|
|
|
|
 |
|
 |
 |
 |
 |
 |
 |
 |
|
|
 Stem Weevil: Sly But Controllable March 1999 Of all the insects that can affect sunflower, the stem weevil might be described as the sly, guerrilla pest of the bunch - covert in the nature of its activity and its damage to sunflower. Larvae will live and feed inside the sunflower plant, often detected only after weevil-weakened stems topple over from wind and/or drought stress, or as plants dry down. Scouting for adult stem weevils can be tricky, because if they sense movement, the adults tend to drop to the ground and "play dead." Scouts must move slowly and deliberately to avoid that reaction. Also, adults are difficult to see on the ground, as they are grayish-brown in color and only about 3/16 of an inch long. Along with its readily apparent harmful effects, the sunflower stem weevil also has been linked to the introduction of pathogens which can lead to Phoma black stem. The connection between Phoma and the stem weevil isn't totally clear, but crop scientists believe the weevil may increase plant stress or act as a carrier of pathogens that can result in the disease - which in turn makes sunflower vulnerable to premature ripening. Some view the stem weevil as the number-one insect problem in the High Plains sunflower production region. "It's probably our most serious problem insect here for oilseed-type sunflower," says Al Jarvi, plant breeder with Cargill Hybrid Seeds at Fort Collins, Colo. In Jarvi's opinion, only dry conditions and weed control have been more challenging to High Plains sunflower production in the 1990s than the stem weevil. "Around 1994 is when we first noticed we really had a problem with this insect," says Stan Pilcher, Golden Plains area extension entomologist for Colorado State University, Akron, Colo. Larval feeding that resulted in sunflower lodging was the telltale sign. There have been infestation hot-spots since, including damage between Burlington, Colo., and Goodland, Kan., and around Akron last year, he says. The stem weevil has been reported in most states west of the Mississippi River and into Canada for many years, says Larry Charlet, research entomologist with the USDA-ARS Northern Crop Science Laboratory at Fargo, N.D. The insect can be found on wild sunflower and other plants as well as on cultivated sunflower. It was first described in 1876 from specimens collected in Texas and California. It was noted as a sunflower pest in Colorado as early as 1921 - and in North Dakota in 1973, when a sunflower field sustained an 80-percent yield loss due to plant lodging. However, the stem weevil has been a fairly isolated problem in the Dakotas and Minnesota since the late 1980s and early 1990s. That comes as no surprise to Charlet, who says insect populations tend to be cyclical, rising and declining in part with weather cycles. Indeed, wet weather in the Northern Plains in the 1990s helped increase weevil mortality. Charlet's research also indicates that a parasitic wasp that overwinters with stem weevil larvae has increased in the Northern Plains, and that has helped to decrease stem weevil populations. Charlet says one of the problems with the sunflower stem weevil is that it can go unnoticed until sunflower lodges from severe infestations. Research in North Dakota has found that a mean infestation of 38 larvae per stem resulted in 28 percent lodging. Fortunately, there are effective options for treatment. Charlet doesn't anticipate serious problems in the Northern Plains this year from the stem weevil. "I would think we would have gotten some indication of buildup in population, and there's no indication of that here," he explains. To keep the pest in check, however, growers should keep a close eye on it - particularly in the High Plains. Timely crop monitoring and scouting will accurately determine whether the stem weevil is present at economic levels. "I would even urge the inexperienced grower to consider using a professional crop scout to look for it, because it's going to be difficult to find," says Al Jarvi. CSU's Pilcher advises using the six- to eight-leaf stage of sunflower as a scouting window for stem weevil. Sampling sites should be at least 75 to 100 feet in from field margins. Walking the standard "X" pattern, scouts should examine five plants per site, stopping at a minimum of five sites per field. The entire plant should be examined, with particular attention given to both sides of leaves, leaf axils, the lower stem, dried cotyledon leaves, and to cracks in the soil around the base of the sunflower plant. Because the larvae live and feed inside the sunflower plant, only by splitting open stalks can they be counted. That can be difficult and time-consuming. Also, larvae can be difficult to eradicate with foliar treatments. So scouting and treatment for the stem weevil is directed toward the adult, prior to egg hatching and larvae entering the plant. High Plains researchers have developed a growing degree day (GDD) model based on accumulated heat units that can help producers determine when to expect adult stem weevil emergence. The model adds daily maximum and minimum tempera-tures for a given locale to correlate GDD with weevil emergence. The model suggests weevil emergence will begin at approximately 300 GDD (± about 30). That level is normally reached sometime between May 7 to June 6 in the High Plains, depending upon weather and one's specific location. The average number of annual GDDs usually ranges between 2,500 and 2,800 in the High Plains region. Emergence studies at five locations in the High Plains indicated planting date risk as follows: High relative risk if sunflower is planted prior to an accumulation of 449 degree days; low to moderate risk between 450 and 599 degree days; and no risk after 600-degree-day accumulation. "We feel the model needs further refinement; but it sure gets us in the ballpark," CSU's Pilcher observes. In addition to more research on the GDD stem weevil prediction model, crop scientists are looking at other improved control measures as well - including ways to trap and monitor populations without physically inspecting plants; enhanced in-furrow and foliar insecticide applications; and preplant seed treatments. - Tracy Sayler  Back to Insects Stories Back to Archive Categories
|
|
