Jun 18, 2021

Strategies for sap beetle management in sweet corn shared

Although there are many species of sap beetles only several commonly become pests in agricultural crops.

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 larva 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 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 in silk and where wet pollen collects including the leaf whorl and axis. Adults also become attracted directly to the corn ear when silk begins to degrade and when kernels ripen close to harvest. In the summer, it takes roughly a month for an egg to mature into an adult, and after July all stages of sap beetles may be found in sweet cornfields. Remember, corn is most attractive to sap beetles just after silking begins and again when kernels ripen in the days leading up to harvest.

Secondary pest

Sap beetles are typically secondary pests of corn but can act as primary pests if populations are high. The insect and plant volatiles released from damage caused by primary pests act as attractants to sap beetles to sweet cornfields. The damage caused by primary pests, such as the corn earworm, then become entryways into cobs for sap beetles.

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 does not impact sap beetles, sap beetles can become a primary pest in sweet corn.

Even when primary pest pressure is low, if there is a large population at the farmscape level, we have seen sap beetles act as a primary pest, causing significant damage even when there is little worm pressure. However, several management methods can be useful at keeping sap beetle populations below action thresholds that would warrant spraying for the insect.

Sanitation and field location

Sanitation within and around sweet corn fields is critical for initial sap beetle control. Fields located near potential breeding sites such as produce refuse piles, compost piles, or woodlots with bacterial ooze may be at higher risk of sap beetle infestation. Corn cobs left on the ground can also act as attractants to sap beetles. Left-over crop material should be tilled deep or completely removed from the field to keep sap beetle population numbers low.

Farms producing a variety of vegetable and fruit crops near sweet corn should be especially vigilant of sanitation. Rotting fruit in the field is an attractant to sap beetles and may be common in years where early fruit maturity can outpace harvest or for you-pick fruit operations. Farms with both strawberries and sweet corn should try to avoid having a population of sap beetles build up in their berry crop. In fact, rotting fruit along with a pheromone has been known to attract sap beetles and serve as a helpful monitoring tool.

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.

Predator conservation

Sap beetles have few natural enemies and little research has been done on the release of predators or bioinsecticides to control sap beetles. Several species of tiny parasitic wasps parasitize sap beetle larvae. The insidious flower bug, Orius insidiosus, feeds on sap beetle eggs. Few other natural predators or parasites are known.

Landscape and field level plant diversity has been shown to host more abundant and more diverse predators of plant pests. While documented occurrences of natural sap beetle control are limited, farmers may consider some best practices to conserve natural enemy populations they may already have or take steps to bolster numbers. These include not using insecticide treatments as preventative maintenance, increasing the complexity of crop rotation, utilizing conservation till management practices, and incorporating conservation strips.

Field monitoring and sampling

To monitor your fields for sap beetle, commercial pheromone traps are available. Used along with bait such as rotting fruit or bread dough, the lures will help you know if populations of sap beetle are increasing. Currently, there are no economic thresholds based on trap counts alone. Place your traps and bait early to catch the initial wave of adult sap beetle emergence. Even if traps are utilized to monitor sap beetles in sweet corn, field sampling throughout the season starting during pollen shed is suggested as the best means to determine sap beetle populations and action thresholds.

A standard sampling method is to examine 100 ears of corn in each field. Five locations within the field should be sampled with 20 ears from each sample location. Corn appears to be most attractive to sap beetles early in the year during silking and after injury from other pests. However, sweet corn begins to become attractive again to sap beetles within 10 days of harvest when kernels generate sugars and silk degrades. Sap beetle populations taper as the season progresses; however, sap beetles can have multiple generations per year, highlighting the importance of field surveying from pollen drop to harvest.

Chemical application

If sap beetle numbers in sweet corn exceed action thresholds, a chemical application may be required for an acceptable harvest. Suggested action thresholds for chemical treatment of sap beetles in sweet corn vary from 5% of ears sampled infested with adults or eggs to 10% infestation of adults and larvae. 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.

On farms with a known history of sap beetle problems, an insecticide application 5-6 days after the first onset of silking is the best timing for maximum protection against sap beetles because it should knock down the first generation. While this timing may be most effective against sap beetles, it may also have the highest impact on bees. Honeybees and other native pollinators will forage for corn pollen and applications made during pollen shed may impact their populations. Avoid using products of highest hazard to honeybees and work with area beekeepers to limit honeybee exposure. Avoid spraying in mid-morning when bees are most active and pollen shed is highest.

In Pennsylvania, consult 2020/2021 Mid-Atlantic Commercial Vegetable Production Recommendations for a list of chemicals labeled for use in sweet corn. Pyrethroids, carbamates, one neonicotinoid (Assail), and a spinosyn product (Blackhawk) 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.

Considerations for organic growers

Organic growers should be particularly careful to follow good sanitation practices as well as planting tight-husked varieties. In general, insecticides are not a highly effective tool for sap beetle control. Good coverage is hard to achieve because the insect is protected. Your best line of defense will be eliminating sap beetle food sources: do not keep cull piles or compost near sweet corn and strawberry fields, and disk in corn debris as soon as possible after harvest.

– Shelby Fleischer, Leah Fronk and Glen Bupp, Penn State University

Sap beetle damage in sweet corn. Note the one adult and several larvae. Photo: Leah Fronk/Penn State University