Distribution. This leaf miner is found commonly in the southern United States from Florida to California and Hawaii, and in most of Central and South America. Occasionally it is reported in more northern areas such as Ohio. Because this insect has been confused with American serpentine leafminer, Liriomyza trifolii (Burgess), and other related species, many location records are incorrect or unconfirmed.
Host Plants. Vegetable leafminer attacks a wide variety of plants, principally in the families Cucurbita-ceae, Leguminosae, and Solanaceae. Stegmaier (1966a) reported nearly 40 hosts from 10 plant families in Florida. Among the numerous weed hosts, the nightshade, Solanum americanum; and Spanishneedles, Bidens alba; were especially suitable in Florida (Schuster et al., 1991). Vegetables known as hosts include bean, eggplant, pepper, potato, squash, tomato, and watermelon. Oatman (1959) reported a similar host range in California, but also noted suitability of cucumber, beet, pea, lettuce, and many other composites. Celery is also reported to be attacked, but to a lesser extent by this leafminer species than by L. trifolii. In Hawaii, damage to onion foliage is a problem for the marketing of scallions (green onions) (Kawate and Coughlin, 1995). Vegetable leafminer was previously considered to be the most important agromyzid pest in North America (Spencer, 1981), but this distinction is now held by L. trifolii. Parrella and Kiel (1984) argued that misidenti-fication accounts for this apparent shift in pest status, and while this may be true, differential susceptibility to insecticides should not be dismissed as a significant contributing factor, or perhaps even the most important factor accounting for increased problems with L. trifolii.
Natural Enemies. Vegetable leafminer is attacked by many of parasitoids, with the relative importance of species varying geographically and temporally. Many species attacking L. trifolii apparently also attack L. sativae. In Hawaii, Chrysonotomyia punctiventris (Crawford) (Hymenoptera: Eulophidae), Halicoptera circulis (Walker) (Hymenoptera: Pteromalidae), and Ganaspi-dium hunteri (Crawford) (Hymenoptera: Eucoilidae) were considered to be important in watermelon (Johnson, 1987). In California and Florida, the same genera or species were found attacking vegetable leafminer on tomato or bean, but Opius dimidiatus (Ashmead) (Hymenoptera: Braconidae) also occurred commonly in Florida (Stegmaier, 1966a; Lema and Poe, 1979; Johnson et al., 1980a). Levels of parasitism are often proportional to leafminer density, and parasitoid effectiveness is easily disrupted by application of insecticides. Steinernematid nematodes can infect L. trifolii larvae when the nematodes are applied in aqueous suspension and the plants are held under high humidity conditions (Broadbent and Olthof, 1995); L. sativae is probably equally susceptible.
Life Cycle and Description. The developmental thresholds for eggs, larvae, and pupae are estimated at about 9-12°C. The combined development time required by the egg and larval stages is about 7-9 days at warm temperature of (25-30°C). Pupal development also requires 7-9 days at this temperature. Both egg-larval and pupal development times are prolonged to about 25 days at 15°C. At the optimal temperature of 30°C, vegetableleaf miner completes its development from the egg to adult stage in about 15 days.
The biology of L. sativae is not well documented, but important elements have been studied by Parkman et al. (1989), Petitt and Wietlisbach (1994), and Palumbo
(1995). The work by Oatman and Michelbacher (1958) probably refers to L. sativae. Keys for the identification of agromyzid leafminers can be found in Spencer and Steyskal (1986).
Foliage punctures inflicted by females during the acts of oviposition or feeding may cause a stippled appearance on foliage, but this damage is slight compared to the leaf mining activity of larvae. The irregular mine increases in width from about 0.25 mm to about 1.5 mm as the larva matures, and is virtually identical in appearance and impact with the mines of L. trifolii. Larvae often are easily visible within the mine, where they remove the mesophyll between the surfaces of the leaf. Their fecal deposits also are evident in the mines. The potential impact of the mining activity is evident from the work of Sharma et al. (1980), who studied the value of treating squash with insecticides in California. These researchers reported 30-60% yield increase when effective insecticides were applied, but as is often the case with leafminers, numerous insecticides were not effective.
Cultural Practices. There is some variation among many crops in susceptibility to leaf mining. This has been noted, for example, in cultivars of tomato, cucumber, cantaloupe, and beans (Hanna et al., 1987). However, the differences tend to be moderate, and not adequate for reliable protection. Nitrogen level and reflective mulches are sometimes said to influence leaf-miner populations, but the responses have not been consistent (Chalfant et al., 1977; Hanna et al., 1987). Placement of row covers over cantaloupe has been reported to prevent damage by leafminer (Orozco-San-tos et al., 1995). The same study evaluated the benefits of transparent polyethylene mulch, however, and found no reduction in leafminer populations. Sometimes crops are invaded when adjacent crops are especially suitable, as was reported by Sharma et al. (1980) in California, where cotton was an important source of invaders. Weeds are a source of flies (Parkman et al., 1989), but also a source of parasitoids.
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