May 29, 2015
Leaf miners seem to be especially noticeable this spring

Leaf miners seem to be especially noticeable this spring in both conventional and organic fields. If you are seeing white meandering tunnels in your beet, spinach and chard leaves you are not familiar with, read on for management tips.

Larvae mine their way through leaves creating blisters that often look like meandering tunnels. In spinach and chard affected leaves are not marketable. Affected beets may not be marketable with tops, but damage is rarely high enough to defoliate to the point of effecting sizing up of beets.

Leaf miner damage is caused by the legless yellow to white larvae which burrow between the layers of the leaves as it feeds. The mature larva cuts a hole in the leaf and drops to the ground to pupate. It emerges 2-4 weeks later as a fly. This fly lays small white eggs, generally on the underside of the leaf. Eggs hatch in 3-6 days. Often there are several larvae within each mine. In Pennsylvania there are several generations per year: taking about 30-40 days per generation Leaf miners over-winter as pupae in the soil or plant debris and emerge as adult flies in the spring.

Either the spinach leafminer or the beet leafminer cause this damage, and both have similar lifecycles. Both were introduced from Europe, probably in the 1800’s. Many weeds serve as hosts, including lambsquarter, pigweed, henbane, and nightshade.

Early detection is important. Check young seedlings weekly for mining on the cotyledons and first true leaves. Look for clusters of small white eggs, mines and hatching larvae. Examine ten plants in ten locations. Be sure to examine the undersides of leaves.

Penn State University Extension

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