Facts:

• The eggs are very small and difficult to see without a hand lens. They look like flat white scales with small yellowish to reddish areas near the center. Hatched eggs appear brighter white; eggs that have been parasitized by a small wasp appear black and will not develop into larvae.
• In the larval stage, the cranberry fruitworm is a smooth caterpillar that is mostly green with some brownish-red coloration on its top surface. It has three pairs of true legs on the thorax and five pairs of fleshy prolegs on the abdomen. Larvae are about 1/2 inch long when fully grown. The larvae attain a length of about 3/8 inch and are usually greenish, sometimes light brown along the back. Once larvae are fully grown, they drop to the ground and spin a hibernation chamber where they overwinter. There is only one generation per year.
• The adults are brownish-gray moths with a wingspan of about 5/8 inch.
• It overwinters as a fully grown larva within a cocoon made of silk and soil particles.
• The cocoons are frequently made under weeds and debris on the soil surface, but they may be deeper.
• The larvae pupate and complete development, with the adult moths emerging after bloom and fruit set.
• The eggs are deposited on the berries, almost always on or inside the calyx cup (blossom end) of unripe fruit.
• Eggs hatch in about five days.

Damage:

• Young larvae move to the stem end of the fruit, enter and feed on the flesh.
• A single larva may feed on up to eight berries to complete its development.
• The larvae move from one berry to another within a cluster and usually web the berries together with silk.
• The inner flesh of developing and ripening berries is consumed entirely by fruitworm larvae. Damaged berries are covered with brown sawdust-like frass and usually webbed together with silk.
• The frass fills the tunnels in the berries that cling to the silk webbing, producing very messy feeding sites, which easily distinguish cranberry fruit worm damage from cherry fruitworm damage.

Scouting:

• Monitoring the flight of adults with pheromone traps will greatly improve the timing of pesticide treatments for this pest.
• Experienced scouts can also monitor the calyx ends for eggs to determine the amount of egg laying.

Control:

• Cultural Control: Elimination of weeds and trash around plants helps by cutting down on overwintering protection for fruitworm cocoons.
• Mechanical Control: Cranberry fruitworm was effectively controlled in the past by picking off infested berries, which are easily detected because of the webbing and their early ripening. This method is still practical in small plantings with light infestations.
• Insecticide Control: Clean cultivation will reduce the population of cranberry fruitworm within a field significantly, but insecticide treatments may still be needed to achieve satisfactory control of this pest.
• Check with your local Cooperative Extension Service office for control recommendations for your area.

Sources:

• Longstroth, Mark. Cranberry Fruit Worm. Michigan State University Cooperative Extension Service. 13 July 1999. Retrieved 14 July 2010.
• Payne, Jerry A. USDA Agricultural Research Service. www.bugwood.org.