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 MOFGA's 2013 Pest Reports - Compiled by Eric Sideman, PhD

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Pest Report - June 6, 2013
Compiled by Eric Sideman, PhD, MOFGA's Organic Crop Specialist


In this report:

Potato Leafhopper

Three-lined potato bug

Wet weather again and nitrogen

Leafhopper burn
Leafhopper nymph (circled)


POTATO LEAFHOPPERS

Potato Leafhoppers (PLH) have made it to Maine. This is the earliest I have ever seen them here. They are in big numbers in some fields, which may be very bad news. They are likely to be moving north so be on the lookout. They primarily feed on beans, potatoes, eggplants, strawberries and alfalfa.

PLH does not over winter anywhere near here. They overwinter way down south on the coast of the Gulf of Mexico, and leapfrog up here in mass migrations. Once they get here there are a few generations during the season and often become a bigger and bigger problem. The first to arrive are females, and they are usually carrying fertilized eggs when they get here. They first land in trees. They were spotted last week in apple trees in Massachusetts. This week I got two calls about them from MOFGA certified farmers in southern Maine, and so I went out to check my potatoes. Yup, there too.

The damage from leafhopper is catastrophic. The bug sucks the juice out of the plant and injects a toxin that clogs the food conducting tissue. The symptoms look like a disease after a while, rather than insect damage, and is frequently mistaken as such. The leaves first get pale, then yellow and then brown from the edges. Then the plant dies. The symptoms are called hopperburn.

The adult PLH is very light green and wedge shaped and tiny (an eighth of an inch long). The best way to find them is to brush the plant, see a bunch of these tiny things fly up, and watch one of the white-looking bugs land. The nymphs are similar to the adult, but have no wings and are even smaller. Nymphs live on the underside of the leaves. If you disturb a nymph you will see it run and it can run sideways as fast as forward. This is a clue that you have PLH and not some other less harmful species of leafhopper.

The adults are flighty. When you brush your crop you will see them fly up. If there is a cloud of them, you are in trouble. Researchers have developed a threshold before treatment is recommended. Thresholds vary but here is one that is typical: Treat potatoes if 5 adults or 15 large nymphs are found on 50 leaves.

Crop rotation does nothing for you since they are coming from far away. Covering your crops with a row cover would work, but these crops are not the type that are usually covered. Effective insecticides are limited. The only material that I have seen work that is allowed in organic production is pyrethrum, so Pyganic is the recommendation that I make. However, it does not work as well as a pyrethrum with PBO, BUT PBO is not permitted in organic production. If you market crops as organic, be sure to use an approved brand. Use the most concentrated mix allowed. Spray late in the day or evening, get good coverage including the undersides of the leaves, and don't wait until it is too late. If you wait to see damage, it is too late. Plants will not recover.

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CUCUMBER BEETLES ON MY POTATOES and TOMATILLO?

No. Those are not cucumber beetles. Potatoes, tomatoes, tomatillos, and sometimes eggplants are attacked by this pest that only superficially looks like a cucumber beetle. This is the THREE-LINED POTATO BEETLE. The adult of this pest is about the same size as a cucumber beetle but has a reddish head and a thorax with two dark spots. The wing covers are dark yellow with three black stripes. Its favorite food in my experience is tomatillo.

The Three-Lined Potato Beetle overwinters as an adult and wakes early in the spring. They are there waiting for you to plant your solanaceous crops. The females soon begin laying eggs that hatch in about two weeks to larvae that look a bit like Colorado potato beetle larvae, except these critters have the endearing practice of carrying a small pile of their own excrement on their back. The larvae mature in about two weeks. There are probably two generations per year.

On most crops the level of the pest does not warrant control. If this pest has been a problem in the past, floating row covers will help you avoid the overwintering adults and that should get you by. Hand picking will work on small plantings. Pyganic and Entrust may offer some relief. Rotenone works well, BUT REMEMBER THAT THERE ARE NO ROTENONE FORMULATIONS THAT MEET ORGANIC STANDARDS.

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WET WEATHER AGAIN and NITROGEN

Crops suffer from wet weather and I am getting calls about yellow/purple, dwarfed corn, broccoli, spinach, etc. Crops suffer for two main reasons. The first is that the roots of nearly all crops need air that they can only get from the spaces in the soil. If those spaces fill up with water there is no air. There is not much you can do about that.

The second reason is nitrogen. Nitrogen is an integral part of the protein molecule and all living things need it. Plants get their nitrogen from the soil and pick it up as dissolved ions, either the nitrate ion or the ammonium ion. Organic farmers and gardeners typically put down nitrogen fertilizer as protein in forms such as soy meal, fish meal, blood meal, livestock manure, etc. A bit of the nitrogen in manures is in the ionic form already, and if you use something like Chilean nitrate, it is all in the ionic form. To get a detailed discussion of these different forms of nitrogen, how they cycle around from one form to another, and how plants pick it up, see the MOFGA Fact Sheet called Providing Nitrogen to Crops at http://mofga.org/Portals/2/Fact Sheets/FS 08 Nitrogen web.pdf.

The important point to make is that lots of rain means loss of nitrogen. Even if you put down fertilizer, it is likely to be lost to a greater or lesser degree by lots of rain. And, lots of rain is in the forecast again. If your nitrogen is in the protein form, it is likely to be OK, but if it is in an ionic form, it is soluble and is likely to be lost. Or if your soil becomes water logged, your nitrogen fertilizer may be lost to the air as a gas. Pay attention, think about how long ago you added the fertilizer and if it has decomposed into the ionic form yet, or not. You may have to side dress your crops with more nitrogen.

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