Onion thrips are the most important insect pests of onions, which cause significant economic losses, by reducing yields up to about 60%.Their damage to young onions is more devastating than on larger plants late in the growing season. However, thrips feeding opens up the onion to secondary infections. Therefore it is important to protect the onions from thrips damage throughout the entire growing season.
Reinfestation of fields can occur from surrounding non-crop vegetation and immigration of thrips from nearby fields.
Onion thrips thrive in hot, dry conditions and are usually more damaging in areas where these climatic conditions prevail for most of the production season.
Onion thrips are extremely polyphagous. They inhabit leaves, shoots, and flowers of many plants.
They feed on many field crops, vegetables, various flowers, bedding plants and weedy plants.
These include alfalfa, asparagus, bean, beet, cabbage, carrot, cotton, cucumber, garlic, kale, leek, lettuce onion, parsley, pea, pineapple, tomato and practically all small grains, among others
Onion is its preferred host and is one of the crops they damage the most.
Onion thrips do not need to mate in order to reproduce. However, the females which do not mate produce females only.
Eggs are laid singly and inserted in the leaves of host plant through the slit made by ovipositors of the female. Hatching takes place within 6-8 days. Nymphs hatch out within 4-9 days. The nymphs are very much similar to the adults. They start feeding immediately after their emergence by lacerating the epidermal tissues of the leaves and swallowing the sap of the host. Nymphal period lasts for 4-6 days during which they pass through four instars. The last two instars are non-feeding stage and during this period they descend to the soil. Prepupal and pupal stages take place at a depth of about 2.5-5.0 cm inside the soil. Prepupal stage lasts for 1-2 days and pupal stage for 2-4 days, after which adult emerges out.
A single life cycle is completed within 11-35days and this is majorly dependent on temperature conditions. Adult female lives for about two to four weeks and can lay 20-200eggs. The average pre-oviposition, oviposition and post-oviposition periods are 6, 23, and 4 days, respectively.
Onion thrips have six to ten generations depending on temperature.
Adults and larvae overwinter in the soil or plant litter on the ground while pupae and prepupae overwinter in the soil.
The adult stage overwinters in the soil in onion fields, in small grain and hay fields.
Adults are more mobile than immature and pupal stages because they can fly. The winged adults are however weak fliers but can fly from plant to plant or be carried long distances via wind. They are elongated with body color varying from yellow to brown, the forewings and hind wings are fringed and pale in color. Mouthparts are piercing-sucking, antennae are 7-segmented, and eyes are gray. Generally, the adult females are 1–1.5 mm long and males are about 0.7 mm in length.
They are microscopic, white or yellow, and kidney-shaped. As they mature, they develop an orange tinge and eventually reddish eye spots become evident. On onion, the average length and width of eggs are 0.23 mm and 0.08 mm, respectively.
They resemble the adults in shape and colour but are wingless and slightly smaller.
Onion thrips have very distinctive feeding behaviors by punching through the leaf surface and then extracting sap from plant cells. During this process, they release substances that help predigest the tissue, then they siphon off the plant contents and consume mesophyll cells, which eventually results in a loss of chlorophyll and reduced photosynthetic efficiency.
Damage appears as silvery patches or streaks on the leaves.
Severe feeding injury by onion thrips is also associated with tiny black “tar” spots, which is excrement from thrips.
Feeding on leaves can also create entry points for plant pathogens.
Plant & Bulb Damage
Generally, onion is most sensitive to thrips feeding injury when plants are young and when bulbs are rapidly enlarging. Water loss from damaged leaf surfaces may cause stress and reduce plant growth and may accelerate leaf senescence, both of which may shorten the period bulbs enlarge. Reduction in bulb yield (smaller bulb sizes) can occur due to severe thrips damage.
Thrips may also feed on onion bulbs following harvest and during storage, and this can cause scars that reduce the aesthetic appearance and quality of bulbs.
Onion thrips may also serve as a vector of Alternaria porri, the fungus that causes purple blotch. A. porri typically enters onion leaves through stomata and the epidermal cell layer, but fungal penetration becomes easier when the leaf surface has been damaged by onion thrips. Therefore, measures against onion thrips should also be considered while planning control of A. porri
Although thrips feeding during the early bulbing stage is the most damaging to yields, thrips must be controlled before onions reach this stage so that populations do not exceed levels that can be adequately controlled.
Several methods can be used to control and/or manage onion thrips. They include the following;
Considering that thrips are piercing and sucking pests, the most effective insecticide to use should have a systemic or a translaminar activity so that, after it has been absorbed by the plant tissues, the insects can die as after sucking the sap.
The following insecticides are therefore recommended in their effective control;
ALONZE 50 EC 5ml in 20L + INTEGRA 3ml in 20 Litres of water.
PROFILE 440 EC 30ml in 20L + INTEGRA 3ml in 20 Litres of water.
- Onion thrips may be difficult to control because the mobile stages of this insect are found mainly in the narrow spaces between the inner leaves where spray coverage may not be easy to accomplish. In addition, the eggs are laid into the leaf tissues where they may escape control. To effectively control them, it is advisable to mix the insecticide with INTEGRA 3ml/20l. This is a sticker, spreader and a penetrating surfactant critical for improving the efficacy of insecticides
- Managing resistance to insecticides is critical given that this is the principal tactic used to manage thrips. Resistance can be mitigated by rotating the insecticides used in a sequence (based on groups or modes of action), and maintaining a thorough coverage to prolong the effectiveness of insecticides
- Timely application of insecticides is very important.