. It then leaves the host remains and pupates in the tunnel nearby. Larval development takes about 8 days and another 8 days is spent in the pupal stage.
The seasonal cycle of this fly is poorly synchronized with that of the European corn borer in the Midwest, generally emerging too early in the spring. It will, however, parasitize the common stalk borer, which allows for survival of the population until the next generation of the corn borer is available to parasitize.
There are usually two generations a year, but three genrations often occur when another host is utilized for the first generation. The winter is passed as second-instar maggots in the hibernating host larvae. A small proportion of first generation larvae may go into diapause and delay adult emergence until the following season.
For many years after its introduction L. thompsoni was the most important parasitoid of European corn borer in many areas of the United States. Parasitization of up to 75% of the second borer generation was recorded in the early years, and it was considered a major controlling factor of borer populations. But there was an abrupt, unexplained decline in populations around 1960 and the fly disappeared from many places. There have been subsequent reintroductions in several locations and L. thompsoni appears to be established once more, particularly from Connecticut west to central Ohio and into South Carolina. Even when it is present, the braconid wasp M. grandii seems to outcompete L. thompsoni. In much of the Midwest E. terebrans and M. grandii are the predominant parasitoids. Also, pathogens such as Beauveria bassiana and Nosema pyraustae may cause significant mortality and affect parasitoid populations, although infection rates do vary from year to year.
- Susan Mahr, University of Wisconsin - Madison
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