Gardener's Guide: Cranberry Fruit Worms

Information on cranberry and cherry fruitworms, devastating insect pests. Learn about grapholita packardii and acrobasis vaccinii.

What is it?

Fruitworms which attack blueberry plants are one of these fruit plants most serious and devastating insect pests. There are two particular types of fruitworms which attack blueberries. They are grapholita packardii or cherry fruitworms and acrobasis vaccinii, or cranberry fruitworms. Both grapholita packardii and acrobasis vaccinii also attack cranberries, cherries and apples. Each caterpillar or fruitworm, can destroy between two and six berries each, which can seriously hinder the yield of the harvest crop.

What does it look like?

Both grapholita packardii and acrobasis vaccinii can be anywhere between 3/8 inch to one full inch in length and at any given time you are likely to find these fruitworms in a wide array of sizes while attacking your blueberry and other fruit plants. Typically the fruitworms which attack blueberries are either a pale yellowish green or they are a pinkish red. Variations between these color schemes sometimes occur with Both grapholita packardii and acrobasis vaccinii. The fruitworms are smooth but look very much like other types of caterpillars, and they will more often than not be found within the insides of the fruit rather than on the foliage of the blueberry plants. Damage from Both grapholita packardii and acrobasis vaccinii shows up in the form of blueberries which are shriveled. Cherry and cranberry fruitworms also leave behind a sawdust like material as they feed which often fills the emptied, shriveled blueberry husks. The shriveled portions of the blueberries may be joined by a silken webbing. This silken webbing causes the fed upon blueberries to appear clustered. Neither grapholita packardii nor acrobasis vaccinii damage the leaves or foliage of the blueberry plants, however the leaves may occasionally have the characteristic silken webbing of both cherry and cranberry fruitworms attached to them in areas near damaged blueberries on the plants.

How does it manifest?

Both cherry and cranberry fruitworms hatch as caterpillars from eggs which adult females have laid. The adult females lay the eggs each year during mid-spring on developing foliage and blueberries. As soon as the fruitworms hatch from the eggs, they bore into the fruit of the blueberries and begin feeding. They bore into the individual berries at the join where the stem and the berry meet and remain inside the blueberry until they are approximately half way to their full mature growth. At this point, both graphholita packardii and acrobasis vacinii move to a second blueberry, usually one which is next to or nearby the first blueberry which the caterpillar had infested. In this way, the fruitworms are able to move from one berry to another to feed without revealing or exposing themselves. Approximately mid-June in the growing season, both cherry and cranberry fruitworms crawl into the soil as full grown fruitworms. Occasionally, acrobasis vacinii and grapholita packardii also move into the nearby plant debris or pruning stubs where they also over winter. Adult moths then emerge again in mid-spring and the adult females start to lay eggs and begin the reproduction process again.

What can you do about it?

Unfortunately, once the blueberries themselves become infested there is no way to kill off the fruitworms which are inside. All infested fruits must be handpicked and destroyed so that the cherry and cranberry fruitworms do not migrate to nearby blueberries to feed and destroy more of the crop. Often, if you are planting a large crop of blueberries it is a good idea to spray the blueberry plants soon after the bloom, but before the blueberries grow to 1/4 inch in diameter. Using an insecticide which contains diazinon will often stave off the infestation of both grapholita packardii and acrobasis vacinii. You should always repeat the initial application again in ten days to be sure to get any worms which may have surfaced a bit later than others.

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