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Pest Status
Background
Chapter 4
Agromyza frontella (Rondani), Alfalfa
Blotch Leafminer (Diptera:
Agromyzidae)
J.C. GUPPY, D.G. HARCOURT, M.O'c. GUIBORD and
L.S. THOMPSON
The alfalfa blotch leafminer, Agromyza frontella (Rondani), was introduced from
Europe to the north eastern United States during the late 1960s (Miller & Jensen 1970)
and first appeared in Canada at St-Armand, Quebec, in 1972 near the United States
border (Harcourt 1973). The insect spread rapidly throughout eastern Canada from
Prince Edward Island to Ontario (Thompson 1974, Guppy & Harcourt 1977), and by
1979 it had threatened to become the most important pest of alfalfa in the 10 eastern
counties of Ontario (Harcourt & Binns 1980) and in the other eastern provinces. It has
continued to spread westward in Ontario and damaging levels were recorded in
Hastings County in 1980 (Guppy 1981).
Hosts include: Medicago, Melilollls, and Trifolium, but alfalfa, Medicago sativa L.,
is apparently preferred (Steyskal 1972, Spencer 1973). The insect overwinters as a
partially developed pupa and the adult emerges about mid-May to oviposit in the firstgrowth
alfalfa. In eastern Canada there are three complete generations a year (Guppy
1981), but a few individuals may complete a fourth. The crop is attacked by both the
adults and larvae, but most of the damage is done by the larvae which feed on the
mesophyll tissues forming blotch mines each of which represents about 27% of the
leaflet area.
The crop is attacked throughout the growing season but damage is more severe in
the first and second cuttings; during the past 4 years, the percentage of leaflets mined
in eastern Ontario averaged 70% and 58%, respectively.
Seven parasitoid species native.to eastern Canada have been reared from larvae of A.
frontella, all of them are hymenopterous species of the family Eulophidae. Diglyphus
websteri (Crawford) and D. begini (Ashmead) were recorded in eastern Ontario and
these two plus D. pulchripes (Crawford) in Quebec. In Prince Edward Island, five
species were found, namely, D. begini, D. intermedius (Girault), Notanisomorpha sp.,
Pnigalio maculipes (Crawford), and Pnigalio n. sp. One larval-pupal parasitoid
Cyrtogaster sp. (Hymentoptera: Pteromalidae) emerged from puparia in Ontario.
Assessment of parasitism has shown that the native species are of little importance
in suppressing alfalfa blotch leafminer populations. However, a hemipteron species
Nabis ferus (Linnaeus), common to eastern Canada, appears to be an important
predator of the larvae. At Ottawa, incidence of attack increases with each generation,
averaging about 60% in the third.
The Beneficial Insects Research Laboratory, Newark, Delaware, United States
Department of Agriculture imported 14 parasitoid species from Europe from 1974 to
1978 for release in the United States (Hendrickson & Barth 1979). By 1978 three
hymenopterous species, Dacnusa dryas (Nixon) (Braconidae), Chrysocharis punctifacies
Delucchi (Eulophidae), and Miscogaster hortensis Walker (Pteromalidae), were
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