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 MOFGA's 2010 Pest Reports - Compiled by Eric Sideman, PhD Minimize

September 22 | August 23 | August 9 | July 26 | July 20 - Late Blight Update | July 19 - Late Blight Is Here | July 7 | June 24 | June 19 - Late Blight Update | June 18 | June 8 | May 25 | May 20 | May 3 | April 22
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Pest Report - June 24, 2010

LEAFHOPPERS: Potato Leafhoppers (PLH) have made it to Maine. 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 do not overwinter anywhere near here. They overwinter way down south and leapfrog up here in mass migrations. The first to arrive are females, and they are usually carrying fertilized eggs when they get here. Then there are a few generations over the growing season before the cold north winterkills them.

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 hopper burn.

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 and watch one of the white-looking bugs land. The nymphs are similar to the adult, but have no wings and are even smaller and 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 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 larvae 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 is 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.

COLORADO POTATO BEETLE: Colorado potato beetle (CPB) adults are showing up in potato and eggplant crops. They have overwintered as hibernating adults and awoke in the spring to look for potatoes or eggplants, and lay eggs. The bright yellow eggs are laid in clumps with about 30-35 eggs each, generally on the undersides of leaves. As with most other insects and plants, there is a direct relationship between higher temperatures (in the range between about 55 and 90 degrees F) and faster rate of development. That includes egg laying, egg hatch, larval growth, and feeding rates. A period of cold, rainy weather slows everything down, but we can expect a surge of adults and shiny yellow eggs to appear with the next hot spell.

Scouting and Thresholds: Walk your fields NOW and look for CPB adults and eggs. A treatment should be considered for adults when you find 25 beetles per 50 plants and defoliation has reached the 10% level. The spray threshold for small larvae is 4 per plant; for large larvae, 1.5 per plant (or per stalk in midseason), based on a count of 50 plants or stalks.

Controls are needed on eggplants when there are 2 small or 1 large larva per plant (if plants are less than six inches) and 4 small larvae or 2 large per plant (if plants are more than six inches).

Potatoes can tolerate 20% defoliation without reduction in yield (or even more, late in the season). Damage to eggplant seedlings from adult feeding is often severe enough to warrant control of the adults. In potato, adult damage in rotated fields is usually not significant, so you may be able to wait until after egg hatch to kill both adults and larvae.

Look on the undersides of leaves for the orange-yellow egg masses. The fresher the eggs, the brighter orange the eggs will appear. Eggs hatch in 7-10 days, depending on temperature. If you want to know when the earliest eggs are hatching, you can flag a few of the earliest egg masses you find with bright tape or flags, and then keep an eye on the hatch.

Hatched larvae go through four stages before they become adults. In the first stage, the larvae are about the same size as the eggs and in the second stage they are about an eighth of an inch long. As the larvae get bigger, they do more feeding. The fourth, or largest, stage does 85% of the feeding damage. It’s a good idea to prevent beetles from ever reaching the fourth instar!

After larvae complete their growth, they drop to the ground and burrow into the ground to pupate. About ten days later the next generation of adults emerges – ready to feed. If they emerge before August 1, they will lay more eggs. After August 1, they feed and head to overwintering sites. Good control in June prevents problems with CPB in August.

Cultural Controls

Crop Rotation. The single most important tactic for CPB management is to rotate potatoes or eggplant to a field that is at least 200 yards from the previous year’s fields. Since the adult that comes out of winter cannot fly well, barriers such as roads, rivers, woodlands, and fields with other crops are helpful. Rotated fields tend to be colonized 1-4 weeks later in the season. Also, the total population of adult beetles is lower, producing fewer larvae to control.
Perimeter treatments or perimeter trap cropping can be applied to potato. One approach is to plant a barrier crop between overwintering sites and this year’s crop and get it in earlier than the main crop; then control early arriving beetles.

Early planting. Green sprouting, also know as chitting, prepares whole seed potatoes to emerge rapidly, gaining about 7-10 days to harvest. This early start makes it easier for the crop to put on growth and size before CPB adults and larvae arrive. It can be combined with raised beds and plasticulture. While it won’t avoid damage altogether, it may reduce the need for insecticides. Refer to the New England Vegetable Management Guide (online at www.nevegetable.org) for more details.

Late planting. Another strategy for beating the beetle is to plant late. CPB adults that do not find food leave the field in search of greener pastures. Planting after mid- June, using a short season variety, often avoids CPB damage and eliminates the need for controls.

Straw mulch. It has been well documented that when potato or eggplants are mulched with straw, fewer Colorado potato beetle adults will settle on the plants and fewer eggs will be laid. This can be accomplished on larger plantings by strip planting in rye mulch, followed by mowing and pushing the rye straw over the plants after they emerge. For smaller plots, straw may be carried in.

Biological Control. There are numerous predators and parasitoids that attack CPB adults (a tachinid fly), larvae (12-spotted ladybeetle, spined soldier bug, ground beetles), and eggs.

Organic controls. Spinosad (Entrust) and azadiractin (AZA-Direct) are two options. Recent studies have shown very good results with spinosad, but please use reluctantly because resistance will build up in populations of CPB. The azadiractin has shown some efficacy, but neem works slowly. Generally it is used to reduce overall damage and reduce numbers but it is not a rescue treatment like spinosad is. NOTE: There is no longer a registered product for agricultural use in organic crops in the US that contains Bt tenebrionis as the active ingredient. Beauvaria bassiana (Mycotrol O) has been shown to suppress CPB populations though does not provide immediate control. And, using Beauvaria bassiana does not jive well with using crop rotation because you need the overwintered adult that remain infected with the fungus to eliminate buying the product new every year.

(Modified from the U Mass Vegetable Notes, an article by - R Hazzard; (sources include: D Ferro (UMass Amherst), J. Mishanec (NYS), J Boucher (CT), J. Whalen (DE), T. Kuhar (VA), G Ghidhu (NJ), New England Vegetable Management Guide, Ohio Vegetable Production Guide)

Blackleg photo by David Buchanan.
(not late blight): I am getting reports of Blackleg and of course the farmers are worried it is late blight. At this time there are not any reported late blight infections in northern New England.

Blackleg typically causes the stem of the potato to turn inky black from the potato seed piece up the stem. The leaves may then turn yellow and leaflets roll upwards and eventually the plant will wilt and die. (See the web version for a picture)

The disease is caused by a bacterium (Erwinia carotovora subsp. Atroseptica). This bacterium does not survive well in soil or really anywhere outside of the potato tuber. The chief source of the problem is planting infected seed. Warm, moist conditions favor disease development. Planting into cool, moist soil followed by warmer conditions favors Blackleg, while planting infected seed pieces into warmer soil favors seed rotting.

Managing Blackleg clearly centers on using clean seed. The level of infected seed in a lot plays a large roll, however even a low level may be a problem because the bacteria can be spread from seed piece to seed piece by seed cutting. Crop rotation is also important, as is sanitation. Rogue out any diseased plants you find.

LATE BLIGHT: As reported earlier in the week, late blight has been found in Connecticut. That still makes only isolated reports in a handful of states, but no major outbreak. Let's hope that the weather cooperates. Remember from earlier discussions, the late blight spores are killed by dry, sunny weather (see the spring issue of the MOFGA newspaper for the whole story about late blight). Last year the materials that performed the best for organic growers who kept late blight away were the copper based fungicides. Remember, if you are an organic farmer you must use a brand that meets the USDA organic standards. A few of these are Champ-WG, Cueva Fungicide Concentrate, and NuCop 50-WP.

Diagram from University of Nebraska-Lincoln Department of Entomology.
The squash vine borer (SVB) is being caught now in high numbers in New Hampshire. If this has been a serious problem for you it is time to do some thing about it. The SVB is much more of a problem in gardens than large plantings, and much more a problem in the southern part of our region than up north.

The squash vine borer is a moth, but unlike most moths it is a daytime flyer. It is a clear-winged moth with a wingspan a bit more than an inch and has a black body. The larvae are wrinkled, whitish caterpillars with brown heads. The female moths lay eggs at the base of squash, pumpkins and gourds and the larvae burrow into the stems. They feed in the stem for about a month and the branch of the plant where they are feeding wilts and dies. With bush type squash the whole plant dies. To distinguish the damage of the squash vine borer from diseases that cause wilting, e.g., Fusarium Wilt or Bacterial Wilt, you should look for frass (caterpillar poop).

The squash vine borer overwinters as pupae or mature larvae in the soil. Adults emerge in the late spring or early summer. The warmer it is the earlier they emerge (they emerge around the time that 1000 base 50 degree days have accumulated).

Disturbing overwintering sites is the first line of defense. Till in crop debris soon after harvest. This is important for summer squash too where the early plantings of squash could be harboring larvae that may lead to a second generation of SVB that will head for your later squash.

Controlling the SVB with insecticidal materials is complicated because once the larvae borrow into the stem it is very hard to manage them. Here is a piece written by Abby Seaman, and entomologist with Cornell University, about using insecticides for managing the SVB:

Before applying any pest control product, make sure to include what you might want to use and how you intend to use it in your organic system plan and get your certifier's approval. (Caution: the use of an unapproved material can result in the loss of certification. Always check with your certifier before purchasing or using a new product or material to ensure that it is permitted for use in your organic farming system. For more information, read the related article, Can I Use This Input On My Organic Farm? http://www.extension.org/article/18321).

Spray timing is critical for effective chemical control because the larvae begin to tunnel into the stem within hours of hatching from eggs, and once inside are protected from the application. Insecticide residues must be present at egg hatch so larvae contact or feed on residues as they enter the stem. Moth flights (and egg-laying) may be predicted by degree-day accumulations as described above or monitored by pheromone traps. Small wire cone traps, nylon mesh Heliothis traps, and Unitrap bucket traps have been found to me most successful for monitoring squash vine borer (Jackson et al. 2005). Traps and lures may be purchased from Great Lakes IPM (www.greatlakesipm.com/) or Gemplers (www.Gemplers.com).

Several insecticide active ingredients approved for organic production are labeled for use against squash vine borer including azadirachtin (neem), neem oil, kaolin clay, geraniol, thyme oil, pyrethrins, and spinosad. Of these, efficacy information is currently available only for spinosad. Trials in Connecticut showed that four weekly applications of spinosad applied during the moth flight resulted in control levels similar to a conventional pyrethroid insecticide.

Stem injections of Bacillus thuringiensis (Bt) are frequently recommended for squash vine borer management, especially for organic gardeners. Trials in South Carolina compared spray application and stem injection of commercial formulations of Bt with a conventional insecticide and an untreated control (Canhalil and Carner 2006). The injection and spray application methods produced similar results, and the Bt treatments provided control similar to that of the conventional insecticide. Note that Bt does not appear on the list of active ingredients currently labeled for use against squash vine borer.

July 7 | Page 8 of 15 | June 19 - Late Blight Update


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