HortiPest

Leafminer | Liriomyza sp.

Host plants and damage

Most Liromyza species are not feeding on economically important plants. However, some species attack a wide range of vegetable crops, including Allium sativum (garlic), Allium (onion), Lactuca sativa (lettuce), Phaseolus (beans), Brassica chinensis (Chinese cabbage), Pisum sativum (pea), Raphanus (radish), and Vigna uniguculata (cowpea).

Adults (for some species only the females) puncture leaves of the host plant causing wounds, which serve as sites for feeding or oviposition. Males feed on spots created by female flies. Feeding punctures are particularly along the leaf margins. However, main damage is through larval feeding. Larvae mine leaves and petioles. Different species of Liromyza produce different shaped mines. In some cases a continuous trail of frass is visible within the mine.

Photosynthesis is often reduced as chlorophyll is destroyed.

Leafminer adults may vector diseases during feeding and egg laying, but they are not inherent carriers of diseases.

Morphology & biology

Leafminer development is strongly influenced by temperature. For example, at a constant temperature of 28 °C one generation can be completed in 14–15 days. Under tropical conditions (20–30 °C) breeding and development will occur throughout the year. Because the precise biology will vary with temperature, and host, the following information gives a general picture of the biology of most pest species of Liriomyza.

Leafminer eggs are inserted by females using their ovipositor just below the leaf surface. Eggs hatch in 2–5 days. Many eggs may be laid on a single leaf.

Eggs hatch into a tiny larva (maggot), which immediately starts eating its way through the leaf. Maggots are legless with no separate head capsule. There are three larval stages. The first stage is transparent, but later stages, depending on the species, turn darker. A fourth larval instar is described for some species between puparium formation and pupation.

Leafminer larvae pupate primarily in the soil beneath the plants. Sometimes, however, the pupae stay hanging on the leaf. The reddish-brown pupa hatch after 8-12 days.

Adults have a characteristic yellow spot on the back (scutellum) but correct species identification is difficult for the amateur. An adult female lives for 1-2 weeks.

Monitoring

Leafminer feeding and oviposition punctures appear as white speckles. Look for leaf mines; they can vary in form with the host plant and the species. Larvae will be found feeding at the end of the mine, or the mine will end with a small slit in the epidermis where the larva has left the mine to pupate.

Adults are small black and yellow flies, which may be detected flying closely around host plants.

Sticky yellow traps can be used to monitor populations in the field.

Pest management

Biological control

Natural enemies, hymenopteran parasitoids like Ganaspidium utilis, are reported to be able to suppress leaf miner populations in Hawaii.

Foliar applications of the entomophagous nematode Steinernema carpocapsae significantly reduced adult development of L. trifolii in Hawaii.

In Europe the parasitic wasp Diglyphus isaea (Walker) is commercialized for glasshouse use to control Liriomyza on tomato.

The release of gamma-irradiated and hence sterile Lyriomyza males in glasshouses has been tested with considerable success.

Laboratory trials have shown that larvae of L. bryoniae can be killed with Bacillus thuringiensis.

Chemical Control

For many species resistance to synthetic insecticides out of different chemical groups have been reported (mainly organophosphates and carbamates). The application of ineffective insecticides to control leafminer is futile. It usually results in a larger leafminer problem because it reduces field densities of leafminer parasites. Many outbreaks are reported as a result of insecticide use. Therefore check the efficacy of insecticides applied and change to a different type of insecticide if necessary.