Leafminers

Liromyzid leafminer adults are small (0.06 inch or 1.5 mm) black and yellow flies having a bright yellow scutellum, a triangular spot on the top rear of the thorax. In the San Joaquin Valley, it is important to distinguish between L. trifolii and L. sativae, which can occur in the same areas, because L. trifolii is much more resistant to most chemical controls. Using a strong hand lens, observe the compound eye of the adult. The eye of L. trifolii is nearly completely surrounded by yellow with only a small portion of black touching the rear of the eye. Liriomyza sativa has approximately one-fourth to one-third of the eye bordered in black at the rear. The top of the thorax of L. sativa is shiny black whereas on L. trifolii it is more dull dark gray colored. The larvae of leafminers are tiny bright yellow maggots about 2 mm long when they emerge from the leaves to pupate. Pupae look like tiny brown grains of rice. There can be many generations a year.

Females insert the white, oval eggs into the leaf tissue by puncturing the leaf at the tip and along the edges. Numerous punctures are made, but only a small percentage contain eggs: the majority serve as feeding sites for the adults who lap up the exudate.

Larvae mine between upper and lower leaf surfaces, creating winding, whitish tunnels that are initially narrow, but then widen as the larvae grow. They may cause leaves to dry, resulting in sunburning of fruit and reduction in yield and fruit quality. In severe infestations, the leafmining may reduce yields or cause plant death. The most serious infestations usually appear late in the season.

Biological control, unless destroyed by treatment for other pests, is often adequate to control leafminers. Good field sanitation can also greatly reduce the numbers of this pest. Evaluate levels of parasitism before making treatment decisions. Also, avoid summer plantings, if possible, in the San Joaquin Valley where leafminers usually do not reach damaging levels until mid-July to early August.

Because of numerous parasites, leafminers are generally not serious pests, but can be sporadic in their attacks. The destruction of beneficials by frequent applications of organophosphates, carbamates, and pyrethroids applied to control other pests can result in leafminer outbreaks. Of the numerous parasites that attack the leafminer in California, the most abundant is the parasitic eulophid wasp, Solenotus intermedius, but Diglyphus spp. and Chrysocharis spp. are probably the most important for controlling leafminers.

Cutting forage crops and deep plowing after harvesting crops aid greatly in reducing leafminer numbers.

In desert production areas, exclude leafminers by applying row covers (plastic and spun-bonded materials) at planting and gradually removing them at first bloom or earlier if needed. Row covers are not recommended for the San Joaquin Valley.

• Do not remove the entire plastic row cover at one time because a drastic reduction in humidity will shock plants and can lead to collapse. Instead vent the covers and remove them gradually. Covers made of spun-bonded materials do not need venting because hot air is able to escape.
• Remove row covers if the air temperature underneath reaches 104° F before bloom.
• Remove row covers before plants grow high enough to touch hot plastic.

Plants that are not stressed for moisture can better tolerate this pest. In the San Joaquin Valley, plant in early spring and avoid summer plantings because leafminers often occur in damaging numbers beginning in mid-July.

Biological and cultural controls are acceptable to use in an organically certified crop.

Start monitoring leaves for mines during the vegetative growth stage. Early season infestations are common, but in most cases are controlled by natural enemies. However, if leafminer populations build to high levels and parasitism is low, an insecticide application may be necessary when seedlings have four to five leaves. In desert areas treatment may be required 2 to 4 weeks after planting and then repeated as needed. Use 3-by-5-inch yellow sticky cards to monitor adults moving into the fields from surrounding crops being harvested. Plastic trays can be used to monitor pupating larvae emerging from the leaves by placing the trays under the plant to catch the larvae as they drop to the ground. No economic threshold values are available yet, however.

If parasitism approaches 50% or more, the chances of the leafminer population being kept below economic levels are excellent. Once larvae have entered the leaf, they are difficult to control with insecticides.