Tomato Pinworm

Tomato pinworm occurs throughout southern California and sporadically in some areas of the San Joaquin Valley and coastal growing areas.

Eggs, seldom noticed because of their small size, are usually laid singly on lower surfaces of leaves. Early instars are light colored and appear smooth even when observed with a hand lens; they lack the obvious tubercles and bristles of newly hatched tomato fruitworms or tobacco budworms. Later instars are most often found in fruit; they usually are gray or yellowish with an irregular band of red or purple across each segment. Larvae either pupate in leaf shelters or drop to the ground to pupate. The slender, brown pupa is usually enclosed in a loose silk cocoon with adhering soil or plant debris. Adult moths are small, about 1/4 inch (6–8 mm) long, and light gray, peppered with small black flecks.

There can be as many as seven or eight overlapping generations per year.

This caterpillar feeds on leaves and creates blotch-type mines, but causes most of its damage when it attacks the fruit. Where abundant, the tomato pinworm may seriously damage foliage and infest nearly 100% of the fruit. Larvae normally enter fruit through the calyx, but when populations are high they may enter at any point on the fruit's surface. They make dry burrows and do not penetrate very far into the fruit. When infested fruit is picked, caterpillars may be difficult to detect unless they have been feeding long enough to create small piles of brown, granular frass at the edge of the calyx. Because the pinworm has many generations per season, it becomes more serious as the season advances. The greatest damage occurs where tomatoes are grown from early in the season to late in the fall or in areas where the seasons for early and late tomatoes overlap.

Successful management requires keeping pinworm infestations below damaging levels in the current season, and reducing the overwintering population that will attack later crops. Important management tools are host-free periods, mating disruptants, insecticides, destroying plant residues after harvest, and destroying other solanaceous host plants in the field's vicinity. Careful monitoring can improve management. In the central San Joaquin Valley, tomatoes planted in late winter rarely require treatment for tomato pinworm.

Parasites, including Apanteles spp., Sympiesis stigmatipennis, and Parahormius pallidipes, can be important in controlling pinworm in unsprayed or lightly sprayed fields. Hold infested foliage in cages or bags at room temperature for 2 to 3 weeks. Note the number of parasites that emerge to pupate to determine the percentage of parasitism.

A host-free period is essential for reducing pinworm populations; the longer the period the better. Destroy residues by burning (where permitted) or plowing-under to help reduce overwintering populations of pinworm. Avoid growing both an early and late-season crop if pinworm is a persistent pest in your area. Shred and disc crop residues immediately after harvest completion, if substantial tomato pinworm populations are present during the first planting and a second crop has been planted in an adjacent field. Crop rotation is not an efficacious management tool unless practiced on an area-wide basis and must also include the removal of solanaceous weeds.

Transplants have been implicated as a potential source of infestation. Check transplants for evidence of pinworm larvae and avoid infested plants.

Biological and cultural control and the use of mating disruptants are acceptable on organically certified produce.

Set out pheromone traps at planting. Use one trap for each 10 acres but no fewer than two traps per field. Distribute traps throughout the field. Check traps and remove any trapped moths twice a week from planting to harvest. When you begin trapping pinworm adults, start monitoring foliage for larvae. If a mating disruption program is being used, pheromone traps will not catch moths.

To survey the foliage, carefully check for mines and folded leaf shelters on all foliage in several sections of row, each 6 feet long, chosen at random throughout the field; record the average number of larvae per row section. Conduct the first survey as soon as seedlings are well established; continue checking weekly until it is necessary to begin treatments. A provisional guideline is to treat when you count an average of 1 to 2 larvae per row section. Also monitor parasitism percentages.

If pinworm populations reach damaging levels, the narrow-spectrum insecticide abamectin can be used. If broad-spectrum insecticide materials are used, it usually is necessary to continue treating throughout the season until final harvest. The time between treatments depends on population levels. Several materials used to control tomato pinworm kill parasites of Liriomyza leafminers, so repeated applications often cause leafminer outbreaks.

Mating disruption

Pheromone mating disruption can be effective in isolated fields and where all tomato fields in an area are treated. In fields surrounded by untreated fields, females may mate in the untreated fields and migrate into treated fields to lay eggs. Where successful, pheromone confusion suppresses pinworm populations without affecting natural controls of pest species. Combine pheromone confusion with a comprehensive program of visual monitoring (pheromone mating disruption will disrupt moths from being attracted to pheromone traps) so you can tell if numbers reach treatment thresholds.