Raspberry fruit is vulnerable to damage from both disease and insects. Good pest control habits can reduce the risk of your plants and fruit being ruined by these bugs. Prune raspberries properly and visually inspect them often. The simple act of keeping wild brambles out of your bushes will reduce the need for insecticides even as it reduces the likelihood of pests. Some pests attack the canes of the plants while others ruin the raspberry fruit.
TIP: Our expert gardening advisor, Susan Patterson adds, "Consider netting your fruit to reduce pest damage."
The least worrisome raspberry fruit pest is probably the picnic beetle, because it only feasts on fruit that's already overripe and neglected. A picnic beetle is black with four yellow-orange spots on its back and is typically about a 1/4-inch long. These beetles aren't drawn to raspberry fruit specifically, but to other plants and fruits in the garden. The best way to prevent damage from these beetles is to pick your raspberries and other fruit as soon as they are ripe. Don't leave fruit on the vine to become overripe. Insecticides aren't very effective against this particular pest.
The fruitworm is distinguishable from the picnic beetle by its shorter length (about an 1/8-inch long), its light brown color and hairy appearance. The female fruitworm lays its eggs on the buds and fruits in the late spring. When the eggs hatch, they look like tiny yellow worms. They eat the fruit during the 6 weeks before they drop from the plants and pupate. They stay cocooned in the soil this way through the winter and then in late spring, usually May, adult beetles emerge and the process begins again.
While the adult beetles aren't destructive, the fruitworm larvae eats the berries. Look for early signs of infestation including fruit dropping to the ground or rotting before harvest time. If present, the larvae are visible on the fruit. The best defense against fruitworms is to turn the soil in early fall to allow the cocooned worms to be removed by weather and predators. You can also pick adult beetles from the plants. If desired, insecticide can be used according to the manufacturer's instructions.
Raspberry Bud Moth
Again, adult bud moths are not a threat to raspberry fruit, but the larvae is destructive enough to prevent the plant from producing any fruit at all. The larvae get inside the plant through an unopened bud and destroy it from the inside out. Careful insecticide use according the manufacturer's directions is really the only way to keep this pest from destroying your raspberry fruit.
TIP: Susan suggests, "Try dusting plants with natural diatomaceous earth to keep pests at bay."