Thrips, Leafminer, Cold Injury
June 17, 2011
In this Issue:
Western flower thrips feed on melons, cabbage, cucumbers, lettuce, tomatoes, and will soon start to build in numbers, in particular on peas. These tiny insects are hard to spot (a hand lens is necessary for identification), and are usually not noticed until significant damage has occurred.
They feed by scratching the leaf tissue and sucking up the cell contents. As a result, they are removing chlorophyll and leaving the plant with a shiny, silvery cast. On cucurbits, thrips feeding causes a white stippling on the leaves. On peas, leaves look as if something toxic has been dripping on the leaves.
Thrips overwinter in grains, clover, alfalfa crops, and weedy areas. They will migrate to vegetable crops in late spring to early summer when cereal crops or weeds are cut or begin to dry.
To monitor, look at the undersides of leaves, or in protected sites to determine presence. No threshold for treatment has been determined for vegetable crops, but in general, it is a good idea to treat when you see more than 5-6 nymphs per leaflet. The best option is to monitor the crop and treat before the population builds to damaging levels.
Treatment: azadirachtin (Azatin), spinosad (Conserve, Entrust, Green LightH, FertiLomeH), Beauvaria bassiana (Botanigaurd), lambda-cyhalothrin (Warrior)
Onion thrips populations are very low now due to the cool moist spring. This insect typically thrives in hot, arid environments, potentially making it the most destructive pest on onions in Utah. They overwinter as adults and start feeding in early spring on volunteer onions and new plantings.
On onions, they feed on new leaves near the center of the onion neck, leaving white to silvery streaks where they have fed. Heavy feeding can cause plant withering, and during July and August (when bulbs are rapidly enlarging), it can also cause reduced bulb size due to loss of plant vigor. Thrips can also vector viruses such as the iris yellow spot virus, which is fairly new to Utah.
Onion growers in northern Utah should be scouting their fields now for thrips activity. To find them, open the leaves of the plant and look in the neck at the newest leaves and quickly count the thrips before they hide. From now to mid July, if up to 15 thrips/plant are found, treatments should be made to prevent later population build-up.
To manage this pest, consider alternate options before pesticides, as onion thrips can quickly develop resistance. Heavy sprays of water (including overhead irrigation) will dislodge and drown the thrips. Also, applying straw mulch, and interplanting with carrots as an alternate host, can reduce thrips populations on onion.
Treatment: azadirachtin (Azatin, Neemix), spinosad (Success, Entrust, Green LightH), spinetoram (Radiant), insecticidal soapH, kaolin clay (SurroundH), carbaryl (SevinH), methomyl (Lannate), permethrin (Ambush, Pounce), spirotetramat (Movento)
Halso for homeowner use
Masses of eggs may be found on the undersides of leaves in clusters of around 25. This pest is so successful because each female can lay up to 500 eggs, it is a voracious feeder, there are two generations, and it adapts to whatever it thrown at it, i.e., insecticides.
Monitor fields or individual plants for Colorado potato beetle adults now by shaking a stem over a white cloth. If adults are found, look under leaves for egg masses. Treatment threshold is 25 beetles per 50 plants and a 10% defoliation level.
Alternative treatments to insecticides include:
When scouting, treat plants if you find more than 1 adult beetle per plant, or 4 small larvae per plant. Keep in mind that healthy and late-season potatoes can tolerate up to 20% defoliation without yield losses.
Treatment: spinosad (Conserve, Entrust, Success, BonideH, FertiLomeH, MontereyH), pyrethrin (Ace Flower and Vegetable Insect SprayH), imidacloprid (Admire), carbaryl (SevinH, Bayer AdvancedH), abamectin (AgriMek), acetamiprid (Assail, Ortho Max Flower, Fruit, and Vegetable Insect KillerH), indoxacarb (Avaunt), spinetoram (Radiant), azadirachtin (Azatin, Abamectin), neem oil (Concern)
The larvae are hairy-green and have a yellow stripe along their backs. As you scout your plantings (examine 5 consecutive plants in at most 10 random locations in the planting), look for these larvae. If present, treat when 10% of plants have active larvae. (Once heads form on broccoli, cauliflower, and cabbage, observe a 5% threshold.)
commercial growers: Bt (on young larvae, Xentari, Agree), indoxycarb (Avaunt), spinosad (Entrust, Success), spinetoram (Radiant), pyrethrin (Pyganic), tebufenozide (Confirm 2F), methoxyfenozide (Intrepid)
residential growers: neem oil, spinosad (Bonide, FertiLome, Monterey), pyrethrin (Ace Flower and Vegetable Insect Spray), carbaryl (Sevin, Bayer Advanced), acetamiprid (Ortho Max Flower, Fruit, and Vegetable Insect Killer)
Flea beetles are tiny black beetles that jump when disturbed. They overwinter as adults and begin feeding in spring. They feed on the underside of leaves causing small holes or sunken pits. They are mostly a problem on new seedlings, and if left unchecked, can cause significant damage. Older plants can usually withstand feeding, although the lower leaves may be affected.
Monitor young seedlings carefully until they have mature leaves. The best time is mid-day when they are most active. Although there can be up to 3 generations, the overwintering adults cause the most damage. Treat when 5% of plants are infested.
Treatment: spinosad (Entrust, Success, MontereyH, Ferti-LomeH, etc.) insecticidal soapH, diatomaceous earth, neem oilH, carbaryl (SevinH), permethrin (Ambush, Pounce)
The adult is a fly and the oblong eggs are white, and the neat rows are easily visible on the undersides of leaves. The maggots burrow inside the leaves, eating cell contents between the upper and lower epidermis. The visible symptom is a winding trail that may enlarge to gray blotches on the leaves. A single maggot can cause significant damage, feeding on multiple leaves during its development.
Maggots feed for a few weeks before pupating in the soil. There are 3 to 4 generations per season. Late May is the first peak period of activity.
Treatment: Look for eggs or the start of new mines to determine when to start treatment. Repeat treatment in 7-10 days. Options include: Agri-mek (abamectin), Coragen (chlorantraniliprole), Beleaf (flonicomid), Provado (imidacloprid), malathionH, insecticidal soapH, pyrethrinH, spinosadH
Plants affected by cold injury can recover, but will grow slower then healthy plants. Good news is that evening temperatures are forecasted to remain in the 50s for much of northern Utah. Colder locations will see temps in the 40s over the weekend, and plants there will still need overnight protection.
Adults have been active for several weeks now, laying eggs on the terminals of raspberry canes. Larvae feed within the stem tissue and later move down the cane to pupate.
The adult is a wasp (Hartigia cressoni and is rarely seen. The male is black, and the female is black with yellow markings. Adults emerge from winter pupation within raspberry canes, and lay eggs on canes by inserting eggs just under the epidermis. Larvae then feed inside the cane. Wilting of the tips may not be evident until extensive feeding has already occurred. This wilting may recover at night, but later in the season, the top terminal usually dies back. In late summer, the larvae move down the cane, and remain in the pith for the winter.
Fields can tolerate low populations, however if left untreated, populations can build and cause quite a bit of damage and frustration. Research at USU has found that there are high levels of natural predation by other wasp parasites.
Monitor plants throughout the season for terminal wilting and prune and destroy the infested plant material. When pruning, be sure that you get the white larva inside the stem. It may be farther down the stem than you think. To get a feel for where the larvae are feeding, slice a few cut stems vertically to locate the larva. Where there is no borer, the pith will be creamy-white. A pith with loose brown material will indicate borer activity (either above or below the cut). Some growers have a “touch” for finding the larva within a stem, and squeeze the cane where it is feeding (thus killing the larva inside), without having to prune the tip off.
Treatment: Carbaryl (Sevin) may reduce the adult egg-laying population. It and should be applied now, and again 2 weeks later; if plants are in bloom, do not spray, or limit spraying to evening hours only to protect pollinators.
The adult beetles lay eggs near the base of the canes, and the larvae hatch and move into the plant tissue. The insect at first forms random, spiraling galleries on the inner bark of canes, and then moves into the center where it moves up or down the cane. Canes may have swellings at the feeding sites, and infested canes may break at weak areas later in the season.
The best treatment option is to remove and destroy infested canes late in the season and over the winter. There are a few insecticides that can be used to kill the eggs and newly hatched larvae, but use caution when spraying flowering plants: treat at dawn or dusk only, to avoid harming pollinators, or ideally, do not spray plants in bloom.
Treatment: Malathion, rotenone + pyrethrin (Pyrellin EC, Bonide Liquid Rotenone Pyrethrin Spray; this material is softest on bees), Diazinon. Treat every 7 days for 3 weeks.
Serviceberries (AKA Amelanchier, shadbush, juneberry, or Saskatoon) are a delicious fruit, tasting like a mix of blueberries and nuts. I’m trying to grow a crop of these myself, and I thought I’d mention an interesting find (in Cache County) in case there are others growing serviceberries.
A majority of the developing fruit an infestation of a fruit feeding insect called Saskatoon sawfly. Adult sawflies emerge in May, and each female lays a single egg inside several flowers of serviceberry and chokecherry. Eggs hatch about a week after petal fall, and the larvae then feed inside the developing fruit. They push frass (sawdust-like excrement) out of an exit hole and consume the entire inside contents of the fruit. Around the end of June, they drop out of the fruit and burrow into the soil to spend the rest of the summer and winter. They pupate in the spring.
Infested fruits will appear black and shrunken at harvest. Losses can be extensive (up to 90% in my garden!), as each larva may hollow out several fruits in a cluster during the course of its lifetime.
Treatments are too late this year, and should begin next year at pre-bloom and at petal fall. Options include Delegate or Malathion, or a pyrethriod labeled for fruits.
Precautionary Statement: Utah State University Extension and its employees are not responsible for the use, misuse, or damage caused by application or misapplication of products or information mentioned in this document. All pesticides are labeled with ingredients, instructions, and risks. The pesticide applicator is legally responsible for proper use. USU makes no endorsement of the products listed herein.