Onion thrips, the primary insect pests of onions, are responsible for causing significant economic losses due to their ability to reduce yields by up to approximately 60%. The damage inflicted by these pests is particularly severe on young onions compared to larger plants during the later stages of the growing season. Moreover, thrips feeding on onions creates vulnerabilities that can lead to secondary infections. Hence, safeguarding onions against thrips damage is crucial throughout the entire duration of the growing season.
Fields can be reinfested by thrips through the presence of surrounding non-crop vegetation and the migration of thrips from nearby fields.
Onion thrips thrive under hot and dry conditions, making them more harmful in regions where such climatic conditions persist for most of the production season.
Onion thrips exhibit a highly polyphagous nature, as they can be found on the leaves, shoots, and flowers of numerous plant species.
They feed on a wide range of crops, including field crops, vegetables, various types of flowers, bedding plants, and even weeds. Some examples of their targeted plants include alfalfa, asparagus, beans, beets, cabbage, carrots, cotton, cucumbers, garlic, kale, leeks, lettuce, onions, parsley, peas, pineapples, tomatoes, and virtually all small grains, among others.
While onion thrips infest various plants, they particularly favor onions and cause significant damage to this crop.
Onion thrips possess the ability to reproduce without the need for mating. However, females that do not mate exclusively produce female offspring.
The eggs are laid individually and inserted into the leaves of the host plant through slits created by the female’s ovipositors. Hatching occurs within a span of 6-8 days, and the nymphs emerge within 4-9 days. The nymphs closely resemble the adults and immediately begin feeding by tearing the epidermal tissues of the leaves and consuming the host’s sap. The nymphal stage lasts for 4-6 days, during which they go through four instars. The last two instars are non-feeding stages, during which the nymphs descend to the soil. The prepupal and pupal stages occur at a depth of approximately 2.5-5.0 cm within the soil. The prepupal stage lasts for 1-2 days, followed by a pupal stage of 2-4 days, after which the adult emerges.
A complete life cycle of onion thrips ranges from 11 to 35 days, primarily depending on temperature conditions. Adult females have a lifespan of about two to four weeks and can lay between 20 and 200 eggs. The average durations of the pre-oviposition, oviposition, and post-oviposition periods are 6, 23, and 4 days, respectively.
The number of generations onion thrips undergo, ranging from six to ten, is dependent on the prevailing temperature. Adults and larvae overwinter in the soil or plant debris on the ground, while pupae and prepupae spend the winter in the soil.
During the winter season, the adult stage of onion thrips can be found overwintering in the soil within onion fields, as well as in small grain and hay fields.
Compared to the immature and pupal stages, adult thrips are more mobile due to their ability to fly. However, their flight capabilities are relatively weak. They can fly from one plant to another or be transported over long distances by the wind. The winged adults have an elongated body, which can range in color from yellow to brown. Both the forewings and hind wings are fringed and have a pale coloration. They possess piercing-sucking mouthparts, 7-segmented antennae, and gray eyes. Typically, adult females measure around 1-1.5 mm in length, while males are approximately 0.7 mm long.
In their early stages, onion thrips are microscopic and have a white or yellow coloration. They possess a kidney-shaped body. As they mature, they undergo changes in color, acquiring an orange hue. Eventually, reddish eye spots become noticeable.
On onion plants, the eggs of onion thrips have an average length of 0.23 mm and a width of 0.08 mm.
The wingless nymphs of onion thrips bear a resemblance to the adults in terms of their shape and color, although they are slightly smaller in size.
Onion thrips exhibit unique feeding behaviors characterized by puncturing the surface of leaves and extracting sap from plant cells. During this feeding process, they release substances that aid in the digestion of plant tissues. They then consume the mesophyll cells, leading to a reduction in chlorophyll and decreased efficiency of photosynthesis.
The damage caused by onion thrips is visible as silvery patches or streaks on the leaves. In cases of severe feeding, small black “tar” spots can also be observed, which are the excrement left behind by the thrips.
Furthermore, the act of feeding on leaves can create entry points for plant pathogens, increasing the risk of plant infections.
Plant & Bulb Damage
Thrips feeding injury on onion plants tends to have the greatest impact when the plants are young and the bulbs are undergoing rapid enlargement. The damage caused by thrips feeding results in water loss from the leaf surfaces, which can induce stress and hinder plant growth. It may also accelerate leaf senescence, ultimately shortening the period during which the bulbs can enlarge. Severe thrips damage can lead to a reduction in bulb yield, resulting in smaller bulb sizes.
Furthermore, thrips can continue to feed on onion bulbs even after harvest and during storage. This feeding activity can leave scars on the bulbs, negatively affecting their aesthetic appearance and overall quality.
Moreover, onion thrips have the potential to transmit Alternaria porri, the fungus responsible for causing purple blotch disease. Normally, A. porri enters onion leaves through stomata and the epidermal cell layer, but when the leaf surface has been damaged by onion thrips, it becomes easier for the fungus to penetrate. Hence, when planning control measures against A. porri, it is important to also consider strategies to manage onion thrips.
While thrips feeding during the initial bulbing stage poses the greatest threat to onion yields, it is crucial to implement thrips control measures before onions reach this stage. This is necessary to prevent thrip populations from surpassing manageable levels.
There are various methods available for controlling and managing onion thrips. These methods include the following:
To effectively combat thrips, which are piercing and sucking pests, the recommended approach is to utilize insecticides with systemic or translaminar activity. These types of insecticides are absorbed by the plant tissues and prove lethal to the insects after they feed on the plant sap.
The following insecticides are highly recommended for effective thrips control:
- ALONZE 50 EC: Mix 5ml in 20 liters of water, and add INTEGRA at a rate of 3ml in 20 liters of water.
- PROFILE 440 EC: Mix 30ml in 20 liters of water, and add INTEGRA at a rate of 3ml in 20 liters of water.
Please note the following important considerations:
- Controlling onion thrips can be challenging since the mobile stages of these insects primarily reside in the narrow spaces between inner leaves, making it difficult to achieve adequate spray coverage. Additionally, their eggs are laid within leaf tissues, making them less susceptible to control measures. To enhance effectiveness, it is advisable to include INTEGRA, a sticker, spreader, and penetrating surfactant, in the insecticide mixture at a rate of 3ml/20 liters. This additive plays a critical role in improving the efficacy of insecticides.
- Managing resistance to insecticides is crucial since it is the primary approach used to control thrips. To mitigate resistance, it is recommended to rotate the use of insecticides in a sequence based on different groups or modes of action. Additionally, maintaining thorough coverage during application is important to prolong the effectiveness of the insecticides.
- Timely application of insecticides is of utmost importance to ensure their effectiveness in controlling thrips.