Vegetable Notes 2024 Vol. 36:13

Squash bugs and eggs were observed in several fields this week on both sides of the state. Squash bugs are large, shield-shaped true bugs that lay masses of round, shiny, bronze eggs, usually on the undersides of cucurbit leaves. Nymphs are light-gray with black legs. Adults and nymphs feed on plant sap with piercing sucking mouthparts and inject a toxin while they feed, which can cause foliage to wilt, turn black, and die—a symptom called Anasa wilt (named for the genus that squash bugs belong to). Damage is most severe on summer squash, zucchini, pumpkins, and Cucurbita maxima crops (e.g. Hubbards). Watermelon, cucumber, cantaloupe, and butternut will resist damage. Row covers will exclude squash bugs but must be removed to allow for pollination. For newly laid eggs and nymphs, consider a foliar application of acetamiprid (Assail 30 SG) which has moderate toxicity to bees (lower than other neonicotinoids). Adults and larger nymphs are more difficult to control, partly because they hide in the lower canopy and near the soil. An organic option for nymphs is a mixture of pyrethrin (a contact toxin) and azadiractin (an insect growth regulator, derived from neem). This would be gentler on bees than a high rate of pyrethrin alone and would include two modes of action. Insect growth regulators work to disrupt the molting process, so they are only useful on immature stages. Treat late in the day when the flowers are closed to reduce risk to bees.

Powdery mildew (PM) is now being reported on cucurbits throughout the Northeast. PM is a fungal disease that produces powdery white sporulation on the tops and bottoms of leaves, as well as on petioles. It is different from the similarly-named downy mildew, which produces fuzzy gray sporulation, just on the undersides of leaves. PM occurs on all types of cucurbits, but there are resistant varieties of many cucurbit crops available. For effective chemical control, protectant fungicides (e.g. sulfur, horticultural oils, chlorothalonil, copper, mancozeb) should be applied weekly before symptoms develop in your crop. Once PM develops in your crop, add a PM-targeted material to your spray schedule and alternate between at least 2 different chemistries. Be sure to abide by application maximums listed on labels. Effective targeted materials include Vivando, the DMI fungicides (e.g. Proline, Procure), Gatten, the Luna fungicides (Sensation, Experience, and Flex), Miravis Prime, and Orondis Opti. See Managing Cucurbit Downy and Powdery Mildews in the May 23, 2024 issue of Veg Notes for more info and example spray schedules.

Cucurbit downy mildew (CDM) was reported on cucumber in southern Ontario, near Buffalo, NY, this week. Last week CDM was reported in New Jersey. There is a large greenhouse cucumber industry in Ontario, so it’s likely that CDM overwinters there, as well as in the southern US where cucumbers are grown year-round. There have been no reports of CDM moving closer to MA from NJ since last week. No pesticide applications are currently recommended for MA growers.

Legumes

Mexican bean beetle (MBB) adults, eggs, and larvae were present in a bean field in Hampshire Co. this week. Adults and larvae feed on legume foliage and can eventually skeletonize leaves. There are several generations per year. This pest can be controlled using the parasitic wasp biocontrol Pediobius faveolatus. The wasp lays its eggs in MBB larvae and the developing wasp larva feeds on the MBB larva from inside. Ideally, you will only need to release Pediobius in the first MBB generation—the wasps that hatch from the parasitized larvae will go on to control the next generation of larvae. Pediobius can be ordered from the NJ Department of Ag. Scout your beans and call to order the Pediobius immediately if you see larvae. If you don’t see larvae yet, flag an egg cluster and check it every few days to monitor when it hatches, then call to order the Pediobius. The wasp can be ordered as live adults, which are ready to parasitize when they are released, or as mummies with developing larvae inside. Wasps will emerge from the mummies within a few days.

Tomato leaf mold was observed in several high tunnels this week. This fungal pathogen produces fuzzy olive-green sporulation in round patches on the undersides of leaves. The leaf usually turns yellow on the top of the leaf, opposite the sporulation. This disease almost exclusively occurs in high tunnel tomatoes, as it requires high heat and humidity. The best way to control leaf mold is by using resistant varieties (see this list compiled by Meg McGrath at Cornell Cooperative Extension).

Hornworms are showing up in greenhouse and field tomatoes now. There are 2 species of hornworm—tomato hornworm and tobacco hornworm—but they can be treated the same. The caterpillars can cause significant feeding damage to tomato foliage and fruit. They camouflage very well in the foliage and are often first noticed because they produce a lot of dark green, pellet-shaped frass. Bt products (e.g. Dipel, Xentari) are effective against hornworms, or they can be hand-picked out of crops. Fun fact: hornworms have stripes that light up under black lights, so if you have a black light handy, take a look through your tomato crop at night with it and see the hornworm stripes glow!

Pith necrosis was diagnosed in high tunnel tomatoes this week. The affected plants had yellowing leaves and side shoots, but the main stem looked green and healthy. Tomato plants with pith necrosis often produce lots of adventitious roots (roots coming from the stems), but this case didn't develop that symptom. Pith necrosis is caused by a weak bacterial pathogen that infects plants that are growing too quickly. Generally, pith necrosis occurs when plants are growing too fast. The disease is favored by high nitrogen fertility, cool nighttime temperatures, and high humidity. The only way to manage this disease is by avoiding favorable conditions, especially excessive nitrogen.

White mold was identified in another high tunnel with tomatoes this week. In tomatoes, this disease is often referred to as “timber rot”, as entire stems turn papery and tan. White mold, caused by the fungus Sclerotinia sclerotiorum, overwinters as sclerotia—masses of fungal tissue surrounded by a hard black rind—in infected crop residues and soil. The fungus will produce lots of fuzzy white mycelium in and on infected plants, and over time, these sclerotia will develop amongst the mycelium. Infected plants will wilt and die, with the stem turning brown. Crack open the stem to find the mycelium and sclerotia. Removing infected plants will reduce the amount of inoculum in the soil. Don’t compost infected plants. Contans is a commercially available biocontrol product for use against white mold. The material is applied in the fall. The current label for Contans does not include tomatoes, but it includes most other vegetable crops, which are also hosts for the disease. Therefore, it can be applied in the fall prior to planting winter greens into a tunnel, and it will control for white mold in the following summer crop.

Potato aphids continue to feed on tomatoes in high tunnels. Some growers have found success using Aphelinus abdominalis, a slower-acting parasitoid wasp than Aphidius ervi, for biological control. Ants, if present in large numbers, will protect aphids to feed on the sugary honeydew they produce and must be controlled before biocontrol can be effective. Household ant baits targeted towards sugar-loving ants are the recommended method for managing high populations of ants. The ants carry the bait back to the colony, resulting in its death. The baits should be placed around perimeters of high tunnels where ant trails are present. 

Colorado potato beetle (CPB): We are nearing the end of the first generation of CPB now, and larvae that escaped treatment are quite large and getting ready to pupate. The large larvae can be hard to kill with pesticides, so physically knocking them off plants may be a better strategy at this point. As the next generation of CPB starts in the next week or two, remember you will need to switch IRAC classes on this next batch of beetles and larvae.

Scarab beetles have emerged and are feeding in crops now. There are several scarab beetle pests of vegetables, including Japanese beetles, Oriental beetles, and Asiatic garden beetles. All have wide host ranges. Japanese beetles especially love basil and can cause severe defoliation. The immature stages of all of these scarab beetles are grubs that live in the soil and feed primarily on grass roots. We often see high scarab beetle pressure in fields that were recently transitioned to vegetables from sod or in fields surrounded by lots of grasses. The New England Vegetable Management Guide lists products for Japanese and/or Oriental beetles in basil and sweet corn. For controls in a crop where these beetles are rarely a pest and therefore not mentioned in the Guide, check the label of commonly used broad spectrum synthetic pyrethroids, carbamates, and neonicotinoids (as foliar spray). Organic options include neem/azadiractin products and pyrethrin (e.g. Pyganic).

Four-lined plant bug damage is being seen in a variety of crops now. This pest is common on plants in the mint family, including basil. The feeding damage looks like large groups of dark or translucent round spots, concentrated around the base of leaves and leaf veins. The bug overwinters as an egg and nymphs emerge in late spring to begin feeding on crops. Chemical management is usually not recommended because the bugs are active for only a short period of time in late spring/early summer. In perennial mint crops, thoroughly cut back growth at the season’s end and destroy this debris in order to destroy the overwintering eggs.

Caterpillars in Brassica Crops

We’re seeing damage now from imported cabbageworm, diamondback moth, and cross-striped cabbageworm in brassicas across the state. Cabbage loopers should start to show up mid- to late summer. Though they may look similar, these four major brassica caterpillar pests are different species with important distinctions among them that can affect management decisions. They differ in size and feeding habits, as well as how susceptible they are to beneficial parasitoid insect species and certain insecticides. Getting acquainted with these pests will help you to know what kind of damage to expect and what to look for when scouting for their different life stages and biocontrols. Feeding damage by any of these caterpillars can reduce yield and marketability of both leafy and heading brassica crops.

​​​​​​Imported cabbageworm or cabbage butterfly (Pieris rapae) is a very familiar white butterfly that can be seen during the day fluttering around brassica fields. Each forewing has a dark border and one or two round black spots. Eggs are laid singly on the underside of leaves, standing upright. They are bullet-shaped, about 1/8-inch in length, and initially pale white, turning yellow as they mature. Larvae are gray-green, slightly fuzzy, and sluggish. They can be very well camouflaged, especially when they lie along the midrib of a leaf. Feeding and resting occur on the underside of leaves, and in cabbage or broccoli, the larvae feed more heavily in the head than on the other leaves. The overwintering stage is the chrysalis (pupa), which is green or brown and smooth, with three pointed ridges on its back. There are 3-4 generations per year.

Click on images to enlarge.

Diamondback moth (Plutella xylostella) adults are tiny (< ½ inch long) brown moths that and rest with their wings folded together like a tent. Historically, DBM did not overwinter in the Northeast, but caterpillars have been reported by May in recent years, suggesting that they may be overwintering in high tunnels or other warm spots. Regardless, this pest also blows in every year from warmer areas to our south. Adults are weak fliers, but populations are known to disperse long distances on wind and annually reinvade areas well into Canada. Eggs are laid singly or in small clusters. Caterpillars go through four instars and are small (< ½ inch when fully grown), light green, and appear segmented, with a forked end and pointed shape. When disturbed, they wiggle vigorously and may drop off the plant on a string of silk. Feeding causes small, round holes that don’t break through the top layer of leaf tissue, leaving translucent films across holes, called “windowpane” damage. Feeding tends to be spread across the foliage and is not necessarily concentrated in the head.

Cabbage looper (Trichoplusia ni) usually does not survive the winter in New England and arrives in migratory flights from farther south. Generally, populations of cabbage loopers are not high until late July or August, though earlier flights may occur in some years or they may not be found at all in others. Adult moths are mottled gray-brown, about ¾ inch long, with a distinct round silver-white mark on each forewing. Since they fly at night, they are rarely seen unless monitored with pheromone traps. If you want to know when moths arrive, use a wing trap baited with Trichoplusia ni lure, placed near the canopy. Eggs are round, pale green or yellow, and are laid singly underneath the foliage. The cabbage looper caterpillar is light green and smooth, with wavy white or light yellow lines down the back and sides and stubby legs at the tip of the abdomen. Full-grown larvae reach 1½ to 2 inches. Cabbage loopers of any size move like inchworms—by raising the middle of their body in a characteristic “loop” shape. Feeding tends to create large, ragged holes in foliage, on both frame leaves and heads. Cabbage looper also feeds on many non-brassicas including lettuce, celery, spinach, and chard, so when they do arrive, scout those crops as well as brassicas.

Cross-striped cabbageworm (Evergestis rimosalis): Formerly restricted to the South, this insect is now a serious problem on brassica crops in southeastern New England. Cross-striped cabbageworm does not generally overwinter in New England but we saw a very early infestation on one farm in MA this spring and are guessing that the caterpillars overwintered in high tunnel brassica crops. One of the major differences between this insect and the other brassica caterpillars is that the eggs are laid in a cluster, and caterpillars feed in a group on one plant so that it’s riddled with big holes like buckshot. CSCW is closely related to European corn borer, and the adults are similar in shape and coloring–straw-colored with a little purple, and crossed by wavy lines. Since the adults fly at night, you will likely only notice the caterpillars and their damage. The clusters of 3 to 25 eggs are yellow, flattened, and attached to the lower leaf surfaces. The caterpillars are light bluish-grey on top and green underneath, with numerous black transverse bands across their backs and a yellow line down each side. Larvae grow to ¾ inch long in 2 to 3 weeks. There are 2 to 3 generations per year, but generally, numbers do not reach damaging levels until late summer. Larvae can produce small holes in leaves until only veins remain, feed in terminal buds and sprouts, or burrow into heads. Plants with larvae are often completely skeletonized while adjacent plants may be left undamaged.

Scouting

It is especially important to check cabbage or broccoli plantings as they begin forming heads. Leafy greens such as collards and kale should be scouted earlier, since all leaves are marketed. Randomly select 25 plants throughout the field and check for caterpillars or fresh feeding damage on the top or underside of leaves. Look for black or green frass and tiny clustered feeding holes. It is often easier to spot the frass and feeding damage first before finding the caterpillar. Classify plants as infested (one or more caterpillars present) or non-infested, and calculate the percentage of plants infested. In the Northeast, there is generally no need to treat young plants until at least 35% are infested with caterpillars. Once heading crops (cabbage, broccoli, cauliflower) begin to form the head, a lower threshold should be used to protect the marketed head; treat heading crops after head formation if 15-20% or more of the plants are infested. The most critical time to scout and apply controls is just prior to head formation. For leafy crops like kale and collards, a 10-15% threshold should be used. Because cross-striped cabbageworm can be so destructive, a lower threshold should be used—treat when 5% of plants are infested with this pest.

Chemical Control

Use selective insecticides to protect beneficial insects that keep aphids under control, eat insect eggs and small caterpillars, and parasitize imported cabbageworm or diamondback moth. Selective products often are most effective when consumed with foliage, so coverage is important. Use at least 50 gallons of spray material per acre; higher volumes provide better coverage. Better coverage of lower leaf surfaces can also be achieved by using drop nozzles. Use a spreader-sticker to prevent sprays from rolling off of waxy leaves. The most effective materials include:

• Diamides (Group 28) including chlorantraniliprole (e.g. Coragen), cyantraniliprole (e.g. Exirel), and cyclaniliprole (e.g. Harvanta)
• Spinosyns (Group 5) including spinetoram (e.g. Radiant) and spinosad (e.g. Entrust) - also effective against flea beetles and onion thrips
• Bacillus thuringiensis (Group 11) including Bt aizawi (e.g. XenTari) and Bt kurstaki (e.g. Dipel DF, many other products) – these materials are highly selective and will ONLY affect caterpillars.

These materials and the aizawai strain of Bt will usually provide better control of resistant diamondback moth than older products. See the cabbage insect control section of the New England Vegetable Management Guide for additional synthetic and naturally derived products and more details.

Cultural and Biological Controls

Incorporate crop residues shortly after harvest to decrease movement to successive plantings and reduce overwintering populations. Populations are suppressed by a wide range of natural enemies, including several species of parasitic wasps. Diamondback moth eggs are parasitized by the ichneumonid wasp, Diadegma insulare, which occurs naturally in eastern North America. D. insulare females require sources of nectar, so maintain wildflower stands near brassica fields. Imported cabbageworm eggs are parasitized by the braconid wasp, Cotesia rubecula, which was introduced to New England from China in 1988 and is now established in Massachusetts. You may see their small white cocoons on brassica leaves. The chalcid wasp, Trichogramma brassicae, will lay its eggs in many species of caterpillar, including all of the brassica pests above (as well as non-target caterpillars, so be cautious if you are maintaining wildflowers that might attract endangered moths or butterflies). T. brassicae wasps are not found in New England but can be purchased from several biological control companies for release in brassica fields. The wasps arrive as pre-parasitized caterpillar eggs that are glued to cards that can be distributed throughout the crop. As of 2022, each card costs around $20-23, and contains about 100,000 wasps, which is enough for up to 1 acre. According to one source of T. brassicae wasps, IPM Labs Inc., some growers will release the wasps in lieu of using any kind of pesticide. Some growers release one card per acre per week for about 4 weeks, while others will release every week for the life of the crop. These biological controls are compatible with many selective and lower impact sprays used for control of caterpillars (Bt, oils, soaps), especially because the wasps are protected from sprays when they are inside of host eggs. Another source, Evergreen Growers Supply, notes that Trichogramma wasps are more effective against moth species that lay their eggs in clusters, so they may be a good option if cross-striped cabbageworm has been a particular problem on your farm.

--Written by R. Hazzard, S.B. Scheufele, and L. McKeag