Pest Report – June 25, 2015

2015

In this issue:
Potato Leafhopper
Imported Cabbage and Other “Worms”
Colorado Potato Beetle
Garlic Bulb (Bloat) Nematode
Squash Vine Borer

Leafhopper nymph
Leafhopper hopperburn on potato leaves


POTATO LEAFHOPPERS

I have received no reports yet, nor seen them in Maine or New Hampshire, but be on the look out for the potato leafhopper (PLH). It is being reported in large numbers in Rhode Island.

They primarily feed on beans, potatoes, eggplants, strawberries and alfalfa. PLH does not over winter anywhere near here. They over winter way down south on the coast of the Gulf of Mexico, and leapfrog up here in mass migrations. Sometimes hardly any make it, and other years there are massive numbers. 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 and were spotted last week in strawberries in Rhode Island. This week they are in the potato fields down there.

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 this 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 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 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. 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 leaf symptoms, it is too late. Plants will not recover.

Top

Cabbage worm egg (circled)
Imported cabbage worm and leaf damage

IMPORTED CABBAGE WORM and others

The white butterfly with the dark marks on its wing that are flying in large numbers around the vegetables now is a troubling site. They are very numerous this year. Becky can find the eggs they lay, and the tiny caterpillars that hatch. I only notice them after I see the holes and chunks chewed out of the leaves. That has not happened yet, but the moths are laying eggs.

The imported cabbage worm is the most common of the three different caterpillars often found feeding on cabbage, broccoli, Brussels sprouts, kale, etc. By the way, it also feeds on lettuce. The pest overwinters as a pupa in crop debris and is a common pest every year.

There are two other caterpillars often seen feeding on brassica crops; the diamondback moth and the cabbage looper. The looper does not overwinter in the north, and any overwintering of the diamondback is spotty at best. Diamondback is being found in southern New England now. These pests are carried up here with storms. Hence problems from these two pests vary from year to year.

All three pests produce multiple generations each growing season. It is important to destroy the crop debris after harvest so as not to harbor future generations that will move onto your later plantings. Controlling cruciferous weeds will help. Row covers will work however these crops do not do well in heat so there could be problems.

Bt gives good control of all three caterpillars. Dipel is a common brand. But, as usual, if you are a certified grower be sure that the formulation you use is approved for organic production. Spinosad works even better, but is more expensive. Be sure to scout the crops and spray only when the pest is present. Bt must be eaten to work and since it degrades quickly in sunlight it should be sprayed only when the pest is actively feeding. There is no protective activity, so there is no point in spraying yet. But, be on the look out because since the moths are here the caterpillars are not far behind.

Top

COLORADO POTATO BEETLE

Colorado potato beetles (CPB) adults are just starting to show up in potato and eggplant crops. 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.

Colorado potato beetle eggs
Colorado potato beetle eggs
CPB larvae and leaf damage
CPB larvae and leaf damage
Adult Colorado potato beetles
Adult Colorado potato beetles

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 larvae 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 may not be significant, so you may be able to wait until after egg hatch to kill both late 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 larvae from ever reaching the fourth instar! The smaller instars are the easiest to kill with pesticides.

After larvae complete their growth, they drop to the ground and burrow into the ground to pupate. About ten days, 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, barriers such as roads, rivers, woodlands, and fields with other crops are helpful. Rotated fields tend to be colonized (from second generation adults that fly well) 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.

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 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 materials for control. 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 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)

NOTE: There are label restrictions on the use of Entrust that have been written in to help avoid build up of resistance. Be sure to follow the label with respect to mixing, AND number of times a particular crop may be sprayed per season. The manufacturer sent out a letter a few weeks ago threatening to limit distribution or some other move because they have been getting reports of excessive use. It was disturbing to me that they pointed a finger at organic farmers, but it is good that they are promoting resistance management. Be sure to follow the label and not give the manufacturer any reason to act.

Damage caused by garlic bloat nematode

Top

GARLIC BULB (bloat) NEMATODE

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 have already seen this problem in garlic this year.  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.  It can only move short distances on its own.  It is primarily moved by growers either moving soil (on tools, boots, etc.), in moving water, in debris from garlic or onion, or most commonly in garlic bulbs used for seed.

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 a portion of the roots is 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 me to get instructions on 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 planted during the rotation period 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.

Top

Hawk moths, adult form of the squash vine borer
Squash vine borer, larval phase, inside a zucchini stem. Photo by Massiv99. Licensed under CC0 via Wikimedia Commons

SQUASH VINE BORER

Squash vine borers (SVB) are showing up in very large numbers in traps in N.H., and I am sure that southern Maine is next. This pest is showing up earlier, and further north each year. Many vine crops are beginning to bloom, and the row covers are being taken off to allow the bees access. That means the borer now has access too. Care should be taken during bloom to avoid using insecticides when the bees are active.

Squash vine borer moths are day-flying. They have a 1.0 to 1.5 inch wingspan and bright orange markings. In flight, they look like wasps. There is one generation each year. Adults emerge in late June/early July. The moths fly slowly in zig-zags around plants and lay eggs singly on stems; eggs are usually found on the main stem near the base, but are also found on leafstalks or on the undersides of leaves. Upon hatching, larvae bore into stems (where they are protected from insecticides). Thick-stemmed squashes are preferred. Unless you use traps or scout fields for evidence of eggs or larvae, the first sign of squash vine borer infestation is often wilting vines in July and August. By that time, it is too late to do anything.

Growers should scout their pumpkin and squash fields weekly for squash vine borer from late June through early August. Examine the base of vines for eggs. The squash vine borer can be killed with an insecticide and the recommendation for organic growers would be spinosad (Entrust, Monterey Garden Spray), or the Bt aizaiwi strain (Agree WG). Appropriate timing is crucial. The insecticide is most effective when applied to young larvae before they bore into the stem. Once they are in the stem the insecticide will be useless. If you see the moths flying, or hear reports from local Extension about SVB flights, or find eggs, then two insecticide sprays, ideally applied to the base of the plants and timed five to seven days apart, will control newly hatching larvae before they are able to bore into the stem.

Again, the timing is crucial and there is no point in spraying before you find evidence, or after the larvae bore into the stem.

If you are a small scale grower, and evidence of larval feeding (sawdust-like frass near entrance holes) is found, then split open the stem to confirm the presence of larvae, which, by the way, may suggest more eggs are being laid so scout. Borers can be removed from vines if detected before much damage is done. Slit the stem longitudinally, remove the borer, then cover the wounded stem with moist soil above the point of injury to promote additional root formation.

 Some growers monitor for the moths themselves using Scentry Heliothis pheromone traps from early June through early August. If more than 5 moths per week are captured, then they make 2 to 4 weekly applications of pesticides. Timing is very important, so this is not recommended for the casual gardener.

(Source material for this from the New England Vegetable Management Guide; UMass Vegetable Newsletter, The Resource Guide to Organic Insect and Disease Management, and the SVB Report from UNH Extension’s George Hamilton)

Top

Scroll to Top