Pests: Spinach leafminer (Pegomya hyoscyami), Beet leafminer (Pegomya betae)
Pest/disease identification and lifecycle, most common symptoms and crops affected:
These closely related species of leafminer attack spinach, beet, chard and some weeds, such as lambsquarters. The adult is a fly that lays its eggs on the undersides of leaves. Eggs hatch in as few as three days, depending on temperature. The tiny, pale maggots tunnel into the interior of the leaf to feed, leaving pale mines that, when numerous, run together to form necrotic, blister-like areas. The damage is cosmetic, not impacting yield of beets, but it ruins the marketability of greens such as chard or spinach. When fully grown, larvae drop out of the leaf and pupate in the soil below. Leafminers have several generations, with the overwintering generation doing so in the soil as pupae, re-emerging the following spring as adult flies.
Rotation is very important, especially when paired with the use of exclusion netting or row covers to exclude the adult fly, otherwise any overwintered pupae can emerge from the soil right where they want to be. Best practices include thorough harvesting where all leaves are removed, as well as destroying crops at the end of harvest to reduce the egg and larval population. Similarly, controlling weeds, especially lambsquarters, chickweed and plantain is important for reducing the nearby leafminer population, and subsequently the number of overwintering pupae. Deep plowing of soil harboring pupae attempting to overwinter can reduce the number of emerging flies the following spring.
Organic pesticides (as a last resort):
Because spinosad (e.g., Entrust for commercial growers, Monterey Garden Spray for gardeners) penetrates leaves to some extent, some farmers claim that it is effective against leafminers. It will be most effective if applied before eggs hatch and larvae enter the leaf.