Leafminers can be a disaster for your plants

If you see the damage, about all you can do is pick off the affected leaves, put them in the garbage and hope for the best because the larva is inside the leaf, and no spray will be able to reach it. Next year, try crop rotation.

At first, I thought the leaves of my beets had gotten a bit sunburned in that first blast of heat we had a few weeks ago.

But the weather cooled, and the leaves looked even worse. An initial examination did not reveal any obvious insects, holes or other eaten portions. The leaves just had large, brownish, almost transparent patches, No other vegetables growing in that part of the garden were affected, and the new, emerging beet leaves looked healthy.

The culprit turned out to be leafminers. These insects tunnel between the upper and lower leaf surfaces, feeding on the soft inner tissue. Immature stages of many other insects tunnel in leaves, too, differing by the pattern of "mine" they create. My particular invader is the spinach leafminer (Pegomya hyoscyami), a fly that feasts on spinach, chard, beets, arugula and even some weeds.

The insect overwinters in the soil as a pupa, emerging as an adult in midspring. The leafminer then lays small masses of tiny white eggs on the underside of older leaves, which is why new growth seems unaffected. When the eggs hatch, the young maggot tunnels into the leaves, feeding for two or three weeks.

When grown, they cut a little hole in the leaf, drop to the ground and again pupate in the soil. Several generations may be produced this way, but most activity ceases by midsummer, when the larvae remain in the soil until the next spring.

The probable reason I didn't see these guys upon first inspection is that they are so tiny. The grayish, adult fly is only one-fifth to one-third of an inch long, and the larva are even smaller. The eggs are almost microscopic, too.

If you see this problem now in the greens mentioned above, about all you can do is pick off the affected leaves, put them in the garbage and hope for the best. Because the larva is inside the leaf, no spray will be able to reach it. But what about next year?

It is recommended that you do some crop rotation. Do not grow plants that will be affected in that soil for two years. You can grow other plants that are not attacked by this insect. If you think your infestation may be originating in weeds bordering your garden, cover your planting with row cover (Remay) as soon as you sow your seeds. This will help keep out any adult leafminer flies looking for a place to set up housekeeping. In the case of spinach, leave the row cover on until harvest. I'm hoping my beets' small, remaining leaves will grow enough to mature.

If these insects have been a problem, you might want to consider growing your spinach in the fall, or even overwintering it, when the leafminer is not active. They usually hatch and begin their work in April or May. I had spinach in the ground early, and harvest was over by the time I noticed the problem with the beets, so the spinach was not affected.

And so much depends on the weather. Gardening is not for sissies, or for the unobservant. You have to pay attention to what is going on in your patch!

Coming up: Christie Mackison of Shooting Star Nursery will help us understand the statement, "This is not the Pacific Northwest," when she teaches about drought-tolerant plants and how to care for them. The class will be held from 7 to 9 p.m. on Thursday, July 18, at the Southern Oregon Research and Extension Center, 569 Hanley Road in Central Point. The cost is $5. Call 541-776-7371 to register.

Carol Oneal is a past president of the OSU Jackson County Master Gardeners Association. Email her at diggit1225@gmail.com.


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