Distribution. There long has been confusion about whether beet leafminer and spinach leafminer are separate species. Now they are considered distinct species, but they are very similar in appearance and biology. Both leafminer species are found in Europe, northern Africa, Asia, and North America. In the United States they apparently occur widely, though absent from the southern and southwestern states. They also occur throughout southern Canada. Spinach leafminer predominates in eastern North America and beet leafminer in the west. These leafminers were accidentally introduced to North America from Europe, probably in the 1800s.
Host Plants. This species is best known for its damage to members of the plant family Chenopodia-ceae, but spinach leafminer also is reported to attack a few plants in the family Solanaceae, Carophyllaceae, and perhaps others. The principal crop hosts of both leafminers are beet, spinach, sugarbeet, and Swiss chard. Alternate hosts are poorly documented, but lambsquarters, Chenopodium album; and redroot pigweed, Amaranthus retroflexus; are cited as hosts. Spinach leafminer also attacks black henbane, Hyoscyamus niger; deadly nightshade, Atropa belladonna; Datura sp.; and Dianthus sp. In studies conducted in France, beet leafminer favored Beta vulgaris for oviposition, and larvae developed only on this species, whereas spinach leafminer oviposited and survived on Chenopo-dium and Datura (D'Aguilar and Missonnier, 1957).
Natural Enemies. Several parasitoids of spinach leafminer are known from North America, including Biosteres anthomyiae (Ashmead), Opius fulvicolis Thomson, O. middlekauffi Fischer, and O. nitidulator (Nees) (all Hymenoptera: Braconidae). In addition, Biosteres spinaciae (Thomson) (Braconidae) has been reared from both leafminer species. Predatory bugs such as Nabis (Hemiptera: Nabidae) feed on larvae by puncturing the leaf epidermis to reach the insect beneath.
Life Cycle and Description. A generation requires about 30-40 days for its completion. The number of generations is reported to be 2-4 annually. Overwintering occurs in the pupal stage and commences beginning in August. Adults of the overwintering generation emerge in April or May. (The spinach leafminer larva in color figure 177 is referred to on page 209.)
Egg. The eggs are elongate-oval and measure about 0.87mm long and 0.31 mm wide. Delicate hexagonal sculpturing is present on the surface of the eggs. The eggs are white. The eggs are glued to the lower surface of leaves, often in clusters of 2-5 but sometimes in larger groups. The eggs are positioned adjacent and parallel to one another. Females have been reported to deposit up to 70 eggs, but this has been poorly studied so the true fecundity remains unknown. Eggs hatch in 3-6 days, and larvae immediately burrow into the leaf. (See color figure 263.)
Because the identity of these leafminers has been confused, there is some uncertainty about leafminer biology. Cameron (1914) and Frost (1924) provided detailed biology of spinach leafminer. Characters to
Beet leafminer puparium.
distinguish beet and spinach leafminer were given by D'Aguilar and Missonnier (1957), Chillcott (1959), and Michelsen (1980). A procedure for culture of beet leafminer was described by Rottger (1979).
The larvae feed between the lower and upper epidermis of the leaf. The form of the mine initially is long and narrow, but soon turns into an irregular blotch. Old mines become dry and brittle. Such mining is rarely of consequence to growth of beet roots, but if beet tops are desired or if leafy crops such as Swiss chard or spinach are grown, the damage can be considerable. These leafminers are mostly known as garden pests, but commercial-scale fields of spinach are sometimes damaged.
The eggs are easily observed by examining the lower surface of leaves. Damage can be prevented by application of insecticides to foliage or to soil; systemic insecticides are particularly effective, but care must be taken to avoid residues on leafy vegetables. Residual soil applications are effective because larvae generally drop to the soil to pupate, thereby contacting the insecticide. This approach requires that the earliest foliage, which supports leafminer development, be discarded. An effective alternative for small-scale production is to cover the plants with screen or floating row cover material; this prevents flies from ovipositing unless they emerge from the soil beneath the cover. Crops
grown early and late in the season often escape the principal flights of flies, thereby escaping injury.
Was this article helpful?