Spinach Leaf Miner

Spinach leafminer seems to be more prominent than usual this spring in Southeastern Pennsylvania. After receiving a call, I started asking growers and many of them have seen it in fields that are not usually affected. I have seen leafminer damage in Swiss chard, beets and even arugula, in addition to spinach this year.

From a distance, damaged leaves will appear to have blisters – white squiggles on the leaves. When you take a closer look, you may be able to see the legless, yellowish-white larva burrowing between the leaf layers. When the larva is mature, it will cut a hole in the leaf, drop to the ground and pupate. Two to four weeks later it will emerge as an adult fly. The flies are small – 1/4-inch long. Apparently, they are gray with black bristles, but without a hand lens they look like any other small fly to me – perhaps a little slimmer. The adult flies lay clusters of small, white eggs on the leaves.

Usually early spring plantings escape damage. This year the soil temperatures warmed up sooner in places – possibly contributing to earlier than normal problems.

What can you do? Start with cultural controls. Once the leafminer larvae are inside the leaf they are relatively protected from pesticide applications. Crop rotation is important for leafminers. Spinach, beets and Swiss chard (and apparently arugula) are hosts. Make sure you plant in an area that has not seen these crops for two to three years. The spinach leafminer flies overwinter in or near spinach fields and emerge in April and May to lay eggs. Planting early or overwintering spinach can help you escape significant damage, especially if you will be harvesting before mid-May. Deep spring plowing can also reduce the numbers of overwintering fly pupae.

In smaller plantings, they recommend using row cover. The row cover keeps out the egg-laying adults. Install the row cover immediately when you plant, and keep it in place until harvest. One note of caution: If you place row cover over beds that have overwintering leafminers or flies enter under the row cover, you are trapping them inside. The naturally-occurring predators and parasitoids would normally be helping you keep the populations down. Instead, you have created a haven for leafminers and they will multiply!

Unfortunately, if you have significant leafminer problems now, it is hard to go back and do cultural controls. If possible, it is a good idea to pick and destroy leaves with leafmining damage before the larvae emerge and lay eggs for the next generation. On a small scale it is possible to scout for and destroy eggs. For organic growers, Entrust is the only labeled product that has shown any efficacy in research trials. It will penetrate leaves and so it has more activity. Other products, including garlic, may be effective; unfortunately research trials are lacking to back them up. I have also heard of growers using beneficial nematodes. Non-organic growers can use abamectin, cyromazine, dinotefuran, permethrin and
spinosad products.

See the 2010 Production Guide for Organic Spinach at www.nyipm.cornell.edu/organic_guide and Commercial Vegetable Production Recommendations at http://horticulture.psu.edu/node/465 for details.