Beet leaf miners

Asked July 2, 2020, 1:31 PM EDT

Hello, I have a raised beet which I planted beets and chard for summer harvest. Both the beets and chard had large brown areas on their leaves that make online photos of beet leaf miner damage. I tried to combat it with neem oil but it hasn’t worked. My question is: I’d like a fall crop of beets and chard. The Summer beets have been harvested and the chard is producing fairly well despite the leaf miner damage. Should I pull out the chard and start a new for the fall crop, using a covering over the crop to keep flies (what I think I read causes the miners) off, or are these leaf minor in my soil? Will they die off with cold weather? Thanks so much! Emily

Lane County Oregon

1 Response

The spinach family leaf miner lays tiny white eggs on the underside of the leaf and as soon as they hatch the fly maggot enters the leaf between its layers and makes the noted damage. Neem oil will not affect the maggot inside the leaf nor any other pesticide we can use. The best bet is to plant in a different area and cover the crops as soon as planted with a row cover, securely fastened down. When the maggots mature, they drop from the leaf and complete their cycle in the soil, so putting them in the same bed again is not advised.
This is the profile of your pest from OSU

Includes spinach leafminer (Pegomya hyoscyami)

Pest description and crop damage Adults are small black to gray flies with yellow markings. The body is covered with long stiff bristles. Larvae are a nearly translucent white or yellow color and about 0.25 inch long when mature. Eggs are white, cylindrical, and laid singly or in small groups. Both larvae and adults damage plants. Larval feeding results in slender, winding trails on the leaves, which form large white blotches if mining becomes severe. Adults can make as many as 100 feeding punctures on a single leaf. Around 5% of these punctures may contain actively feeding larvae. Excessive mining renders leaves unmarketable, reduces photosynthetic capacity, and provides easy access for disease organisms.

Biology and life history Leafminers overwinter as pupae in the soil. Adults emerge in late May, mate, and females lay eggs on the undersides of beet and chard leaves or on lambsquarters. The eggs hatch in about 4 days, and the small maggots eat into the leaf. There may be several maggots in a leaf. When mature, larvae fall to the ground and pupate just under the soil surface. Adults emerge in 10 to 25 days and begin laying eggs for another generation. There are at least three generations each year. Natural enemies, especially parasitic wasps, commonly reduce populations of leafminers, unless they are killed by insecticides applied to control other pests. To avoid killing beneficials, choose selective pesticides for treating other pests, whenever possible. Other parasites attack leafminers, but because leafminers feed within the leaf, they generally are protected from most predators. Row covers work well in excluding egg-laying female flies.

Home gardeners: Remove and destroy affected leaves.