Gardeners in the southern U.S. are probably already familiar with the tomato pinworm. But those of us north of the Mason-Dixon line had better familiarize ourselves with this tiny pest quickly because it’s becoming more common. I had trouble with tomato pinworms in my Pennsylvania garden for the first time last summer, and unfortunately, they’re back again this year.

About Tomato Pinworms

Tomato pinworms (Keiferia lycopersicella) don’t survive the regular freezing winter temperatures we have here in the north, unless we have an unusually warm winter. However, tomato pinworms easily overwinter in greenhouses and go on to emerge in the spring and lay eggs on a new generation of plants. The biggest reason we’re seeing these pests in the north is probably because more tomato transplants are started in the south and then shipped around the country to be sold at various retailers, including many big box stores. As infested plants move from one region to another, so does this pest. Learning to identify the tomato pinworm is key to controlling it. Northern gardeners should also purchase tomato plants only from local growers or grow their own from seed. Because adult tomato pinworms are highly mobile and can move quickly from garden to garden, encourage your neighbors to do the same.

Identifying Tomato Pinworms

As adults, pinworms are very small, night-flying moths that are gray and nondescript. Their wingspan is a mere half-inch, but female moths lay small groups of eggs on tomato leaves. Occasionally they may also lay eggs on eggplants and peppers as well.

Tiny caterpillars hatch a few days later and begin to tunnel between layers of leaf tissue in the same manner that leafminers do, leaving blotches or squiggly lines on infested tomato leaves. Eventually, as the caterpillars grow, they exit the layers of leaf tissue and curl or fold leaves around themselves as they continue to feed. This foliar damage, however, is not typically problematic; it’s when the larvae near pupation that the real trouble begins.

Eventually, the caterpillars make their way to the developing fruits, chewing small holes into the stem end of the tomato. They feed inside the growing tomatoes for several days, tunneling out the tissue and leaving a “rotten” spot behind.

How To Control Tomato Pinworms

Aside from purchasing locally grown plants, you can also help prevent pinworm issues by removing infested tomatoes immediately and destroying the pinworm larvae. Watch closely for tomatoes with tiny holes in the top of the fruits. At the end of the growing season, discard of any fallen fruit by putting it into the garbage, not the compost pile.

Be sure to inspect your tomato plants on a weekly basis through the spring and summer. Look carefully for any curled leaves that are folded over a small caterpillar or leaves with tunnels through the tissue. If you find either of these signs, remove the infested foliage from the plant and burn or throw it into the garbage.

Large tomato plantings can be sprayed with the organic pesticides Bt or spinosad if you spot the pinworms before they tunnel into the tomato fruits. Once inside the fruit, however, tomato pinworms are extremely difficult to control with pesticides.