Figure 2. Pupae in base of allium plants. They have been in the pupal stage throughout the summer, and should emerge as adults this fall. Photo: L. Donoval.
They’ve been in the pupal stage all through the summer, and will re-emerge as adults in early fall. Last year, emergence started around the last week of September, and continued well into November. The life cycle is shown in Figure 1.
Figure 1: Allium leafminer life cycle. Adults are due to emerge in September. (Schematic: C. Sanden)
The allium leafminer infests plants in the Allium genus, including leeks, onion, garlic, chive, shallot , and green onion. We’ve also found feeding signs on wild garlic, garlic chives and an ornamental– Allium ‘Globemaster’. From past experience, we expect leeks to be especially at risk.
This pest has been expanding its geographic range, and is now in at least 27 counties in Pennsylvania (Figure 3), and in several neighboring states.
Fig. 3. Counties known to have allium leafminer. Counties shaded yellow have records confirmed by PDA. Counties in green are based on records confirmed by Extension. This pest is also now confirmed in several neighboring states.
Initial signs of this pest are a linear series of puncture marks made by the female (Figure 4), and infested plants sometimes exhibit a distorted, wavy growth pattern.
Figure 4. Female puncturing leaf with ovipositor. Note linear puncture marks, which is a sign of adult activity. Photo: T. Elkner.
The adult fly (Figures 5) has a charismatic orange face, yellow “knees” (end of femurs), and matte black body. At 3mm-4mm, the adult is small (larger than a fruit fly but much smaller than a house fly). Eggs are laid in or on plant tissue, and larvae are well embedded into the plant tissue.
Figure 5. Adult. Top view. Note yellow patch on head, and yellow ‘knees’. Photo: A. Megroz.
Row covers during the flight period should prevent damage. Also, insecticides labelled for leafminers in various allium crops are available. Check labels to ensure the crop is listed, and for rates and days-to-harvest intervals.