Sunflower weevils are small, long-snouted beetles that feed on sunflower seeds or stems, causing yield losses that range from minor to severe depending on the species and infestation level. Three species matter most to growers: the red sunflower seed weevil, the gray sunflower seed weevil, and the sunflower stem weevil. Each attacks a different part of the plant, causes different kinds of damage, and calls for different management approaches.
Red vs. Gray Seed Weevil Identification
The two seed weevil species are easy to tell apart once you know what to look for. The red sunflower seed weevil is the smaller of the two, covered in fine orange-to-red hairs that give it a warm coloring. Over time, those hairs rub off and the weevil can appear nearly black. It has a black snout with small, bent antennae and black legs.
The gray sunflower seed weevil is noticeably larger. Small gray hairs cover most of its body, and its legs are gray rather than black. It shares the same black snout and antennae structure as the red species. Both species produce cream-colored, legless larvae that curl into a C-shape when disturbed. These larvae are what actually destroy seeds, feeding inside them as they develop.
How Each Species Damages Seeds
Gray seed weevil larvae bore into seeds at the base, destroy the developing ovary, and consume the contents entirely. What’s left is a swollen, hollow shell so lightweight that it blows out during harvest. Most of the damaged seed never makes it into the grain bin.
Red seed weevil larvae are sneakier. They feed inside the seed without completely consuming it, so infested seeds look normal from the outside and weigh about the same as healthy ones. That means conventional threshing and cleaning won’t separate them. When the larva matures, it chews a small exit hole near the top of the seed and leaves. But dead larvae stay behind inside, and for confection (snack) sunflowers, those contaminated seeds can trigger quality downgrades. In oilseed varieties, larval feeding reduces both kernel weight and oil content.
A useful rule of thumb from Kansas State University: roughly 27 damaged kernels result for every adult red seed weevil present at early bloom. That damage adds up quickly in a heavily infested field.
The Sunflower Stem Weevil
The stem weevil attacks the stalk rather than the seeds. It produces one generation per year. Adults prefer to lay eggs on plants between the 8-leaf and 12-leaf stages, starting around the lower leaf nodes and moving higher on the stalk when competition for egg-laying sites is heavy. The puncture wounds darken and form blackened callous tissue on the stem surface.
Larvae tunnel through the interior of the stalk. In central Kansas studies, researchers found little direct impact on plant productivity even with 50 to 80 larvae per stalk. The real problem is lodging: larval tunneling weakens the stalk, and a fungus that causes black stem rot follows the feeding galleries deeper into the plant. Adults actually carry fungal spores into the stalk when they feed, and the disease spreads through the tunnels the larvae create. The combination of tunneling and fungal rot can weaken stalks enough that wind knocks plants over before harvest, which is where most yield loss occurs.
Mature larvae overwinter inside the lower portions of dead stalks and emerge as adults the following spring, often triggered by rainfall.
Scouting for Seed Weevils
Scouting should begin when yellow ray petals first become visible on the developing bud (the R4 growth stage) and continue until about 70% of pollen shed has occurred. To get a reliable count, walk roughly 75 feet into the field and examine five random sunflower heads. Move to another spot about 75 feet away and check five more. Repeat until you’ve looked at 25 plants total, then calculate the average number of adult weevils per head.
You can count weevils by rubbing the face of the head to disturb them. If that doesn’t flush them out, a quick spray of aerosol insect repellent across the head will drive them to the surface where they’re easier to spot.
Economic Thresholds
The number of weevils that justifies treatment depends entirely on whether you’re growing oilseed or confection sunflowers. For oilseed varieties, the economic threshold is 4 to 6 adult red seed weevils per head. Some sources place it slightly higher, at 10 to 15 weevils per flower, depending on seed price and control costs.
Confection sunflowers have a far lower threshold: just 1 red seed weevil per head. The confection industry demands that seed damage stay below 3 to 4% of kernels, and because infested seeds look normal and aren’t removed during cleaning, even low-level infestations can cause quality problems.
Spray Timing and Chemical Control
Timing matters more than product choice. The ideal window for treating red seed weevils is when most plants have reached about 40% pollen shed. In practice, North Dakota State University recommends considering treatment when more than half the plants are just beginning to show yellow ray petals through 30% pollen shed, with the remaining plants still in the bud stage.
Spraying at the bud stage is a common mistake. While insecticides applied that early will kill weevils on contact, the treatment isn’t cost-effective for three reasons: seeds haven’t developed enough for egg-laying, eggs inside the female weevils aren’t yet mature, and adult weevils are still emerging from the soil. You’d be spraying before the real wave of infestation arrives.
Pyrethroid insecticides are currently the only chemical class available for red seed weevil control, since chlorpyrifos has been removed from use on field crops. However, field failures with pyrethroids have been reported in recent years, particularly in South Dakota. University entomologists now recommend applying at the highest labeled rate to reduce the risk of poor performance. Heat also plays a role: temperatures above 90°F shorten how long pyrethroids remain active on the plant, and some active ingredients break down faster in heat than others.
Cultural Management Strategies
For the stem weevil, planting date is the most effective cultural tool. Most eggs are laid before mid-June, so sunflowers planted after the first week of June typically escape stem weevil infestation. Research across both the northern and southern Great Plains has confirmed that delayed planting lowers larval densities in stalks. Tillage is less effective because overwintering larvae are protected inside the stalks, though burying stalks through fall or winter tillage can reduce adult emergence the following spring.
There’s a tradeoff to keep in mind. Delaying planting avoids stem weevils and sunflower beetles, but later-planted fields can become more attractive to the red sunflower seed weevil, which is active later in the season. Growers in areas where both pests are present need to weigh which poses the greater economic risk and adjust planting dates accordingly.