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Stem Weevil: Sly But Controllable

Monday, March 1, 1999
filed under: Insects

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


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


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


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


"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

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