Strawberry insect pests of the home garden
Published in Yard & Garden Brief, April 1999
In Minnesota, the most common insect pests of strawberries are tarnished plant bugs, strawberry bud weevils, slugs, picnic beetles, and spittle bugs.
Fig. 1 Tarnished plant bug
As strawberry flower buds appear in spring, the adult females emerge from their overwintering sites in weeds and lay eggs. Nymphs hatch and feed on the flowers and/or the developing seeds, resulting in misshapen fruit at harvest time. Most damage occurs when the fruit is green however the bugs are in the fields up through harvest. Once deformed fruit is noticed, it is too late for control.
Strawberry seeds, which are found on the surface of the fruit, actually trigger fruit development. If seeds are injured, the fruit will be malformed. Tarnished plant bug feeding produces enlarged, hollow, straw brown seeds. These seeds are clustered at the end of the berry. Poor pollination or light frost may also be responsible for malformed fruit. These injuries produce smaller than normal seeds.
Fig. 2 Tarnished plant bug
To control tarnished plant bugs, remove weeds and debris around the berry patch to reduce potential adult overwintering winter sites. Keep the garden as weed free during the summer as possible.
Check plants at least twice a week before bloom for signs of tarnished plant bugs. An insecticide application is considered necessary when an infestation reaches an average of 1 nymph per plant. An easy way to check for tarnished plant bugs is to hold a white paper plate below the foliage and tap the plant with your hand. Watch for insects that fall onto the plate. Tarnished plant bug nymphs can be distinguished from aphids, a common pest, by speed of movement. Aphids move very slowly and tarnished plant bug nymphs move very quickly. It is not necessary to monitor for tarnished plant bugs after green fruit have formed.
Effective insecticides to use include carbaryl (e.g., Sevin), diazinon, malathion, and permethrin. Make one insecticide application just prior to bloom. Avoid spraying when strawberries are in bloom to protect pollinating insects.
Strawberry bud weevils, Anthonomus signatus, are also called strawberry clippers or strawberry weevils. They are reddish brown, with black patches on their backs (fig. 3). They are about 1/10" in length with curved snouts about half as long as their body length.
Fig. 3 Strawberry Bud Weevil
Crops are attacked in early spring as adults emerge from overwintering sites in woodlots and along fence rows. Using their snouts to puncture the immature flower buds, they feed on pollen. Females lay a single egg in each bud then girdle the buds to keep them from opening, thus protecting developing larvae. Girdled buds may drop to the ground or hang from the plant.
Eggs hatch into grubs after a week then remain in buds from three to four weeks until maturing into adults. When adults emerge, they move to various flowers to feed on pollen. By mid-summer, they move to overwintering sites where they remain until spring. Only one generation occurs per year.
Start monitoring plants as soon as the strawberries form flower buds. Check plants at least twice a week. It is not necessary to monitor once the buds are open. The presence of one cut bud for every 4-5 plants indicates the need for insecticide application. Careful timing using carbaryl (Sevin), diazinon, or permethrin provides the best control. It's too late for control when strawberries are in bloom.
Fig. 4. Slug
Fig. 5 Picnic Beetle
Fig. 6. Spittlebug on strawberry
Slugs are not insects but are mollusks, related to oysters and clams. They are often described as looking like snails without a shell (fig.4). Slugs are approximately ¼ to 2 inches long.
Slugs rasp away at plant tissue with their mouths. Signs of slug damage are small, deep holes in the fruit, usually under the cap and irregular holes in foliage. Slugs leave silvery slime trails which may also be seen on foliage. Damage is usually noted in the morning, as slugs generally feed at night. They may also be found feeding during the day when it is cloudy. Slugs are more common during periods of wet weather.
Remove mulch, leaves and other debris from the garden to reduce hiding places and overwintering sites. In some cases, it may be more important to leave mulch in place to help conserve moisture and occasionally to combat weeds. Water garden plants deeply and reduce the frequency of watering. This allows the foliage and soil surface to dry between waterings which helps manage slugs.
Trap slugs by placing boards or newspaper in the garden. Slugs will hide under these objects during the day. In the morning, remove these traps and kill the slugs found under them. Another option is to sink cups or pans filled with beer into the ground so that the top of the container is even with the ground. The beer attracts slugs into the container where they drown. Replace the beer after a rain or every few days.
If nonchemical methods are not effective, try metaldehyde bait. Apply the bait to the soil near the strawberries. Do not apply bait to the plants.
Picnic or sap beetles are¼ inch long, black with four yellowish-orange spots on their backs (fig. 5). They feed on overripe or rotting fruits and vegetables that remain in the garden.
Sanitation is the best control. Remove fruits and vegetables as they ripen; do not allow them to fall to the ground and rot. Insecticide control is not recommended because you must wait a specified time between insecticide application and harvest. By the time it is safe to pick strawberries, the picnic beetles have returned to the garden.
Spittlebugs are the immature form of froghoppers. They are among the easiest insects to identify. They get their name from the clear, bubbly foam masses in which they hide (fig. 6). These masses of 'spittle' are about 1/3 - 3/4 inches in diameter.
Spittlebugs damage plants by piercing the stems and feeding on the juice. They work from ground level up to the newer growth. Spittlebug feeding can cause small berries and stunted plants. Leaf blades and petioles often appear distorted. This damage to the plants is temporary and the plants will outgrow it.
Physically crushing and removing spittlebugs is the best control. Insecticides are unnecessary because spittlebugs are normally not an important pest. Also, insecticides do not penetrate the spittle mass well and has little effect on spittlebugs inside.
Carefully observe the waiting period. Protect pollinators — do not spray strawberries or other fruits when they are blossoming. If suggestions in this publication contradict label recommendations, the label is the final authority on how to use that specific product.
Caution: Always use insecticides strictly in accordance with label precautionary statements and directions. The label should state that the insecticide may be used on strawberries or generally on fruit.