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MOFGA's Pest Report - June 7, 2012
Compiled by Eric Sideman, PhD - MOFGA's Organic Crop Specialist

This is the time of year that scouting is the most important pest management strategy on the farm or garden. This is when hibernating pests are waking from their winter nap and others are working their way north from warmer wintering grounds. Managing the pests now when they are in small numbers is more likely to be successful, and in some cases is the only way to save the crop.

In this issue:

  • Late Blight update
  • Garlic bloat nematodes
  • Cucumber beetles
  • Potato beetles
  • Squash bugs

Late Blight Update: There are more reports of late blight coming in from various places in NY and PA. It is very important to be scouting your fields now both for your own good and for the good of your neighbors. The weather we are having is very favorable for late blight to spread and we must do our best to catch it early.

If you need a refresher on how to identify late blight and manage it, see the following fact sheet: http://plantclinic.cornell.edu/factsheets/lateblight.pdf

Garlic infected with garlic bloat nematode. Note missing roots. Photo by Eric Sideman.

Garlic Buld Nematode (Ditylenchus dipsaci): The stem and bulb nematode, also called the garlic bloat nematode, is a new garlic pest in the Northeast, first appearing in New York in 2010 and now being found throughout the Northeast. It has been spread by infested garlic seed. I am now getting reports of growers finding it in their plantings. The microscopic worms feed by piercing root and leaf cells with their stylet. Leaves of severely infected plants turn yellow and dry prematurely. Plants may be stunted. The roots may be missing and the basal plate may appear to have a dry rot similar to Fusarium basal plate rot.

The pest is favored by wet, cool conditions. Although the pest is not active in hot dry weather, such weather may exacerbate symptoms. The nematode survives freezing and hot weather in soil and plant debris.

Most garlic naturally shows some yellowing of the tips of older leaves, so don't get worried right away. But, if you see some plants in your field that are significantly different than most with lots of yellowing and they seem stunted, pull them up and check the roots. If the roots are mostly missing, please get in touch with me and I will tell you where to send the sample for identification of this microscopic pest. DO NOT IGNORE THE PROBLEM, IT WILL GET WORSE. AND, MOST IMPORTANT, IF YOU SEE ANY DO NOT SELL ANY GARLIC SEED. THIS COULD BECOME A MAJOR PROBLEM AND WE DO NOT WANT THAT.

Cultural Control: The best way to avoid garlic bulb nematode is to use your own garlic for seed, IF it is not yet infested. Monitor for symptoms of infestation during the growing season and submit suspect plants to a diagnostic lab for confirmation. Contact the lab to get instructions how to take and where to send the sample.

If it is determined that you do have the problem, DO NOT use your own garlic for seed. Even bulbs that show no symptoms may have low levels of infestation. Obviously, do not sell any garlic for seed from a potentially infested lot. Do not replant garlic in an infested field for at least four years. Other hosts include all Alliums, celery, parsley, and salsify. Mustards, sorghum-sudan grass, and other bio-fumigant cover crops have been shown to reduce nematode populations in a field. These nematodes can survive in dry debris so carefully clean equipment and storage areas. There are no materials that offer control. (Reprinted from a draft of the soon to be published second edition of the Resource Guide to Organic Insect and Disease Management).

Cucumber beetles. Photo by Eric Sideman.

Striped Cucumber Beetle: (Modified from article in Mass. Veg. Notes written by Ruth Hazzard & Andrew Cavanagh). Striped cucumber beetle is showing up in south central Maine. Striped cucumber beetle is our most serious early-season pest in vine crops. These beetles spend the winter in plant debris in field edges, and with the onset of warm days and emergence of cucurbit crops, move rapidly into the field. Densities often are very high, especially in non-rotated fields or close to last year’s cucurbit crops. Management cannot wait because adult beetle feeding on seedlings (cotyledons and young leaves) is the most serious, and can cause stand reduction and delayed plant growth. Also, the striped cucumber beetle vectors Erwinia tracheiphila, the causal agent of bacterial wilt, and this may be more damaging than direct feeding injury. Crop rotation, transplants, and floating row cover are cultural controls that help reduce the impact of cucumber beetles. By far, floating row covers offer the best results. You must get the row cover on at transplanting or seeding. Early management is important to avoid early season infection with wilt. Cucurbit plants at the cotyledon and first 1-4 leaf stage are more susceptible to infection with bacterial wilt than older plants.

Perimeter trap cropping gives excellent control with dramatic reduction in pesticide use (see 2011 Pest Report for short article).

Beetle numbers should be kept low, especially before the 5-leaf stage. Scout frequently (at least twice per week for two weeks after crop emergence) and treat after beetles colonize the field. Early spot treatments of field edges can be helpful. The threshold depends on the crop. To prevent bacterial wilt in highly susceptible crops, we recommend that beetles should not be allowed to exceed one beetle for every 2 plants. Less wilt-susceptible crops (butternut, most pumpkins) will tolerate 1 or two beetles per plant without yield losses. Spray within 24 hours after the threshold is reached. Proper timing is key. The best approach is to avoid the beetles with a row cover or the use of Surround (see below for instructions on Surround), but if you missed the arrival of the beetles then pyrethrin should be sprayed very early in the morning, at dawn or before, to catch the beetles before they become active with the day’s warmth, and to give the pyrethrin time to work before the sun degrades it. Evening spraying can work for that, but often the beetles are flighty then and you see them flying up and away as you move down the row. Do not spray during the day when bees are active.

OMRI-listed insecticides available for use in striped cucumber management include kaolin clay (Surround WP), pyrethrin (Pyganic Crop Spray 5.0 EC), and spinosad (Entrust). In 2009 spray trials comparing these three products at the UMass Research Farm, kaolin was the most effective in reducing beetle numbers and feeding damage. There was a trend toward Surround WP being more effective when Pyganic or Entrust was mixed with it, but never significantly better than Surround alone. Other studies have shown more efficacy from pyrethrin and spinosad. Remember, Surround should be applied before beetles arrive because it acts as a repellent and protectant -- beetles do not “recognize” the plant and so do not feed -- not a contact poison. With direct-seeded crops, apply as soon as seedlings emerge if beetles are active. Transplants can be sprayed or dipped into the solution before setting out in the field.

I heard from a grower who tried out a vacuum and reported that it seemed to work well. He suggests using a vac with a low setting and/or a broad nozzle so that you don't damage plants. Also, do this earlier in the morning when they are sleepy and slow and be sure to look in their hiding places under the plants.

Three-lined potato beetles. Photo by Eric Sideman.

Cucumber beetles on my potatoes and tomatillo? No. Those are not cucumber beetles. Potatoes, tomatoes 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.

Colorado potato beetles. Photo by Eric Sideman.

Colorado Potato Beetle: (Leptinotarsa decemlineata). Colorado potato beetles (CPB) adults are just starting to show up in potato and eggplant crops. The bright yellow eggs will soon be 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, like we have just had, 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 soon and look for CPB adults and eggs. 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.

Colorado potato beetle larvae - 4th instar. Photo by Eric Sideman.

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, but see below under timing control.

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

Cultural Controls

  • Crop Rotation. The single most important tactic for CPB management is to rotate potatoes and 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, 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 than non rotated fields. Also, the total population of adult beetles is lower, producing fewer larvae to control.
  • 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 if you are isolated and beetles from later generations in distant fields do not fly in.
  • 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 a 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 (OMRI listed products). Organic farmers face a particular challenge at this time because there is one very effective product, spinosad (Entrust), and other options (azadiactin, pyrethrin, and Beauvaria bassiana) are less effective. Consider trying these options anyway, to delay resistance to spinosad. Beauvaria bassiana (Mycotrol O) has been shown to suppress CPB populations over time, though it does not provide immediate control. Bt tenebrionis is no longer available. Cultural practices and natural biocontrols have become even more important in this situation.

Timing for Entrust. To get the most mileage out of the fewest applications of Entrust, time the first spray for when the earliest larvae are reaching the forth instar, or about half to 2/3 grown (see photo in web version of the Pest Report). This timing will catch the largest possible number of larvae while preventing significant feeding damage. (modified from the Umass 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)

Potato Leafhopper Update: Potato leafhoppers are as far north as Massachusetts. Scout your potatoes, beans and strawberries. For details on description, life history, and management see the June 23, 2011 issue of the Pest Report.

 Squash bugs and eggs. Photo by Eric Sideman.

Squash Bug: Squash bugs are now being seen in southern Maine. Squash bugs (Anasis tristis) are serious pests of pumpkins and squash throughout North America. Damage and survival are low on watermelon, very low on cucumber and muskmelon, and highest on squash and pumpkin. Both adults and nymphs feed by inserting their beak and sucking juices from plant tissue. Toxic saliva injected during feeding causes foliage to wilt, then turn black and die; the severity of this damage is directly related to density of squash bugs on each plant. Often I get calls by growers who believe they have a disease. Later in the season, squash bugs may feed on the fruit, causing them to collapse or become unmarketable.

Adults are 0.5 to 0.75 of an inch long, flattened and grayish-brown. They hibernate in trash in and around the garden for the winter and emerge in the early summer to feed a bit and lay eggs. Eggs are laid in clusters usually on the underside of leaves and are orange when first laid, but turn bronze-colored before they hatch. The wingless nymphs are similar in appearance to adults, and are whitish when small, with a brown head, and grayish white when larger with black legs. There is one generation per year in the Northeast.

Black plastic, straw mulch, and reduced tillage systems encourage higher populations, probably by providing good hiding places.

Squash bug young nymphs and eggs. Photo by Eric Sideman.

Squash bug numbers are reduced by clean cultivation in the fall, and crop rotation. Infestation is delayed by row covers. If possible, rotate cucurbit crops between fields as far apart as possible. Keep headlands mowed and free of trash to reduce overwintering sites.

Squash bugs are unusually difficult to control with insecticides. Scout undersides of leaves for squash bug adults and eggs. Crush the eggs. You may not want to crush the bugs as they stink when you do.

If you miss some eggs and have to spray, time squash bug sprays to kill young nymphs just after hatch, because this stage is the easiest to control. Treat late in the day when the flowers are closed to reduce risk to bees. Neem products have proved effective.

For adult bug control, insecticides applied to the base of the plant are most effective, possibly because bugs tend to cluster. But, squash bugs are virtually impossible to control later in the season when nymphs are large and the canopy is dense. So now is the time to manage this pest.
(Source material from New England Vegetable Management Guide; Handbook of Vegetable Pests, A Capinera; ATTRA. Adapted by Andy Cavanagh & R Hazzard, UMass Extension, and modified for this Pest Report by me)

For more information, contact:
Eric Sideman
Organic Crops Specialist
Maine Organic Farmers and Gardeners Association
Phone: 603-269-6201

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