The beet leaf miner (Pegomyia betae) is the larva of a small, rarely seen fly, as is its lookalike cousin, the spinach leaf miner (Pegomyia hyoscyami)*. But the damage caused by leaf miners is highly visible: translucent tunnels and patches on beet, Swiss chard and spinach leaves as well as those of other plants of the Amaranthaceae family (formerly the Chenopodiaceae family), like the common weeds lamb’s-quarters (Chenopodium album) and redroot pigweed (Amaranthus retroflexus).
*Since the beet leaf miner and the spinach leaf miner are practically indistinguishable and have the name life cycle, host plants and treatments, I’ll simply refer to beet leaf miner throughout this article. Who knows which one you really have?
Beet leaf miners are often fairly discreet, causing damage to a leaf or two here and there and not really affecting the harvest to any great degree. But then, of course, there are years when the population seems to explode and almost every plant over half a continent is affected. And not just a leaf or two per plant, but most leaves, often with several leaf miners working each leaf. In such bad years, beet leaf miners can completely obliterate the harvest.
Even in the worst years, though, you’ll notice that the insect has less impact on beets (beetroots) than on Swiss chard and spinach. That’s because they don’t attack the beet’s root (the part usually harvested), but only the leaves. Since the plant usually has enough green tissue to carry on photosynthesis in spite of their presence, you may still end up with nice, sweet beet roots. With plants whose leaves are harvested, though, like Swiss chard and spinach, there is often little left to harvest.
Beet leaf miner is an easy pest to recognize. The leaves of the infested plant become marked first with translucent tunnels, then these grow to become blotches. The upper leaf surface takes on the texture of parchment while the lower one eventually turns brown. Further inspection will reveal one or more small whitish larvae (maggots) inside the leaf. The blotches are caused by the larvae as they eat the cells that make up the inside of the leaf, leaving only the upper and lower epidermis intact.
The female beet leaf miner hatches from a pupa that overwintered underground. Depending on your climate, she’ll likely be active in late April or May, earlier in mild climates. She lays eggs on the underside of the leaf of the chosen host plant. After about five days, the tiny larvae emerge from the eggs and either settle on the leaves of that plant or on neighboring plants. They pierce a small hole under the leaf and crawl inside. Then they start to feed.
At first, they form tunnels, then enlarge those into larger and larger blotches, especially when the tunnels of several larvae merge as is often the case. The blotches can even come to nearly cover the leaf. After two or three weeks of feeding, the larvae exit the leaf, drop to the ground at the foot of the affected plant and form a small brown pupa underground. After 3 to 5 weeks, adults emerge from the pupae and the cycle begins again.
Each generation lasts about 30 to 40 days and there are usually three generations per year (more in climates with very long growing seasons), the first being generally by far the most damaging. The last generation remains in their pupa all winter … and the cycle starts again the following spring.
This insect has many natural predators, including parasitic wasps, most of which are too small to be readily visible, but they are behind the sudden population crash that inevitably follows a banner beet leaf miner year.
How to Prevent Damage
As soon as your beets, Swiss chard and spinach have 4 to 6 true leaves, start checking the underside of the leaves for eggs. They are tiny, oblong, about 0.7 mm in length, white and found in clusters of 3 to 12. Just knock them into a cup of soapy water to remove them. Or squash them.
Placing sticky yellow traps near plants can also be useful if you put them out at the 4 to 6 leaf stage. They are only effective on the flies, of course, not the larvae. Note that these traps attract pest insects, not beneficial ones.
Another possibility is to exclude them from your plantings. For this to work, you need to sow beets, Swiss chard and spinach in a location where there were none the previous year and cover the spot, after sowing, with floating row cover to prevent the female flies from even approaching them. If you don’t rotate your crop, this method won’t work, though, since the pupae overwinter where the host plants grew the year before and will be trapped under the row cover when they hatch, able to feed at will.
Or try simply delaying sowing beets and their relatives. If you sow at the end of May or in June rather than early in the season, the first generation of female flies—the generation that does the most damage!—will have gone elsewhere. Any damage due to subsequent generations is not likely to be of much concern.
Too Late, The Miners Are Already at Work
It’s too bad that no one warns gardeners when a bad year for leaf miners is expected. If so, we could do something to prevent them. Most of us, however, just react when we see the damage, so…
If there just a few leaf tunnels here and there, simply cut off the affected leaves and you’ll nip the infestation in the bud. Or squish the larvae inside the leaf: there’s a special satisfaction in that!
If most of the leaves are affected, though, the easiest solution is to pull and destroy the plants. (If you do so, remember you can harvest and use any intact leaves.) Then resow. There is almost always plenty of time for this, as spinach, beets and Swiss chard are all fairly fast growing.
In general, there is little use spraying conventional insecticides once damaged leaves are visible, because the larvae will already be safe inside the leaf where most pesticides can’t reach them. However, if you notice small, grayish flies near your young plants, you might be able to kill a few with insecticidal soap or neem spray.
Theoretically, you could use systemic insecticides—ones that penetrate plant tissues and are carried throughout the plant by the plant’s sap—to make the leaf toxic to leaf miners … but remember you and your family will be eating these plants later. Do you really want to make vegetable plants poisonous? In any case, systemic insecticides are no longer available to home gardeners in most countries.
Beet leaf miners: the damage they cause can sometimes seem pretty discouraging, but most gardeners quickly learn to live with them.