Iowa State University Extension gardening experts answer questions on damaging worms:
Worms are devouring the needles on my mugo pine. What should I do?
The “worms” that are eating the needles on your mugo pine are the larvae of the European pine sawfly. European pine sawfly larvae are grayish green. Two light stripes and one dark stripe run down the sides of the body. The legs and head are shiny black.
The larvae feed mainly on mugo, Scotch and Austrian pines, though other pine species are occasionally damaged. They do not feed on spruce or fir. Larvae typically appear in mid to late May in Iowa and are usually gone within a few weeks.
European pine sawfly larvae feed on needles produced in previous years. (The needles on most pines persist for two to five years.) They do not harm the new needles developing on the branches. As a result, the damage is mainly aesthetic. Larval feeding does not destroy the affected branches. The branches simply have fewer needles than normal.
To keep damage to a minimum, the larvae of the European pine sawfly can be controlled by pruning off and discarding infested branches, knocking the larvae off affected branches into a bucket or other container and destroying them, or spraying them with an insecticide, such as Sevin.
I occasionally find small, white worms in my cherries. What is the best way to control them?
The small, white “worms” are probably the larvae of the cherry fruit fly (Rhagoletis spp.) Cherry fruit flies lay eggs on developing cherry fruit in May. Damaged fruit appear shrunken and shriveled when ripe, and usually contain one off-white larva (maggot) that is slightly longer than one-quarter of an inch.
Cherry fruit fly damage varies greatly from year to year. It may be more practical to tolerate some damage and loss of usable fruit than to attempt effective preventive control.
To prevent maggots from appearing inside the fruit, the tree must be thoroughly sprayed with a labeled insecticide when the adults emerge and before the females lay their eggs inside the young fruit. Because the flies emerge over an extended period of time, several sprays will be needed. You can monitor fruit flies with yellow sticky traps hung in the tree in early May. Check traps daily after the first fruit fly is caught and repeat the spray application until flies no longer appear.
Check for home orchard sprays and other insecticides at your local garden center. Carefully read and follow label directions.