Hexomyza simplex (Loew, 1869)
The only agromyzid mining in asparagus roots. Body black, shining with fine pubescence. Wing length: 2.2 - 3 mm. The male genitalia are rather characteristic compared to related species of the related genera Hexomyza, Ophiomyia and Melanagromyza.
H. simplex is not the only fly whose larvae feed on asparagus stems. The Tephritidae species Plioreocepta poeciloptera (Schrank, 1776) is another pest species. Tephritidae do not match to the general description of agromyzid larvae and adults given in the 'higher taxa' and 'morphology' modules. The tephritid larvae normally damage the asparagus stems and roots more severely than Hexomyza because they are not restricted to the stem surface.
BIONOMICS AND HOST PLANT
The larvae feed exclusively superficially on asparagus stems and roots. The eggs are laid on the base of the stalk near the surface or the soil (Fink, 1913). After hatching, the larva usually mines about 30 cm upwards into the stem, turn and continues mining downwards. Pupation occurs just below the epidermis either above or below ground level.
In Western Europe adult flies normally emerge from early June to mid-July. In the USA emergence is from mid-May and in California and southern states certainly considerably earlier (Spencer, 1973). There are two or sometimes maybe three generations per year. Bakel and Bethe, 1974 in the Netherlands observed that the second generation is much less abundant than the first. Probably many of the first generation puparia enter diapause and emerge not until the subsequent year.
DISTRIBUTION
Widespread in Western Europe and North America. Probably, Hexomyza simplex occurs in any area, where asparagus is cultivated. Although the species originally has been described from the USA, it is very probable that simplex was introduced from Europe together with it's only host plant asparagus. Asparagus officinalis is native to Europe and Asia.
ECONOMIC IMPORTANCE
Although the abundance of the fly on asparagus cultivars is not always high, simplex should be considered as a serious pest because the mines facilitate the infection of the plant with fungi (Fusarium oxysporum, F. moniliforme = Gibberella fujikuroi) causing root and crown rot (Bakel and Bethe, 1974, Gilbertson et al., 1985). Through epidermal splits produced by feeding mining flies, the fungus can easily invade into the plant. Usually pesticides have to be applied to control both fungi and the miner flies (Damicone et al., 1987).