Hypera postica (Gyllenhal) -- Coleoptera, Curculionidae
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First found in the United States near Salt Lake City, Utah in 1904, Hypera postica is believed to have invaded from Europe (Titus 1907, 1910). The weevil was confined to 12 western states until 1952 when it was detected in Maryland (Bissell 1952). From Maryland it spread rapidly and is now found throughout North America.
There is one generation per year and winter is spent as aestivating adults and as eggs. Eggs hatch in spring about the time that alfalfa begins to grow. In the Midwest, larval feeding continues through May when pupation occurs. After emergence adults leave the field for available cover where they undergo summer aestivation. In autumn adults return to the field and begin laying eggs (Manglitz & App 1957).
Parasitoids were first introduced from Europe into the United States in 1911, and by 1919 they were well established in many areas of the western United States (Chamberlin 1924). Bathyplectes curculionis (Thomson) is the most widely distributed and most successful introduced parasitoid in the Midwestern U. S. During the 1960's and 1970's, both B. curculionis and B. anurus (Thomson) were released in Illinois by USDA personnel and are now found in most midwestern populations of the weevil (Dysart & Day 1976) (also see Michelbacher 1940a,b; Hamlin et al. 1949, Poinar 1963, Dysart & Puttler 1965, Streams & Fuester 1967, Hagen & Mangalitz 1967, Brunson & Coles 1968).
A fungal disease of alfalfa weevil larvae was found in Ontario, Canada in 1973 (Harcourt et al,. 1974), and was similar to that reported active on cloverleaf weevil, Hypera punctata (Arthur) by Arthur (1886). The fungus is believed to be Erynia phytonomi (Thomson) and actually differs from that attacking cloverleaf weevil. It was found to spread rapidly out of Ontario to other portions of North America (Muka 1976, Puttler et al. 1978, Barney et al 1980, Los & Allen 1983, Nordin et al. 1983). It is now considered to be the major naturally occurring biological control agent of the alfalfa weevil throughout most of its range (Carruthers & Soper 1987). A similar fungus causes comparable mortality in Hypera variabilis in Israel (Ben Ze'ev & Kenneth 1982).
Erynia phytonomi overwinters in the soil as thick-walled resting spores that germinate in springtime to produce germ conidia, which infect weevil larvae. Conidia produced by infected larvae are responsible for the horizontal transmission of the disease (Ben Ze'ev & Kenneth 1982). Younger larvae tend to produce conidia and older larvae resting spores (Barney et al. 1980). Brown & Nordin (1982) developed a detailed model of this disease and estimated that the first incidence occurs in Kentucky after an accumulation of 220 to 290 degree days. Then the alfalfa weevil population has to reach a threshold density in order to allow for sufficient horizontal transmission for an epizootic. Brown & Nordin (1982) estimated this threshold to be 1.7 weevil larvae per stem. Mortality rates caused by the fungus are often quite high (30-70%) at the time of peak larval occurrence and often 100% later in the season (Morris 1985). It is restricted in effectiveness as a biological control agent because it often appears late relative to currently recommended harvest dates (Ambrust et al. 1985). Brown & Nordin (1982) proposed using computer-directed harvest dates that are earlier than normally recommended. The microenvironment in windrows promotes an earlier than normal epizootic and reduces the need for insecticides.
The appearance of the fungus as a major mortality factor after the two above mentioned parasitoids were established poses the question of how these all will now coexist, especially as they attack the larval stage. About five days elapse from infection to death in diseased larvae and parasitized larvae die within 10 days. Such time periods suggest that an alfalfa weevil larva infected and parasitized simultaneously would probably die from the fungus before the parasitoid completed its development. Field studies indicate that the disease has a negative impact on the two parasitoids (Los & Allen 1983, Loan 1981, Morris 1985).
REFERENCES: [Additional references may be found at: MELVYL Library ]
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