Garden Plots

Beets and chard clobbered by leaf miners

Every year, insects surprise us. Sometimes, it’s by their absence. So far, there have been few stink bugs and yellowjackets.

But this summer, chard, spinach and beet greens were attacked by an explosion of the beet/spinach/chard leaf miner. Why so many this year? We really don’t know, but the general consensus is that the mild winter may have played a role in getting them off to an early start.

Leaf mining starts with small gray-to-black flies with yellow stripes. You probably will never see them. The adult flies mate and the female deposits her three or four hexagonal eggs on the undersides of nice leaves. Sometimes, she sucks sugar from the leaf herself.

Eggs hatch in three to 10 days, and the little legless maggots burrow into the leaf and then proceed to mine out all the juicy bits between the upper and lower leaf surface.

What they leave, besides some frass (aka poop), are sizeable papery leaf epidermis blotches. These blotches can get quite large when several maggots sometimes join in a feeding frenzy.

From a farmer’s standpoint, leaf miner-damaged leaves are unsaleable and can represent quite a loss. (An intrepid home gardener will eat the undamaged portions of the leaf with no concerns.)

On sugar beets, the maggots reduce the amount of sugar produced.

After feeding, the maggots pupate. Sometimes they stay within the leaf, comforted by its protection. But generally, they drop to the ground and seek out a 2-inch or so crack in the soil to hide them while they undergo their conversion to a fly. That takes 10 to 25 days.

There can be three to four generations per summer. The last set of maggots winters over in the soil and emerges in late April to early May. Usually, damage is worse on your first crop, since they are often attacked just when they are really starting to grow.

So what can a gardener do?

Since the maggots often overwinter in the soil below their feeding site, rotate spinach, beets and chard each year. Protecting the crop with row covers from start to finish works great IF you rotate your plots. Otherwise, you trap the soil-emerging flies inside the cover.

Insecticides don’t really work for home gardeners, since the proper timing is hard to determine. Other methods that may help include squishing the tiny maggots in small blotches when you first see them. Alternatively, remove and destroy large, infested leaves.

Sometimes, tiny beneficial wasps will attack the maggots, but they are fairly protected inside the leaf. I have seen yellowjackets trying to get to active leaf-feeding maggots to eat them.

Chip Bubl works at the OSU Extension/Columbia County. Call 503 397-3462, or email chip.bulb@oregonstate.edu.  

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