These two tiphiid wasps are among several natural enemies imported for the biological control of Japanese beetle. Tiphia popilliavora is native to Japan, where it attacks the larvae of the Japanese beetle (Popillia japonica), as well as to Korea and North China. In its native areas of Korea and North China, T. vernalis is parasitic mainly on scarab species in the genus Popillia other than Japanese beetle. T. popilliavora was the first of the imported parasites to be released. Fewer than 100 adult wasps from Japan were first released in New Jersey in 1921-22, but they quickly established and their progeny and strains from Korea and China were distributed over the infested areas of 10 states by the end of 1950. The Korean strain seems to be most adapted to American conditions, emerging later in the season than the Japanese strain. T. vernalis was first released in New Jersey in 1925, and by 1953 over 2,000 colonies of this species were distributed over 15 states. These two species do not disperse rapidly, as compared with many other parasitoids, so it was necessary to actively move the wasps to new areas. Of almost 50 species of natural enemies that were imported and released, only five became established; these two are the most effective and widely distributed.
The adults are 1/4" long shiny black wasps. The adults of T. vernalis emerge during May and early June and feed on aphid honeydew. The female wasp searches out the overwintering third instar host larva in the soil, stings it to temporarily paralyze it, and then deposits a single egg between the last thoracic segment and the first abdominal segment. Each female may deposit up to 25 eggs during 25-30 days. In 5-7 days the larva hatches and makes a feeding puncture at the egg site. It feeds externally at that site until after the fourth molt, when the host dies and the entire body, except the head and legs, is consumed. Larval development takes 18-30 days. They pupate in the fall in the soil cell of the host, and overwinter in their cocoons as almost ready-to-emerge adults. There is a single generation per year.
The biology of T. popilliavora is similar to T. vernalis, but the adults emerge in August and early September and feed on nectar from the blossoms of umbelliferous plants, especially wild carrot, rather than honeydew. The eggs are deposited singly between the 5th and 6th abdominal segments. The females prefer to oviposit in third instar larvae; if the smaller, second instar larvae are parasitized they produce mainly male wasps. Because the egg-laying period of T. popilliavora is often at a time when the hosts are predominantly second instars, they produce a very high percentage of males, thus limiting population growth of the wasp. This species produces an average of 50 eggs over 20 days or more. The wasp overwinters as a full-grown larva inside its cocoon spun in the soil cell of the host. Pupation occurs the following year.
Under favorable conditions, T. vernalis can parasitize up to 60% of beetle larvae in an area and T. popilliavora somewhat less. The abundance of both species is greatly influenced by the availability of adult food sources. The females feed in the morning and then fly relatively short distances to deposit their eggs. This is why parasitism by both species tends to be greatest in grassy areas near weedy borders that contain aphids or wild carrot.
Although the effect of these wasps on Japanese beetle has been substantial in older infested areas, its impact is obscured by the presence of milky disease which can cause significant beetle mortality. The bacteria that cause milky disease do not infect wasp larvae, but may indirectly affect wasp populations by killing a parasitized host before the wasp larva completes its development. This loss affects T. popilliavora most because soil temperatures at the time of parasite activity (fall) are more favorable to the rapid development of the disease than in the spring when T. vernalis is active.
- Susan Mahr, University of Wisconsin - Madison
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