Understanding & Attacking The (Stem Weevil) Enemy
(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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