THRIPS AND THRIPS MANAGEMENT
 Thrips are present season long in leafy vegetables, but are usually most abundant during the spring/summer when temperatures begin to increase. They are most important in head, leaf, romaine, spring mix lettuces, cabbage and spinach because of the cosmetic scarring they cause to leaves and contamination of harvested plant parts. Thrips can build up in weedy areas, and other surrounding crops, moving to lettuce in large numbers when host plants begin to dry down. Further, once adults disperse onto plants, they can readily and rapidly reproduce and colonize in high numbers. The developmental rate of thrips is uncertain on leafy vegetables, but field observations suggest that they can complete development from egg to adult in less than 3 weeks when termperatures average 70°F.
Species Complex
Western flower thrips, Frankliniella occidentalis, and Onion thrips, Thrips tabaci, are the main thrips species that occur in leafy vegetables. Because western flower thrips are generally more difficult to control with insecticides than onion thrips, it is important to correctly identify species composition. Identifying thrips can be very difficult because of their small size and similarities in color. Adult western flower thrips are about 1/20 inch (1.5 mm) in length, and immatures are generally light yellow in color. Western flower thrips have reddish-orange ocellar pigmentation and eight-segmented antennae. Onion thrips ar slightly smaller than western flower thrips, being only 1/25 inch (1.2 mm) long, and their body is yellow with brown blotches on the thorax and abdominal terga. The legs are yellowish-brown, and the antennal segment I and the base of segments III to V are brownish-white, the rest of the antenna is brown. Their ocellar pigment is gray, and they have seven-segmented antennae. If necessary, contact an Extension advisor or specialist for assistance in identification.
Monitoring
Like aphids, thrips can disperse onto crops at any time, thus it is important that fields be monitored regularly. Thrips can generally be found throughout the plant, feeding on the undersides of leaves, but prefer to hide in complex plant parts, flowers and other folded tissue where they are difficult to detect and reach with insecticides. There are several methods for sampling for thrips on leafy vegetables:
- Direct observations involve careful examination of plant parts for the presence of thrips and feeding scars, and can be done when sampling for aphids and other pests. Care should be taken to carefully examine folds in leaf tissue near the base of the plant for immatures. If 3-5 thrips are found on a small plant, there are probably 3 times as many hidden within folds in the leaves or that had dispersed from the plant.
- Sticky traps, placed on field margins, can indicate when adult thrips begin to disperse into fields from adjacent vegetation. Adults fly when temperatures exceed 63-65°F during the day when light intensity is moderate-high. Also allows for identification of species.
- Dislodging involves beating or jarring plants to dislodge thrips onto sheet or sticky surface where they can be counted and identified. Should be done during the morning when adults are less active. Can be time-consuming and does not measure scarring.
- Management. Cultural management has only a limited impact on thrips populations because of their ability to rapidly disperse from native vegetation, weeds and crops. Further, there are few natural enemies that effectively control thrips. Consequently, control with insecticides is often the only viable control alternative.The following points may be helpful when attempting to chemically manage thrips populations in leafy vegetables:
 There are no defined action thresholds, but experience suggests that applications should be initiated when population numbers are low (head lettuce and cabbage), and when scarring on young leaves is first observed (leaf and romaine lettuce; spinach), particularly when temperatures are increasing. Most often, thrips control is a by-product of treatments applied for other insects. Permethrin (Pounce, Ambush), methomyl (Lannate), diazinon, cypermethrin (Ammo), and spinosad (Success) all have good activity against thrips. Note: Always consult the products label before recommending or applying any insecticide.
Frequency of applications will depend on residual of products and immigration of adults from surrounding vegetation. Plants should be sampled at 3-5 day intervals following treatment. In recent trials, the most efficacious insecticides tested were only able to maintain thrips populations at constant levels, and did not reduce numbers significantly. This should be taken into consideration when determining when to treat.
Plant size and temperature may be important factors contributing to the efficacy of these products. The larger the plant, the more difficult it is to obtain good coverage underneath the leaf and near the base of the plant. Research has shown that increasing spray volumes improves efficacy against immature thrips on larger plants. Also, higher temperatures drive thrips development, but may also influence their activity to more readily come in contact with the insecticides.
Information adapted from: Yuma County Farm Notes.
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TIPBURN IN VEGETABLES
 Tipburn is a physiological disorder in which developing plant tissues collapse due to insufficient calcium. Calcium ions are necessary for cell wall construction in a plant. When soil and environmental conditions impede calcium uptake and assimilation, meristematic tissues may be starved for calcium. Tipburn is the result.
Tipburn may occur in many leafy vegetables - lettuces (leaf- and head-types) and cabbage (red, green and Chinese). Black heart in celery, blossom-end rot in peppers, tomatoes and watermelon, and glossy curd and tipburn in cauliflower are all related to insufficient calcium in the affected plant.
Symptoms of calcium deficiency vary depending on the host. Where leaves are affected, the initial symptom is a blanching of the tissues, usually at the leaf margin. The blanching is followed by a drying out of the tissues and necrosis (death and browning). These symptoms are often internal in vegetables that form a head of leaves or are at the center of the head or stalk as that is where the young developing tissues are located. Also, the initial blanching symptom in the tissues may occur several days before necrosis is evident. Thus, the damage is done before the problem is visually detectable. A further complicating factor is that the necrotic tissues may be invaded by secondary organisms, which can cause a watery, soft rot. Bacterial soft rot, which can be a problem in the field or post-harvest, may at times be secondary to a calcium deficiency disorder.
Factors affecting calcium uptake and assimilation by plants are both environmental and genetic. Research has shown that some varieties (lettuce and cabbage, for example) are better at mining and assimilating calcium than others. Weather factors are also important. Calcium deficiency often occurs in the spring when soils are still cold, but air temperatures and sunlight are favorable for rapid transpiration and plant growth. Calcium does not solubilize readily in cold soils. Under these conditions plants are stimulated to grow faster than calcium can be absorbed and translocated. Acid soils are also unfavorable to calcium uptake. Current information indicates that calcium availability in soil is low until the pH is above 7 at which point solubility and availability begin to increase. Nitrogen (N) also interacts with calcium (Ca). High N levels in plant tissues exacerbate tipburn. The N source also has an effect. Ammonium N induces higher rates of tipburn than urea N. Another factor is total soil salts. High salt levels in the soil tend to induce tipburn.
Addressing calcium deficiency disorders is difficult. Calcium is a notoriously insoluble element. And most plants do a poor job of assimilating Ca through foliage. Celery is an exception. Foliar calcium has been shown to reduce the incidence of black heart in this crop if it is detected in the early stages in a field.
The best recommendation is to use tipburn-resistant cultivars, especially during spring/early summer when tipburn tends to be a problem. Other cultural practices include avoiding moisture stress, especially at heading time or early fruit maturity. And manage water to keep soil salts low (leaching) and alleviate stresses created to soil salts.
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RISK MANAGEMENT STRATEGIES FOR VEGETABLE GROWERS
Agri Logic, Inc., has announced that it will hold a second listening session at the Far Western Tavern, Guadalupe, at noon on May 2, 2002. This meeting is specifically directed to growers of asparagus, broccoli, carrots, cauliflower, celery, garlic, artichokes, lettuce (all types), and spinach. Agri Logic, under contract with USDA, is continuing to acquire information to address the feasibility of a risk management strategy for vegetable crops. Lunch will be provided. If you are a vegetable grower of one or more of the above crops, please plan to attend this meeting. If you are not there, your comments and opinions will not be heard. Everyone’s input is important!
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