ONION HEALTH MANAGEMENT & PRODUCTION but still tend to aggregate on the newer, emerging leaves. Feeding reduces chlorophyll content of the leaves, with the result that the plants take on a silvery appearance. Damage also occurs to the onion bulb, with thrips infesting the bulb beneath the dry scales, potentially promoting decay losses during storage and causing cosmetic damage to the outer scales that is particularly noticeable on red cultivars. Thrips are difficult to control because they live in tight spaces and cracks between plant structures. They also have a short reproductive cycle and high fecundity that allows them to produce multiple generations in a season, produce several protected life stages (eggs within leaves and prepupae in the soil), attack a wide host range, and are very mobile as adults. Onion thrips and tobacco thrips have been documented as vectors for Iris yellow spot virus (IYSV). IYSV can reduce plant and bulb size and can cause plant death. Once a thrips larva obtains the virus during feeding, it remains infective for the rest of its life as it moves between host plants. Rainfall is the most important natural control of onion thrips. Thrips may be crushed by rain drops (or overhead irrigation water) and pupae can be trapped by crusting soils. Thrips problems tend to be greatest following extended periods of hot, dry weather. Some insects prey on onion thrips, such as predatory thrips, minute pirate bugs and big-eyed bugs. Predator populations typically do not increase until later in the season when economic losses from thrips feeding have already occurred. Growers sometimes avoid planting onions near grain fields as a way to reduce thrips migrating from drying cereal fields. Applying reduced levels of nitrogen to the onion crop may delay the time onions are colonized by thrips and may reduce the size of the population during the season. This tactic has other benefits (less expensive, and may reduce excess nitrogen in the environment) and should be used with other management practices. The impact and degree of thrips damage varies depending on the varieties, but varietal selection is not very effective as a stand-alone approach in controlling thrips populations or preventing damage. Straw mulch provides protection for beneficial insects and, when used in combination with a bio-pesticide, may help reduce thrips populations. This strategy may be more feasible for small plantings of onions, as time and labor for spreading mulch can be expensive. Crop rotation can also be used as a thrips management strategy. Avoiding consecutive years of onion production in a field and growing corn before onion can reduce thrips populations. Corn is a major nitrogen user and depletes the amount of nitrogen carried to the following year when onions are planted, compared to wheat. Within a season, thrips densities were greatest on alfalfa, moderate on wheat, and least on corn when fields adjacent to onion were sampled for thrips populations. Weeds have also been shown to be reproductive hosts for onion thrips, and can contribute to thrips populations within and across (over winter) seasons. Resistance of thrips to insecticides has become an increasingly severe problem in recent years. The problem of insecticide resistance is likely to increase in the future and is the most serious threat to longterm management of this insect. It is, therefore, important that onion growers undertake management practices that slow resistance development. This includes: 1) using insecticides only when needed, based on field surveys; and 2) rotating between insecticide chemical classes (but not in combination!). When used in combination with other cultural management approaches, such as crop rotation, reduced nitrogen application rates, selection of cultivars that are less susceptible to damage by thrips, and weed management within and adjacent to fields, onions can be grown in a more sustainable manner with less thrips population pressure. Vegetable Leafminer The vegetable leafminer is occasionally associated with onion as it is with most vegetable crops. However, under the proper conditions it can become a significant pest, particularly for green onions.
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