Running a biological pest programme indoors
On our glasshouse benches we have not used a systemic insecticide in years. Fungus gnats, thrips and spider mite still turn up, as they do in every collection, but they are kept below the point of damage by living predators released on a schedule and checked against sticky traps. The same approach works on a windowsill, in a humidity cabinet or across an office, as long as you keep three things straight: the right agent for the pest, conditions the agent can work in, and a record that tells you whether numbers are falling. This sheet is that record.
Identify the pest before you buy anything
Each biological agent is specific. Small black flies rising from the compost when you water are fungus gnats (Bradysia spp.); the adults are a nuisance, but it is the larvae in wet substrate that chew fine roots and seedlings. Silvery streaks, black specks of frass and distorted new leaves point to thrips, whose larvae feed on foliage and then drop into the substrate to pupate. Fine pale stippling on the upper leaf, with webbing and tiny moving dots on the underside, is two-spotted spider mite (Tetranychus urticae). White woolly clusters in leaf axils and along petioles are mealybugs. A x10 hand lens and the undersides of three leaves will tell you more than any photograph.
Match agent to pest
- Steinernema feltiae are microscopic nematodes watered into the substrate, where they enter and kill gnat larvae and thrips pupae. They need moist substrate at roughly 15–25°C and a second application about 14 days later to catch the next generation.
- Stratiolaelaps scimitus (formerly Hypoaspis miles) are predatory mites that live in the top centimetres of substrate and eat gnat larvae and thrips pupae. They establish and persist, so they suit a collection that has gnats every winter.
- Amblyseius swirskii and Neoseiulus cucumeris come in slow-release sachets hung in the foliage. They hunt young thrips larvae and keep releasing mites for several weeks. Hang them out of direct sun, which dries and overheats the sachet.
- Phytoseiulus persimilis is the specialist spider-mite predator. It eats all stages of the mite and dies out when the food runs out, so release it onto infested plants rather than as prevention.
Mealybugs are harder to control biologically on a domestic scale; isolate the plant, remove clusters with a cotton bud and keep records on the sheet while you decide on next steps.
Check the room can support them
Predatory mites are living animals with a comfort range. Below about 18°C, or in air drier than about 50% relative humidity, their eggs hatch poorly and populations build slowly, which is exactly the situation in many centrally heated British homes in winter. Enter your room temperature and humidity and the sheet prints a viability check: OK, or a warning to group plants, add a humidity tray or move the programme to a warmer room. Nematodes need moist, not wet, substrate; water before and after drenching and keep the pots out of full sun for a day.
Leave a gap after sprays
Many sprays sold for houseplants harm the predators you are about to buy. Pyrethroids can stay toxic on leaves for weeks, oil-based coatings smother small mites, and systemic neonicotinoids carry through the plant. Check the label and the supplier's compatibility advice, and only release live agents once the exclusion period has passed. The sheet asks you to confirm this and prints a residue warning box either way, so anyone else caring for the plants knows not to reach for a spray mid-programme.
Fix the substrate as well as the pest
Fungus gnats are a symptom of substrate that stays wet. Larvae feed on decomposing organic matter in dense peat and fine compost, so a collection kept in tired multipurpose compost will keep producing gnats however many nematodes you add. A reset into a chunky, high air-filled porosity bark mix, or into an inert mineral semi-hydro substrate, removes the habitat. Tick the reset option and the sheet adds a bare-root checklist. Our biological control guide covers the whole method.
Count, do not guess
Yellow sticky traps catch gnats and thrips adults; blue traps are more attractive to thrips. Number each trap, write the number on the sheet next to the plants it serves, and record the adult count once a week for six weeks. A falling trend means the programme is working. A flat or rising one after three weeks means conditions are wrong, the agent is wrong, or something is reinfesting the collection, often a new plant that skipped quarantine.