The lower developmental threshold is the temperature at which development halts; the upper threshold is where it slows or stops because heat becomes harmful. Lower thresholds are usually estimated by extrapolating a straight line of development rate against temperature down to zero. For many greenhouse mites and insects the lower threshold sits around 8-12 C and the upper around 32-40 C, but each species and stage differs.
In practice
Between the thresholds, warmer means faster generations. Near the upper threshold, mortality and egg failure rise, so the fastest growth (highest r_m) usually sits a few degrees below it. Grow rooms at 24-30 C sit right in the fast zone for spider mites, thrips and aphids. Indoor nights rarely drop below a pest's lower threshold, which is why indoor populations never pause.
Don't confuse with
- Critical thermal limit: the temperature that kills, not just stops development.
- Diapause: a programmed dormancy, triggered by day length as well as temperature.
Tetranychus urticae, the commonest cannabis mite indoors: a 0.5 mm sap-feeder that stipples leaves pale and spins webbing, breeding fastest in hot, dry rooms.Slender insects about 1-2 mm long, chiefly western flower thrips (Frankliniella occidentalis), that rasp leaf surfaces, leaving silvery streaks dotted with black frass.Small soft-bodied sap-sucking insects of the family Aphididae that cluster on stems and leaf undersides, excrete sticky honeydew and multiply rapidly by live birth.A programmed, hormone-controlled dormancy that a pest enters in response to cues such as short days and cool temperatures, ahead of bad conditions.The daily length of light versus dark; photoperiod cannabis flowers when nights grow long enough, rather than at a fixed age.A unit of accumulated heat above a pest's lower developmental threshold; development finishes when a stage's degree-day total is reached, whatever the calendar says.A record of survival and egg-laying at each age for a cohort of pests at set conditions, from which development time, fecundity and r_m are calculated.
