Question: I’m finding tiny white worms in my red raspberries. I’ve never had this problem before. Do you know what is going on? Is there anything I can do? My family loves raspberries. S.T.
Answer: The worms may be larvae of Spotted Wing Drosophila (SWD), which is new insect pest of Wisconsin fruit crops. SWD is a type of vinegar or fruit fly. Most vinegar flies only affect overripe or damaged fruit, but SWD harm healthy fruits. SWD are most likely to affect skinned fruits like raspberries, blueberries, strawberries, blackberries, cherries and nectarines. Grapes, plums, peaches, pears, apples, and tomatoes have a moderate risk of infestation.
A native of Eastern Asia, Spotted Wing Drosophila was discovered in California in 2008. By 2013 SWD had been detected in 46 states, Canada, and Mexico. Last year SWD was confirmed in 25 Wisconsin counties and suspected in another four, including Kenosha.
Spotted Wing Drosophila adults look like other vinegar flies. Entomologists use small characteristics, such as wing spots and serrated ovipositor, to distinguish SWD from other Drosophila species. Larvae are small, legless, cream colored, and up to 1/8 inch long. Fruit damage is caused when female SWD cut a slit through the skin of healthy fruit to lay eggs. Larval feeding causes infested fruit to collapse within two to three days.
Good cultural management may help reduce reproduction and survival of SWD. Remove uncultivated or wild host plants like dogwoods, honeysuckle, elderberry, plums and apples. Keep SWD off plants by covering with fine-mesh (less than 1/32-inch) netting or floating row cover before fruit begins to ripen. Harvest fruit as soon as it ripens. Overripe fruit on the plant or fruit rotting on the ground must be removed and destroyed to reduce egg laying and larval development. Kill larvae in infested fruit by sealing fruit in a zip-top clear plastic bag and laying the bag in the sun for several hours. Do not compost infested fruit.
It is not safe to use insecticides for larvae in the fruits. Spotted Wing Drosophila adults can be effectively controlled with spinosad or pyrethrum, which are organic, or a number of synthetic insecticides. If you choose to use an insecticide, carefully follow all safety precautions. Use a product labeled for fruit crops, use exactly as directed on the label, and adhere to the number of days between spraying and harvest.
Barb Larson is horticulture educator for Kenosha County University of Wisconsin Extension. Barb has a Master’s of Science in horticulture from the University of Wisconsin-Madison. If you have a plant or gardening question, email Larson at firstname.lastname@example.org or call her at 262-857-1942.