Small pests in greenhouses can cause significant damage! Thrips, mites, and whiteflies can only be effectively controlled with the right management strategies.

The major pests commonly encountered in greenhouses are thrips, mites, and whiteflies. Despite their small size, they cause significant damage; not only do they directly harm plants, but they can also serve as vectors for viral diseases and other pathogens. These three pest groups share several common characteristics—high population densities, severe generational overlap, tiny body sizes that make them hard to detect, and steadily increasing resistance to pesticides—each of which poses substantial challenges to effective control.

Thrips

Thrips are quite common in the production of eggplants, peppers, and cucurbit vegetables. Both adults and nymphs feed by rasping and sucking the sap from tender plant tissues—such as shoot tips, leaves, flowers, and fruits. Affected plants develop elongated, grayish‑white chlorotic spots on the leaf surface; severe infestations can lead to premature flower drop, leaf desiccation, and the loss of apical buds on new shoots. Damaged leaves exhibit curled margins that fail to expand, taking on a wavy appearance, with pale yellow‑green veins and yellow, scraped‑like lesions in the mesophyll, resembling mosaic patterns. Eventually, the affected leaves turn yellow, become brittle, and easily fall off. When the apical bud of a new shoot is damaged, growth at the meristem is inhibited, resulting in bushy branching or stunted apical development. Young fruits—such as eggplants, chayotes, cucumbers, and watermelons—become hardened upon infestation; in severe cases, this leads to fruit drop, significantly reducing yield and compromising fruit quality.

The peak occurrence of thrips typically falls in autumn or during the winter months of November and December, with a second peak from March to May. Thrips prefer dry conditions; temperatures around 25°C and relative humidity below 60% are conducive to their development. Excessive moisture is lethal, and at 100% humidity combined with 31°C, all nymphs perish. Adult thrips are highly active, excellent fliers, and photophobic, spending most of the day on the undersides of leaves or in soil crevices. They only become active on host surfaces during overcast weather, in the early morning, at dusk, and at night, which poses significant challenges for their control.

Mites

In recent years, mites have caused extremely severe damage to vegetables such as peppers, eggplants, and loofahs. The red spider mite, white spider mite, and yellow spider mite—commonly referred to by vegetable farmers—are all types of mites. A characteristic of mite infestations is that, in the early stages, they appear in localized patches within greenhouses.

Often, adult and juvenile mites congregate on the tender parts of plants to feed on sap, leading to deformities and stunted growth. When fruits are damaged, their skins become rough and prone to cracking; for example, in eggplants, cracked skin exposing the seeds is a clear indication of damage caused by tea yellow mites.

When mites infest the tender upper leaves, they cause pale yellow or white spots, severely impairing photosynthesis. The undersides of affected leaves exhibit a grayish-brown, oily sheen; the leaves become stiff and grow abnormally, with symptoms resembling those of viral diseases or hormone toxicity.

In greenhouses, the major mite pests of vegetables include the tea yellow mite, the two-spotted spider mite, and the crimson spider mite. Different species exhibit distinct life‑history traits. The tea yellow mite is most prevalent in warm, humid conditions during early spring and on overwintering vegetable crops, with particularly severe infestations on peppers and eggplants. Its optimal growth and reproduction occur at 22–28°C and 80%–90% relative humidity; high temperatures inhibit its development. In contrast, the two-spotted spider mite and the crimson spider mite prefer drier environments and thrive over a wider temperature range; hot, dry conditions favor their proliferation, leading to frequent outbreaks from late spring through early summer.

Whitefly

The most common whitefly species include the tobacco whitefly and the greenhouse whitefly. These pests affect crops in three main ways: first, by direct feeding—piercing and sucking plant sap to extract nutrients, thereby damaging plant tissue; second, through indirect damage—whiteflies secrete honeydew during their activity, which fosters the growth of other fungi and leads to sooty mold on leaf surfaces, impairing photosynthesis; and third, by transmitting viruses—while feeding, whiteflies can also carry and spread viral pathogens.

Adults prefer calm, warm weather and exhibit a strong attraction to yellow. Unhatched whitefly eggs are concentrated mainly on the undersides of leaves in the upper and middle tiers of the plant, as well as on lateral branches, whereas most of the older leaves below the middle tier have already hatched, leaving only eggshells behind. Therefore, vegetable growers should prioritize these areas when applying pesticides.

Integrated pest management approach

1

 

Clean greenhouses to reduce the initial pest population.

Agricultural control measures for small pests are of great importance; during each crop‑rotation period, greenhouses should be thoroughly and systematically treated. Weeds, dead branches, and fallen leaves provide ideal hiding places for pests, so weeds inside the greenhouse should be promptly removed, and dead or decaying plant material cleared away to eliminate sources of infestation. Additionally, during the fallow period when no vegetables are grown, the absence of host plants presents an opportune time to target pest eggs; this downtime can be used to disinfect the greenhouse—its soil, support columns, and hard‑to‑reach crevices and corners. In summer, a high‑temperature fumigation can effectively kill pests and their eggs on the columns and at ground level. In winter, the fallow period is short—often just one or two days—so growers can use fumigants to treat the greenhouse, which is highly effective in reducing the initial pest population.

2

 

Install insect screens at the ventilation openings in advance.

To regulate temperature and humidity inside the greenhouse and prevent the buildup of harmful gases, ventilation is essential—particularly during the warmer months of spring, summer, and autumn, when vents are opened wide. However, this period also coincides with peak pest activity and dispersal. To keep pests out, growers are advised to install insect‑proof netting at ventilation openings as early as possible. The mesh count should be chosen based on local conditions: a mesh that’s too fine can hinder airflow and cooling, while one that’s too coarse may allow small insects to slip through, negating its protective effect. In most cases, a white insect‑proof net with a mesh count of 40–60 is recommended. Ensure the netting is fitted tightly, with no gaps, and minimize lifting it during use to prevent adult insects from entering. Regularly inspect the greenhouse film for tears and repair them promptly, leaving no opportunities for pests to gain access.

3

 

Use chemotaxis to lure and kill pests.

Pests such as whiteflies and aphids exhibit a strong attraction to yellow, while thrips are drawn to blue. Yellow sticky traps can be used indoors to trap and kill whiteflies and aphids, while blue traps can lure and capture thrips. These traps are easy to use, low-cost, environmentally friendly, and effective. However, whether using yellow or blue traps, the hanging height is critical. For seedlings, yellow traps should be hung 10–15 cm above the plants; as the plants grow and approach the trap’s height, the trap should be raised accordingly. When setting up blue traps, the hanging height should be level with the crop canopy. Additionally, after prolonged use or once the traps become fully covered with pests, their adhesive efficacy declines significantly, so they should be replaced promptly.

4

 

Select insecticides appropriately.

Thrips control agents: For preventive control, you can use bifenthrin, acetamiprid, thiamethoxam, and other products. If an infestation occurs, consider applying agents with high efficacy against thrips, such as spinosad or triflumizole.

Mite control agent: Spirodiclofen at 1,500-fold dilution + silicone adjuvant; quinomifos + abamectin–dimefox + silicone adjuvant

Pest control agents for whiteflies: Spirotetramat + dinotefuran or bifenthrin + silicone adjuvant

Source: Public Welfare Plant Protection, Northern Vegetable News – Cultivation Guide

Editor of this issue: Jiang Jiang