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Home greenhouse

Under glass, pests keep breeding long after the garden has gone quiet. RHS says glasshouse red spider mite is usually a problem from March to October but can strike at any time in a heated greenhouse, that whitefly is active all year round in greenhouses, and that fungus gnats breed all year indoors.

The biological controls work in a small house on two conditions. Introduce them early: RHS notes that Encarsia cannot give instant control, so it goes in before plants are heavily affected. And keep the house warm enough: the mealybug destroyer is only likely to succeed from May to September. Yellow sticky traps catch the parasitoid wasps too, so use them to watch the trend, not to control anything.

crop context · lean-to glasshouse, summer

Six pests you will meet

ALL YEAR

Greenhouse whitefly

Clouds of adults, sticky honeydew and sooty mould; a generation takes about three weeks at 70 °F (21 °C).

ReleaseEncarsia formosa1–10 /m² per release, weekly
ALL YEAR

Fungus gnat

Larvae damage seedling roots and the base of soft cuttings; established plants usually cope.

ReleaseSteinernema feltiae250k–500k /m² of compost surface

Season calendar

NORTHERN HEMISPHERE · FROST-FREE GLASS · QUALITATIVE
JFMAMJJASOND
Spider miteJan: QuietFeb: QuietMar: BuildingApr: BuildingMay: Pest pressure highJun: Pest pressure highJul: Pest pressure highAug: Pest pressure highSep: Pest pressure highOct: BuildingNov: QuietDec: Quiet
WhiteflyJan: QuietFeb: QuietMar: BuildingApr: BuildingMay: Pest pressure highJun: Pest pressure highJul: Pest pressure highAug: Pest pressure highSep: Pest pressure highOct: BuildingNov: QuietDec: Quiet
AphidsJan: QuietFeb: QuietMar: BuildingApr: Pest pressure highMay: Pest pressure highJun: Pest pressure highJul: BuildingAug: BuildingSep: BuildingOct: QuietNov: QuietDec: Quiet
ThripsJan: QuietFeb: QuietMar: QuietApr: BuildingMay: Pest pressure highJun: Pest pressure highJul: Pest pressure highAug: Pest pressure highSep: BuildingOct: QuietNov: QuietDec: Quiet
Release windowJan: QuietFeb: QuietMar: Release windowApr: Release windowMay: Release windowJun: Release windowJul: Release windowAug: Release windowSep: Release windowOct: QuietNov: QuietDec: Quiet

Starter program

A starting point for a small, frost-free house with nothing sprayed in it recently. Encarsia attacks whitefly and Phytoseiulus eats only spider mites, so identify the pest before you order.

  1. AT FIRST WHITEFLYEncarsia formosa1–10 /m² weekly, until most whitefly scales have turned black
  2. AT FIRST MITESPhytoseiulus persimilisOnto infested plants in sheltered spots (RHS); repeat weekly, at least twice
  3. AT FIRST COLONIESAphidius colemani0.25–4 /m² per release, at least three releases
  4. AT POTTINGSteinernema feltiaeDrench the compost; repeat weekly to every 10 days while larvae are found
  5. MAY–SEPTEMBERCryptolaemus montrouzieri5 beetles per infested plant, onto colonies with egg sacs
Size this program for your area →

Setting the house up

In a small house, the jobs between crops do as much as the releases.

Empty and clean between seasons

UConn suggests a fallow of at least four weeks with the house completely empty. Red spider mite females spend the winter in frame cracks, canes and plant debris (RHS).

Wait out old sprays

Many insecticide residues, especially pyrethrins and organophosphates, can harm natural enemies for up to three to four months (UConn).

Check new plants

Many pests arrive on bought plants. UConn advises inspecting everything that comes in and isolating infested plants.

Damp down against mites

Syringing plants and keeping humidity high reduces the risk of red spider mite, but will not control it on its own (RHS).