Thrips are tiny, slender insects that commonly plague indoor and outdoor plants, causing silver-stippled leaves and distorted new growth. Focusing treatment solely on the foliage fails to eliminate the entire population, leading to rapid reinfestation. Successfully eradicating thrips requires targeting the vulnerable life stages that reside within the growing medium. This article focuses on effective biological and chemical methods for eliminating the larval and pupal stages of thrips found in soil.
Why Thrips Hide in the Growing Medium
Thrips have a complex life cycle that necessitates treating both the plant canopy and the soil below. Adult female thrips typically insert their eggs directly into the soft tissue of leaves, stems, or flowers. Once hatched, the thrips develop through two active, feeding larval stages on the plant foliage.
After the second larval stage, most pest species drop from the plant and burrow into the soil or potting medium to complete their development. These non-feeding stages are referred to as the pre-pupa and pupa. These immobile stages are protected underground, sheltering them from foliar sprays and allowing them to mature into flying adults. Ignoring these soil-dwelling pupae guarantees the infestation will cycle back, rendering canopy treatments ineffective for long-term control.
Non-Chemical and Biological Soil Controls
One effective non-chemical approach involves beneficial entomopathogenic nematodes. The species Steinernema feltiae (S. feltiae) actively seeks out and infects thrips pupae in the soil. These microscopic roundworms enter the pest’s body, release symbiotic bacteria, and cause death within 48 to 72 hours.
To apply S. feltiae, the nematodes are mixed with water and applied as a soil drench, ensuring continuous agitation to prevent settling. The growing medium must be kept consistently moist for at least seven days following application, as moisture is necessary for the nematodes to move effectively. For best results, application should occur when soil temperatures are above 42°F (6°C), preferably in the morning or evening to shield the nematodes from UV light.
Another physical control method involves applying Diatomaceous Earth (DE) as a top dressing on the soil surface. DE is a naturally occurring powder made from the fossilized remains of diatoms. When thrips larvae or newly emerging adults crawl across the soil surface, the sharp, microscopic silica shards physically abrade their outer layer, causing dehydration and death.
This barrier is effective against pupae as they emerge and larvae as they drop down to pupate. A light, even layer of food-grade DE should be maintained across the soil surface, especially after watering. For severe infestations, remove and discard the top one to two inches of soil, which contains the majority of the pupae. Alternatively, soil can be sterilized before reuse by baking it at 180°F for 30 minutes or by using solarization to destroy remaining life stages.
Targeted Chemical Soil Drenches
For chemical control, a targeted soil drench directly eliminates soil-dwelling thrips stages. Spinosad, an insecticide derived from the fermentation of a naturally occurring soil bacterium, is widely recommended. While often used as a foliar spray, Spinosad is highly effective when mixed according to label instructions and applied as a generous soil drench.
The drench should be applied until the entire growing medium is fully saturated and water runs out of the bottom of the pot, ensuring full penetration. Spinosad acts as both a contact and ingestion poison, affecting the thrips’ nervous system and causing paralysis. Since it is a microbial product, apply Spinosad in the evening, as direct sunlight can rapidly degrade its active ingredients.
Systemic insecticides offer another chemical approach, working by being absorbed through the plant roots and distributed throughout the vascular system. When thrips larvae or emerging adults feed on the plant, they ingest the toxic compound. Ingredients such as imidacloprid or dinotefuran are often used for this systemic treatment where legal and available.
Systemic treatments can take time, sometimes weeks, to fully translocate to the leaves and flowers in effective concentrations. This method is advised only for ornamental, non-edible plants, as systemics should not be used on food crops due to residue concerns. A proper drench involves fully watering the plant with the solution until saturation.
Integrated Management and Reinfestation Prevention
Successful, long-term thrips control requires an integrated strategy that addresses all stages of the pest’s life cycle simultaneously. Soil treatments, whether biological or chemical, must be combined with foliar treatments to kill actively feeding larvae and flying adults on the plant. For example, a Spinosad soil drench can be paired with a foliar spray or a predatory mite release to maximize population reduction.
Monitoring the adult population with sticky traps indicates the success of the soil treatment. Bright yellow or blue sticky traps, placed near the plant canopy, capture flying adults. They help determine if thrips are emerging from the soil or if reinfestation is occurring from outside sources. If the number of trapped adults remains high after a soil drench, a follow-up treatment is necessary to break the continuous life cycle.
Preventative cultural practices also maintain a healthy, pest-resistant growing environment. Avoiding overwatering is helpful, as excessively wet soil can create an ideal environment for thrips pupation. Ensuring good soil drainage and aeration helps keep the soil surface drier, making it a less inviting place for larvae to pupate. Removing nearby weeds and plant debris, which serve as alternate hosts, reduces the overall risk of reinfestation.