Agromyza frontella (Rondani, 1875)
Wing length: 1.6 - 2.2 mm.
Similar to the European species Agromyza nana and Agromyza drepanura. Readily separable from nana by the more blackish mesonotum and abdomen. Leaf mine initially linear along the margin of the leaflet (Spencer, 1973).
The scales of stridulatory organ not gradually enlarging (von Tschirnhaus, 1972).
Differences to drepanura
Two reclinate upper frontorbital bristles.
Male cheeks (genae immediately below the lower margin of the eyes) less broad.
Surstyli with a small number (4-6) of widely spaced spines (Zlobin, 2000).
North American specimens
Presence of strong presutural dc; termination of the costa before vein M 1+2 (ending midway between veins R4+5 and M2+3); pubescent third antennal segment; ochrous to dark squamal fringe (Spencer, 1973).
Aedeagus rather hyaline at apex. Two terminal tubules are visible (Ag frontella aedeagus.pct). The apical structures were apparently disregarded by Spencer, 1973.
Resembles Agromyza nana and Agromyza vicifoliae Hering (Spencer, 1973).
Larval mandibles of almost identical size, each with two hooks. Anterior spiracles with 8-12 posterior spiracles with 3 bulbs each. The larvae consume the whole mesophyll within the mine. (See also the general description of the Genus Agromyza). Puparium is colored light brown in summer and dark brown in winter.
The leaf mine is restricted to one leaflet within the trifoliate alfalfa-leaves. It is initially narrow and linear, following the margin of the leaf towards the apex, then turning back and developing into a blotch in the area of the mid-rib. The larva produces a full-depth-mine (Helgesen and Baxendale, 1979).
After its accidental introduction to North America (overview: Venette et al., 1999), Agromyza frontella was subject of a remarkably high numbers of ecological and physiological studies, especially between 1975 and 1990. Thus, Agromyza frontella together with Liriomyza trifolii can be considered as the best studied agromyzid fly worldwide. Only a selection of literature could be cited here.
The females lay 25-294 eggs (average 141) (Hendrickson and Barth, 1978). For oviposition the upper leaves are generally preferred (Quiring and McNeil, 1987). It may be noteworthy that virgin females attract males by a sex pheromone (Carriere and McNeil, 1988; Carriere and McNeil, 1990).
Egg and larval development depends strongly on the temperature, the length of the egg stage varies between 18 days (at 10°C) and 3 days (at 25°C). The duration of the larval development varies between about 6 and 17 days (Guppy, 1981). The same author observed that the larvae did not survive at 30°C.
For pupariation the larvae drop from the host plant after leaving the mine and pupates in the soil, 2.5 - 15 cm below ground level. In North America there are 2-4 generations per season (Venette et al., 1999). If not interrupted by hibernation, the pupation period takes about 16-57 days (Guppy, 1981).
Hendrickson and Barth, 1978 reported average development times (including egg and puparial stage) for different temperatures: 16,7°C: 52 days; 20°C: 36 days; 22,8°C: 31 days; 25.6°C: 27 days.
Additional detailed studies on the life cycle of the alfalfa blotch leafminer were undertaken by Mellors and Helgesen, 1978 and Mellors and Helgesen, 1982). Nechols et al., 1983 investigated the parameters governing the diapause. They showed that the emergence in spring can be predicted by measuring the soil temperature in autumn and spring.
Since one larva is restricted to a single rather small leaflet of the host plant, competition between larvae can occur if more than one individual feed on the same leaflet. In such cases cannibalism and aggressive behaviour of larvae was observed (Quiring and McNeil, 1985). The same workers (McNeil and Quiring, 1983; Quiring and McNeil, 1984 a, b) suggested an oviposition-deterring pheromone (ODP) secreted by the female after oviposition. However, the ODP can reduce the oviposition of more than one egg into a leaflet but cannot prevent oviposition completely.
Medicago sativa L. (lucerne, alfalfa); also Medicago falcata L., Medicago lupulina L. and Melilotus spp.
It could be shown that alfalfa is more suitable than hop medic (Medicago lupulina) but the latter plant can serve as alternate host. It is suspected that the in the USA widespread Medicago lupulina can help to maintain a high population of Agromyza frontella (Vegiard et al., 1985).
Widespread in Europe, also known from Afghanistan and Israel (Zlobin, 2000) introduced into North America in 1969 (Miller and Jensen, 1970). It was first detected in Massachusetts and quickly spread throughout the northeastern USA and eastern Canada. The spread is continuing, the alfalfa blotch miner is meanwhile also present in Wisconsin, Illinois and North Dakota (Venette et al., 1999).
The economical important host plant of Agromyza frontella is Medicago sativa (lucerne, alfalfa), which is widely used as fodder plant. Therefore the fly is often called Alfalfa blotch leafminer (abbreviated ABL). By feeding, the larva usually causes the abscission of the whole leaflet unless only a part of it was actually consumed (Hendrickson and Barth, 1978). If the plants were strongly infested the alfalfa yield and the protein content can be reduced significantly (Venette et al., 1999).
In Europe recognizable damage is rare but in some years this species can be of some abundance (Spencer, 1973, Drea et al., 1982). In the USA and Canada, where the species were accidentally introduced, damage on alfalfa can be more serious.
Especially immediately after introduction the infestation was high but soon after establishment the populations decreased to a nearly sub-economical level (Harcourt et al., 1987). This is partly due to the successful establishment of several European parasitoids, of these especially the eulophid wasp Dacnusa dryas. (Harcourt et al., 1988, Letendre et al., 1991). On the other hand there are also some indigenous predators and parasitoids that cause some mortality in the alfalfa blotch leafminer (Harcourt et al., 1987; Hendrickson and Barth, 1979). However, the economic importance of Agromyza frontella is still a matter of debate. Hendrickson and Day, 1986 reported "highly significant" loss. Certainly, there are many local and temporal differences in infestation of alfalfa.
Even if there is some frontella infestation, some authors do not recommend insecticide applications because the treatments did not result in significant yield increases, even though highly significant differences in agromyzid densities were evident (Thompson, 1981, Daley and McNeil, 1987).