If you notice thrips on your plants it is important to treat them immediately. This will help break the life cycle and prevent it from spreading to other plants.
Thrips are tiny wingless insects that resemble lobsters up close with narrow fringed wings. They feed by puncturing plant tissues. Heavily infested leaves, flowers or growing tips may develop silvery streaks.
Pest Identification
The thrips you’re seeing may be difficult to identify as they are smaller than other common greenhouse pests and often blend in with dust particles or hide under leaf surfaces. They also wriggle around the plant rather than fly or hop, making it harder to spot them on the surface of the leaves and flowers. Thrips also pierce the outside layer of a leaf to feed and then suck out the juice inside. This gives the leaves a crinkled appearance. Damage from thrips can also look like rust spots, streaked leaves or flower petals, distorted fruits and scabby buds.
Thrips overwinter in decaying plant debris and bark, so in early spring, they emerge to feed on plants and lay eggs. The first two life stages are larvae and nymphs, which have long narrow fringed wings. Then they wriggle out of their hole and start feeding by piercing the outer surface of a leaf to extract the juices inside. Eventually they reach full size, then drop to the ground or into soil to pupate. After several days, adult thrips hatch and start laying new eggs.
Once the thrips hatch, they can move quickly from plant to plant. This is why it’s important to treat all the plants in a glasshouse or garden when you see signs of damage from this pest.
Regular sampling will help you track the thrips population in your greenhouse or gardens. This will help you decide when to apply insecticide. The graph in Figure 7 shows that thrips populations increased between June 7 and 14 on the tomato crop in area 3.
Samples can be collected by shaking plants over a sheet of paper or using a commercial beat sheet. They can then be poured into soapy water or placed in the freezer to kill the thrips. Alternatively, a beat sample can be taken directly onto a microscope slide and examined under magnification. This is the best way to count both adult and immature thrips, but it’s time consuming. A hand lens is a helpful tool for identifying thrips and other small insects that are hard to see.
Life Cycle
Some species of thrips infest and damage a wide range of garden and greenhouse plants. The thrips in this order (Thysanoptera) are tiny, wingless insects that feed by piercing the surface of plant parts and sucking out the juices. Infested plants become mottled or distorted. Some species, such as greenhouse thrips, western flower thrips, and citrus thrips, are common pests in greenhouses. Others, such as avocado thrips, blueberry thrips, and onion thrips, attack only a few of the plants they infest. Identifying the specific species of thrips infesting your plants may reveal that they are harmless and control measures are not necessary.
Female thrips cut a small slit in the surface of leaves, flowers, or stems where they lay their eggs. In warm, indoor conditions, eggs hatch within a few days. The larvae that emerge are wingless and creamy to pale green in color, although some have a yellowish hue. As they grow, the larvae gradually change color and develop wing pads as they reach each instar. Instars are followed by pupal stages that are inactive and do not feed. The third stage is called a prepupa and the fourth stage is a pupa. When full grown, the nymphs leave their host plants and burrow down into the soil where they incubate until winged adults emerge to begin the cycle anew.
Thrips are very mobile, moving rapidly on foot and in the air. They disperse easily among plants and can spread between greenhouses. Light-blue cards coated with a film of oil suspended two feet above indoor plants can be used as indicator traps for the presence of thrips. When the traps are inspected, the dark spots of fecal excrement indicate the presence of thrips.
In many cases, the most effective insecticide for thrips is a systemic agent that is absorbed by roots and moves through the plant to kill infested tissues. Neonicotinoids are a common systemic insecticide that is effective against thrips. However, their effectiveness is hampered by thrips resistance to the neonics. The use of a longer-persisting product such as dinotefuran, available to professional applicators, or imidacloprid, which is widely used in commercial production, can help delay resistance development.
Damage
The damage caused by thrips and the viruses they vector can be quite noticeable, especially on ornamental plants or vegetable crops with light-colored flowers or leaves. The rasping mouthparts of the nymphs and adults cause distorted and discolored buds, stems and leaves. A heavy infestation can also suck sap from the plant, leaving silvery or gray speckled areas that often appear dotted with black specks (the excrement of the thrips). The damaged plant tissues lose their vigor and produce less food for the rest of the plant.
The females of many species lay their elongated eggs on or into plant tissues, and the larvae feed until they graduate to a non-feeding prepupal or pupal stage. These stages can occur openly on leaf surfaces or within distorted plant tissue such as in galls made by some species such as Cuban laurel thrips and myoporum thrips. The time from egg to adult may be as short as two weeks under warm conditions.
In the wild, thrips are controlled by other pests and predators and their populations rarely reach damaging levels. Indoors in artificial conditions, however, thrips can be a real nuisance and a big problem for houseplants.
Depending on the species of the plant, the first signs of thrips are usually yellow or brown spots on the leaves. These spots will grow and become more pronounced over time. The leaves may then begin to curl, shrivel and fall off. In a severe infestation, thrips may feed by puncturing the cells of the plant and sucking out their contents. This results in a silver-grey or black speckled appearance on the underside of the leaves, and the plant becomes weak and wilted due to a lack of chlorophyll.
The thrips are poor fliers, so they tend to remain in the vicinity of an infested plant. Treatment with insecticidal soaps, such as the winter savory oil, can give good control. A highly refined spray such as superior horticultural oils will also work well, although it must be applied frequently to break the life cycle of the thrips.
Control
Thrips are small, slender insects with fringed wings. They suck plant juices, attacking flowers, buds, leaves and stems. They leave behind a sticky, varnish-like exudate and distorted foliage. Although some thrip species are beneficial predators that feed on other insects and mites, the vast majority of them damage plants.
In the garden, thrips can be controlled by reducing their habitat. Remove weeds, mulch and other debris where they might hide. Keep bushes, shrubs and trees away from structures where they might be sheltered, such as fences or walls.
Indoors, use a trap. Cut strips of corrugated cardboard and place them on the surfaces where thrips are a problem. Tape the edges to prevent them from blowing away. The next morning, check the traps. When thrips are caught, pick them up and toss them into a bucket of water with a squirt of dish soap. The thrips will drown in the soapy water. Repeat as needed.
Other organic controls include sprinkling diatomaceous earth around the soil and on plants. This dust has razor-sharp edges that cut the outer skin of pests, absorbing their moisture. It also kills them by blocking the pores through which they breathe.
For light infestations, a gentle spray of water from a hose can physically dislodge thrips and re-wet the leaves. Wiping the stems, petioles and leaves with a wet cloth also works to remove them without any chemicals or materials.
A contact spray of an insecticidal soap, such as Safer’s, will also work to control thrips. Apply thoroughly to cover buds, flowers and growth tips, and spray the undersides of leaves, where many thrips hide. Follow the product label to avoid damage to other plants and to avoid suffocating beneficial insects that might be on or around the plants you’re treating.
Superior horticultural oil sprays, which are highly refined oils that suffocate the insect, also provide good control for thrips. Follow the product label to avoid damaging other plants that may be sensitive to the oil.
