Alfalfa Blotch Leafminer in Alfalfa
Crop Injury: Alfalfa blotch leafminer (Agromyza frontella) originated in Europe and was first detected in 1968 in the U.S. Populations are generally kept low by parasitoid natural enemies. Adult flies cause “pinholes'' in alfalfa leaves, which are tiny holes all the way through the leaf. Larva tunnel within leaf tissue, creating question mark or comma-shaped mines with patches of dark frass inside. Mines begin small and increase in size towards the leaf tip.
Pest Description: Adult flies are about ⅛ inch long, hunchbacked and black with small white structures on each side of the body. Larvae, which are located inside leaf mines, are very small and yellow.
Alfalfa blotch leafminer leaf mines
Whitney Cranshaw, Colorado State University, Bugwood.org
Alfalfa blotch leafminer and leaves with mines
Whitney Cranshaw, Colorado State University, Bugwood.org
Life Cycle: Alfalfa blotch leafminers overwinter as pupae. Adults emerge in early spring, mating and laying eggs in alfalfa fields. Three generations usually occur annually in North America, with the second generation generally causing the most crop injury.
Scouting: Look for pinholing and mines in alfalfa leaves.
Management: Natural enemies generally keep leafminer levels below economic thresholds. Because leaf mines protect larvae from contact with insecticides and adults are highly mobile, insecticide use is rarely beneficial. Insecticide application can also interfere with natural suppression of other insect pests. Thirty mines per alfalfa stem is the economic threshold for insecticide application or when 30 percent of leaves have pinholes.
Prepared and reviewed by the Alfalfa Pest Management Working Group.
Alfalfa leaves with alfalfa blotch leafminer mines. Whitney Cranshaw, Colorado State University, Bugwood.org
Alfalfa leaves with alfalfa blotch leafminer mines. Whitney Cranshaw, Colorado State University, Bugwood.org
Alfalfa leaves with alfalfa blotch leafminer mines. Whitney Cranshaw, Colorado State University, Bugwood.org
Alfalfa blotch leafminer and leaf mines. Whitney Cranshaw, Colorado State University, Bugwood.org
Gallery images courtesy Whitney Cranshaw, Colorado State University, Bugwood.org.
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