Temperature, humidity and pest population growth in cannabis: every major pest compared

Short answer. Temperature sets the speed of every cannabis pest. Between about 20 and 30 C (68-86 F), spider mites, broad mites, thrips and aphids double every 2-4 days in lab life tables, while mealybugs and fungus gnats take 6-8 days. Humidity matters less to the pests directly than to the predators and fungi that kill them. So choose set-points for the beneficials as much as against the pest, and time every repeat treatment from development time at your own temperature. Each pest's article has the full life table; this page puts them side by side.

All life-table figures here are from lab studies at constant temperature, mostly on hosts other than cannabis (bean, apple, cucumber, grape, eggplant). Only the cannabis aphid figures come from hemp. Treat them as the unchecked upper limit of what a pest can do; real rooms with predators, crowding and plant defences run slower.

How fast does each cannabis pest multiply at 25 C?

At 25 C (77 F), the intrinsic rate of increase (r_m) of common cannabis pests ranges from about 0.08 per day for citrus mealybug to about 0.53 per day for cotton aphid, a doubling time from 8 days down to 1.3 days. Aphids, broad mites and spider mites are the fast group; thrips and whiteflies are intermediate; mealybugs, fungus gnats and moths are slow per day but lay many eggs.

Notes: R0 is the net reproductive rate (daughters per female per generation). A dash means no published value was found. Where a range is shown, values differ by host plant; host quality alone can change r_m by half, as the spider mite data on apple versus peach show (see spider mites).

Why does temperature matter so much for pest growth?

Temperature matters because insects and mites cannot regulate their body temperature, so every step of development and egg-laying runs on the heat around them. Between the lower developmental threshold and the optimum, development rate rises roughly in a straight line with temperature, which is the basis of degree-day models ^[2].

Heat raises r_m in two ways at once. Generations get shorter, and females start laying sooner. Since r_m is roughly the natural log of R0 divided by generation time, halving generation time doubles r_m even if each female lays the same number of eggs ^[1].

Above the optimum, the picture flips. Egg-laying and survival fall faster than development speeds up:

• Cannabis aphid on hemp: r_m 0.355 per day at 25 C, 0.244 at 28 C, 0.019 at 30 C ^[12].
• Broad mite: 0.31 at 25 C, 0.12 at 32 C ^[6].
• Western flower thrips: 96% of larvae died at 35 C, with development fastest near 31 C ^[8].
• Two-spotted spider mite: still 0.30-0.32 per day at 30 C ^[3][4]; daughters per female fell sharply only at 35 C ^[3].

That is why spider mites are the signature pest of hot cannabis rooms. They keep accelerating up to temperatures where aphids and broad mites are already slowing down.

How much faster do pests grow at 30 C than at 20 C?

Pests with published data grow roughly 1.5 to 2 times faster at 30 C than at 20 C, except those whose optimum is below 30 C. The worked example below starts with 10 females and uses N(t) = N0 x e^(r_m x t), the unchecked exponential model from life tables ^[1].

Method. Doubling time = 0.693 / r_m. Population at day t = 10 x e^(r_m x t). For example, spider mites at 30 C for 21 days: 0.302 x 21 = 6.34; e^6.34 = 568; 568 x 10 = about 5,700. Sources: spider mite ^[3], broad mite ^[6], cannabis aphid ^[12], tomato russet mite ^[7], thrips ^[9], mealybug ^[16].

What the numbers mean. These are ceilings, not forecasts. They assume unlimited food, no predators, no crowding and a constant temperature. Their use is comparative: three weeks of neglect at 30 C produces about 20 times more spider mites than three weeks at 20 C. See intrinsic rate of increase and doubling time.

How do developmental thresholds and degree-days help a grower?

Developmental thresholds and degree-days let a grower convert one temperature's life cycle into another's. Days per stage = degree-days required / (temperature minus lower threshold), valid between the lower and upper thresholds ^[2]. See developmental threshold.

The 30 C column for the spider mite and broad mite is close to measured values (about 6-7 days and 3.3-3.5 days), which is a useful sanity check ^[3][6]. Linear models overestimate speed near the upper threshold, so trust them least above about 30 C.

How does humidity affect cannabis pests and their enemies?

Humidity affects cannabis pests far less than temperature does, but it decides which predators and pathogens can work. The clearest effects are on predatory mite eggs and on insect-killing fungi, both of which need high humidity, and on spider mites, which do better in dry air.

Spider mites prefer dry air. In greenhouse cucumber, spider mite densities were up to three times higher at about 55% RH than at about 80% RH ^[5]. Dry air helps mites shed the excess water they take in while feeding; humid air slows feeding and egg-laying. Low humidity is a contributing factor, not a cause.

Broad mites prefer humid air. UConn and other extension sources give 80-90% RH at 21-27 C as favourable; see broad and russet mites.

Hemp russet mite is contested. Extension sources disagree; the tomato russet mite proxy grew fastest at 30-60% RH ^[7]. See hemp russet mite.

Fungus gnats and shore flies respond to substrate moisture, not air RH. A wet top layer of the medium drives them; see fungus gnats.

Predatory mite eggs need humidity. Phytoseiulus persimilis egg hatch was about 8% at 40% RH versus nearly 100% at 80% RH at 27 C ^[22]. The humidity at which half the eggs of persimilis and Neoseiulus californicus die was about 66-70% RH in Wageningen trials ^[5]. Amblyseius swirskii lost half its eggs at about 63% RH (see broad and russet mites).

Insect-killing fungi need humid leaf surfaces for hours to germinate, which conflicts with flower-room humidity targets. See entomopathogenic fungi.

What are the climate windows of the main beneficials?

Each beneficial has a temperature and humidity window, and releasing it outside that window wastes money. The table below summarises extension and research values; supplier technical sheets give similar ranges.

How do short days and diapause affect a flower room?

Short days can trigger diapause in some pests and beneficials, so a 12:12 flower room is a different biological environment from an 18:6 veg room, not just a different light schedule. See diapause.

• Two-spotted spider mites. Females reared under short days and cool temperatures can enter diapause, turn orange-red, stop laying and hide in debris and cracks; they are the survivors that restart the next run. Many greenhouse strains diapause weakly. See spider mites.
• Orius insidiosus. In a lab study at 20 C, 57% of females entered reproductive diapause at 12:12 and nearly all at 10:14 ^[24]. Use non-diapausing strains in flower rooms or rely on predatory mites.
• Aphidoletes aphidimyza. The aphid midge needs about 15.5 hours of light to avoid diapause ^[26]. Night-break lighting to prevent it would disrupt cannabis flowering, so the midge is a veg-room tool.
• Cannabis aphid. Sexual forms and eggs can appear indoors after the flip to short days; see cannabis aphid.

How should grow-room set-points be chosen for pest control?

Choose set-points that keep the crop first, then nudge the room toward the predator's window and away from the pest's optimum. The crop's needs win every conflict: late-flower humidity stays at 40-50% RH regardless of what mites prefer, because bud rot costs more ^[21].

Leaf temperature matters more than air temperature. A canopy hot spot 3-4 C above the room under a lamp runs spider mites at the 30 C row of the growth table while the thermostat reads 26 C. Check with an infrared thermometer, and see VPD and leaf VPD.

How do you set re-application intervals from these numbers?

Set the repeat interval of a contact treatment that misses eggs to fall after the eggs hatch and before the survivors become egg-laying adults. Take egg development time as the earliest repeat and egg-to-adult time (plus the pre-oviposition period) as the latest, then aim for the middle. The IPM programme explains the logic.

These intervals are calculated from non-cannabis lab data (except the cannabis aphid) and are a starting point to confirm by scouting, not a label instruction. Whatever the interval, follow the label and local law, and rotate mode-of-action groups by pest generation; see mode of action.

What are the common myths about temperature, humidity and pests?

• "Mites die in winter." Myth for indoor rooms. Rooms never drop below spider mite thresholds of about 8-14 C ^[3][4]; outdoors, diapausing females survive in debris.
• "Just raise humidity to beat spider mites." Myth as a cure. High RH slows mites and helps persimilis ^[5][22], but it does not eliminate them and is dangerous in flower.
• "Hotter always means more pests." Myth. Cannabis aphid, broad mites and thrips decline above their optimum ^[6][8][12]; spider mites keep going.
• "A cold room stops pests." Myth. Cooling slows them; development continues down to thresholds of roughly 6-13 C.
• "Lab r_m values predict what will happen in my room." Myth. They are upper bounds on non-cannabis hosts, best used for comparisons and timing.
• "Temperature makes the most difference to pest growth." Established ^[1][2].

Common mistakes

• Using air temperature at the thermostat instead of leaf temperature at the canopy.
• Keeping one weekly spray interval all year, whatever the room temperature.
• Releasing humidity-sensitive predators into a dry flower room.
• Relying on the aphid midge or diapausing Orius under 12:12.
• Treating lab r_m values as forecasts instead of upper bounds.

Evidence

• Temperature-driven development and the degree-day method: Established ^[1][2].
• Pest life-table values: Supported from lab studies, mostly on non-cannabis hosts; only the cannabis aphid data are from hemp ^[12].
• Humidity effects on spider mites and predatory mite eggs: Supported by greenhouse trials and lab data ^[5][22].
• Diapause in Orius and Aphidoletes under short days: Established in the cited studies ^[24][26]; cannabis-specific trials absent.
• Hemp russet mite growth figures: proxy only (tomato russet mite) ^[7].