Sap Beetle Management in Sweet Corn

Of these, the dusky sap beetle, Carpophilus lugubris, is the most common sap beetle pest of sweet corn.

Eliminating rotting material and preventing crop damage is the primary way to keep sap beetles out of sweet corn and minimize the need for insecticides. Field sanitation is key on farms with diverse production, especially where sweet corn is grown beside berry and fruit crops. Rotting fruit, such as strawberry, can become an attractant to large numbers of sap beetles that may cross over into sweet corn.

Sap beetles overwinter in wooded areas and protected locations. They can feed on sap from wounds on trees, and tend to build where adults and larvae can feed on ‘free sugars’. In agricultural fields, adults or pupae also overwinter in crop refuse, decomposing corn ears, or decaying fruit on the ground. Adults appear as soon as corn begins to silk or just after. Adult sap beetles feed on corn silk, pollen, and will also chew on tassels.

Adults lay eggs where they feed on silks and where wet pollen collects, including the leaf whorl and leaf axils. The eggs hatch into small, cream-colored larvae that resemble tiny worms or maggots and feed on developing kernels inside the ear. Adults are also attracted directly to ears as silks begin to dry down and kernels ripen near harvest. During the summer, it takes about one month for a sap beetle to develop from egg to adult, and after July, eggs, larvae, pupae, and adults may all be found in sweet corn fields. Sweet corn is most attractive to sap beetles shortly after silking begins and again in the days leading up to harvest as kernels mature.

Secondary Pest

Sap beetles are typically considered secondary pests of sweet corn because they often rely on feeding damage caused by primary pests to gain access to ears. Injury from corn earworm and European corn borer larvae is especially important, as insect feeding and associated plant volatiles attract sap beetles to fields. The tunnels and feeding wounds created by these primary pests then serve as the primary entry points for sap beetles, allowing them to invade and feed within the ears.

Conventional spray programs designed for corn earworm and European corn borer likely controlled sap beetles in the past. With the onset of transgenic corn that can produce Bacillus thuringiensis proteins, European corn borer populations have become much reduced, reducing the frequency of corn damage and insecticidal spraying in cornfields. In the absence of these spray programs, and because Bt corn has no effect on sap beetles, sap beetles can become a primary pest in sweet corn.

Don't assume low worm pressure means sap beetles won't be an issue. When sap beetle populations are high in the surrounding area, they can attack ears directly and cause substantial damage on their own. The good news is that a combination of management tactics can help keep populations in check and often prevent the need for an insecticide application.

Sanitation and Field Location

Sanitation is one of the most effective ways to reduce sap beetle pressure. Fields near cull piles, compost piles, or fruit crops with overripe or rotting fruit may be at greater risk because these materials attract sap beetles and allow populations to build. Corn cobs and crop residue left in the field can also serve as breeding sites. Harvest sweet corn on a regular schedule to avoid over mature ears, and remove or deeply incorporate crop debris after harvest. Growers with strawberries or other fruit crops near sweet corn should also remove unmarketable fruit whenever possible to help limit sap beetle populations.

Variety Selection

Sweet corn varieties with tight, long husks that mature after field corn has dropped pollen have demonstrated resistance to sap beetles. Though these varieties sustain less injury than susceptible varieties, they are not immune. Sap beetles tend to be attracted to earlier maturing sweet corn. Therefore, sweet corn that matures after surrounding field corn has dropped pollen tends to have a lower infestation. Super sweet corn varieties may be more susceptible to sap beetle damage because of poor tip coverage from corn husks and the higher concentration of sugar in the developing kernels.

Minimizing Tip Exposure

Tip exposure is a major determinant of sap beetle damage. Ear tips can be exposed when prolonged drought is followed by wet weather. These conditions can cause the cob to grow for a longer period than the husk. This provides direct access to kernels by sap beetles and increases the likelihood of damage. Weather monitoring and proper irrigation may help to combat the incidence of tip exposure. Irrigation to ensure consistent water uptake during cob maturation under variable weather patterns is important for optimal growth and minimizing the risk of tip exposure.

Scouting and Thresholds

Begin scouting for sap beetles when silks start to wilt. Each week, check 20 primary ears at five locations throughout the field, paying close attention to the ear tips for larvae. Treatment is recommended if you find an average of two or more larvae per 100 plants and harvest is more than four weeks away. Another option is to begin scouting at pollen shed and treat when 5% of ears contain sap beetle adults, eggs, or larvae.

Pheromone-baited traps can also help monitor sap beetle activity. Place traps at field edges and use a bucket containing rotting fruit as bait. Because sap beetles release aggregation pheromones that attract additional beetles, trap contents should be emptied and refreshed every few days.

For farms with a history of sap beetle problems, an insecticide application may be warranted 5 to 6 days after the first silks emerge. This timing targets the earliest sap beetle activity and can help reduce populations before they build and cause significant ear damage. However, applications made during silking may also increase the risk to pollinators visiting corn. To reduce this risk, avoid using products that are highly toxic to bees and avoid spraying during mid-morning and other periods when bee activity is highest.

Predator Conservation and Non-chemical Strategies

Natural enemies of sap beetles are limited, but some beneficial insects can help suppress populations. Tiny parasitic wasps attack sap beetle larvae, and the insidious flower bug (Orius insidiosus) feeds on sap beetle eggs. While natural control alone is unlikely to prevent economic damage, conserving beneficial insects may help reduce pest pressure. Growers can support beneficial insect populations by avoiding unnecessary insecticide applications, rotating crops, reducing tillage where practical, and maintaining conservation strips or other areas of plant diversity around fields. These practices can increase the abundance of natural enemies and contribute to overall pest management.

Chemical Application

It is important to remember that the control of sap beetles with insecticides can be difficult because adults and larvae are protected inside the ear. Timing is critical to ensure maximum knockdown of sap beetles before they become established in the ear.

In Pennsylvania, consult the Mid-Atlantic Commercial Vegetable Production Recommendations for a list of chemicals labeled for use in sweet corn. Pyrethroids, carbamates, and one neonicotinoid (Assail), are labeled for sap beetles in sweet corn. Efficacy trials from Delaware in 2013 showed that multiple (5) applications were required to reduce the percentage of damaged ears by sap beetle relative to the untreated control.

Always read and follow the product label, including any pollinator protection statements, use restrictions, and application timing requirements.

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