New Pest - Allium Leafminer
Feeding marks from a scallion in my garden - ruler for scale.
There is a nasty new pest wreaking havoc in Pennsylvania. Imported from Europe and first spotted in the state in 2015, the Allium Leafminer is attacking onions, garlic, leeks and ornamental Alliums.
This destructive insect is a tiny (1/8 inch) fly with an orange patch on the head. Usually the first damage noticed is a line of tiny dots down the length of the foliage caused by feeding and egg laying. The eggs hatch into larvae that travel down inside the leaf blade, eating their way into the bulb of the plant.
Plants will be distorted, weakened and prone to rotting. There are 2 generations per year – early spring and fall. This serious pest has destroyed entire crops of onions at Pennsylvania farms. Just for perspective – I noticed this pest on my chives and leeks this fall and have had two nursery customers mention that they have it as well. It is here in Delaware County.
For food crops, prevention is usually the best approach. Allium crops can be netted to exclude the flies and prevent egg laying in late winter/very early spring (March - or when you sow or plant) for about 8 weeks and again in fall, around September. Crop rotation is important. If Alliums were grown previously in a location, pupae of leaf miners that wintered over can be trapped under the netting. Any infested plants should be discarded, not composted.
Another approach may be to time your plantings to avoid the egg laying periods of the females. So planting garlic well into November instead of October for instance.
Netting is not really feasible for ornamental Alliums. One option is to apply spinosad (an organic insecticide) after white feeding and egg laying dots appear. Apply it again in two weeks. Allium leaves are waxy and repell water so its helpful to add a surfactant (like insecticidal soap) to help the insecticide stick to the leaf. Since you are not spraying any blossoms, impact on pollinators should be avoidable.
Want more information? Here are some links:
Penn State: Allium Leaf Miner and growing Garlic
University of Maryland Extension: Allium Leaf Miner Resource
Operation “save the Alliums!”