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Understanding & Attacking The (Stem Weevil) Enemy

Monday, March 1, 1999
filed under: Insects

(An Outline of the Biological Nature & Control Measures for This Insect Pest)

Insect Biology & Development

Mature sunflower stem weevil larvae are about one-fourth inch long,

creamy white in color with a small, brown head, legless, and normally

found in a curled - or C-shaped - position within the stalk.

The larvae overwinter in chambers constructed in the lower stalk or

root crown of the sunflower plant. Larvae pupate in these chambers in

the spring in infested stalks, from which adults emerge by chewing

through the stalk.

Adults are about 3/16 inch in length, grayish-brown in color. They

have varying-shaped white spots on their wing covers and thorax, as well

as black snout, eyes and antennae.

Adult sunflower stem weevils emerge from overwintered stalks and root

crowns in early to mid-April in the High Plains and mid- to late June in

the Northern Plains.

The adults are present in the fields in the Northern Plains until late

August, with peak densities occurring in mid-July. In the High Plains,

they're typically gone by the end of July or first of August. Scouting

is recommended from late June to mid July in the Northern Plains, and

during June into July in the High Plains growing region.

Eggs are initially deposited around the first-node (cotyledon) plant

stage, and the height of egg placement in the stalk increases over time.

Larvae feed until early August, then descend to the lower portion of the

stalk or root crown by late August. Then they form overwintering

chambers by chewing cavities into the base of the stem. There is only

one generation per year.

Lodging is a good indicator of stem weevil activity. But bear in mind

that plant lodging also can be influenced by other factors, e.g., stalk

diameter, stem thickness, sunflower head weight, wind velocity and

direction, and incidence of disease.

Stem Weevil Control Measures

Population levels of 25 to 30 larvae per stalk (or over 80 per stalk in

irrigated sunflower, according to High Plains research) can result in

significant stem damage and lodging - especially under windy conditions,

higher plant populations or drought stress. Loss may occur at lower

densities when drought conditions cause smaller-diameter stalks.

Breakage normally occurs at or slightly above the soil surface, since

that is where the larvae will have constructed their overwintering


Delaying planting until early June to mid-June can be effective in

reducing weevil larvae in sunflower stalks. However, be mindful of the

possible effect on yield and oil content - especially in more-northerly


Since damage results from lodging of larval-infested stalks, anything

that promotes strong, healthy stems likewise helps to reduce losses.

Even with the same number of larvae, plants with stems of increased

diameter and greater stem density will be less likely to break. So

lower plant populations, adequate fertilization and proper soil moisture

should thus aid in decreasing sunflower lodging.

Tillage that adequately breaks and buries stalks of overwintering

larvae (at least three to six inches deep) may decrease populations the

following season by reducing adult emergence. However, area-wide

tillage would be needed to impact the number of weevils migrating into

each season's new sunflower fields. Also, the value of standing stalks

for better field moisture and reduced soil erosion must be factored in

when tillage decisions are made.

Both foliar and systemic insecticides will reduce stem weevil larval

populations and stalk lodging, although treatment may not consistently

result in higher seed yields.

Certain producers may want to consider an at-planting systemic

treatment of Furadan 4F (some mix this insecticide with liquid starter

fertilizer) - especially with early planted sunflower and in areas that

had significant stem weevil infestations the previous year.

When it comes to the foliar insecticidal treatments (e.g., Asana XL,

Warrior, Furadan, Lorsban, Baythroid), it's important to begin spraying

prior to most of the egg laying. If that is not done, succeeding larval

populations will not be adequately reduced. In the Northern Plains, that

timing typically is late June to early July. Sunflower growers in the

High Plains are encouraged to use growing degree days as a yardstick for

anticipating emergence and egg laying - and hence the scouting and

treatment. Banded treatments are effective and economical for

row-planted sunflower fields.

Economic threshold for treatment is one adult weevil per two plants;

or, one adult weevil per three plants when stalks are small on account

of high plant populations or drought stress. - Tracy Sayler

(Stem weevil development and control information was provided by Stan

Pilcher, Golden Plains area extension entomologist, and Mike Koch,

research associate, with Colorado State University, Akron, Colo.; Larry

Charlet, research entomologist with USDA's Northern Crop Science

Laboratory, Fargo, N.D.; and Frank Pears, extension entomologist,

Colorado State University at Fort Collins.

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