# Caterpillars on cannabis: species, degree-days and control

> Caterpillars on cannabis are mostly an outdoor, late-season problem: corn earworm budworms bore into colas and start bud rot, European corn borer and Eurasian hemp borer tunnel stems and buds, and armyworms and loopers chew leaves. Each species has a degree-day clock (lower thresholds about 10-14 C), so trap the moths, scout for eggs and hatchlings, exclude moths with netting, and use Bt or other biologicals on newly hatched larvae before they bore in.

- Canonical: https://genetixbay.com/knowledge/pests/caterpillars.html
- Category: Pests · Level: Intermediate
- Evidence: Supported · Status: Draft: awaiting expert review
- Stage: Veg, Transition, Flower, Ripening · Systems: Soil, Living soil, Any system
- Updated: 2026-10-31 · Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay Knowledge Base (genetixbay.com/knowledge)"

## Questions and answers

**What caterpillars attack cannabis?** The main caterpillars on outdoor cannabis and [hemp](https://genetixbay.com/knowledge/glossary/hemp.md) in North America are the [corn earworm](https://genetixbay.com/knowledge/glossary/corn-earworm.md) (Helicoverpa zea), which bores into buds, the European corn borer (Ostrinia nubilalis) and Eurasian hemp borer (Grapholita delineana), which tunnel stems and buds, and leaf-chewers such as beet armyworm and cabbage looper. Tobacco budworm joins corn earworm in the Southeast. Indoors, caterpillars are uncommon and arrive through open vents or doors.

**Why is there brown mush inside my [cola](https://genetixbay.com/knowledge/glossary/cola.md)?** Brown mush inside a cannabis cola is [bud rot](https://genetixbay.com/knowledge/glossary/botrytis.md), and if dark [frass](https://genetixbay.com/knowledge/glossary/frass.md) pellets and tunnels are present a caterpillar started it, usually a corn earworm budworm. Larvae bore into the bud and their wounds and frass let Botrytis in, so the loss is often more rot than chewing. Rot without frass is plain bud rot. Cut out the damaged section with a margin and bag it.

**When is Bt most effective on caterpillars?** Bt ([Bacillus thuringiensis](https://genetixbay.com/knowledge/glossary/bacillus-thuringiensis.md)) is most effective on young caterpillars that are still feeding on the plant surface, because it must be eaten. Utah State advises using it on corn earworm larvae under about 12 mm (0.5 in), and stem borers such as European corn borer and hemp borer are exposed only for days after hatching. Bt often lasts under seven days in sunlight, so repeat it during flights.

**What is a [degree-day](https://genetixbay.com/knowledge/glossary/degree-day.md) model for caterpillars?** A degree-day model adds up heat above a species' lower developmental threshold to predict hatch, larval stages and moth flights. Corn earworm needs about 422 Celsius degree-days above 12.6 C from egg to adult, cabbage looper about 440 above 10 C, and beet armyworm about 490 above 12.2 C. European corn borer flights in Vermont start near 374 Fahrenheit degree-days, base 50 F.

**How fast do caterpillars develop on cannabis?** At a steady 25 C, corn earworm takes about 34 days from egg to adult, beet armyworm about 27 days on sugar beet in the lab, and cabbage looper about 29 days, based on UC [IPM](https://genetixbay.com/knowledge/glossary/ipm.md) degree-day models and published life tables. At 20 C the same generations take roughly 44 to 63 days. These are crop and lab figures; no cannabis-specific development study has been published.

**When should I spray for European corn borer in hemp?** For European corn borer in hemp, University of Vermont Extension advises targeting newly hatched caterpillars before they bore into stems, typically mid-June in the Northeast, and suggests treating if 15% or more of plants show larvae or fresh feeding, rechecking after 4 to 7 days. Egg laying begins at about 450 Fahrenheit degree-days, base 50 F. Follow the label and local law.

**Do Trichogramma wasps work on cannabis caterpillars?** Trichogramma egg [parasitoids](https://genetixbay.com/knowledge/glossary/parasitoid.md) work against European corn borer in corn, where Cornell and UConn trials with Trichogramma ostriniae at 30,000 to 120,000 wasps per acre gave 65 to 100% egg mass parasitism. That species gives little control of corn earworm, which needs Trichogramma pretiosum. No release rates have been established for hemp, so treat it as promising but unproven in cannabis.

**Do I need to spray loopers and armyworms on cannabis?** Usually not. The Pacific Northwest Insect Management Handbook says damage from caterpillars other than corn earworm, such as cabbage looper and alfalfa looper, is usually minor on mature hemp and generally does not require control. Loopers and armyworms chew ragged holes in leaves; a healthy plant tolerates moderate leaf loss. Act when young plants are being stripped or larvae move into flowers.

**Can netting keep caterpillars off cannabis?** Yes. Fine insect netting over outdoor plants, or screened vents and doors on greenhouses, keeps the egg-laying moths off cannabis entirely, which no spray can match. It is grower practice with little cannabis-specific research, but strong in principle because every caterpillar starts as an egg laid by a flying moth. Fit netting before the first flowers and before local moth flights start.

**Is there a treatment threshold for caterpillars on hemp?** There is no established treatment threshold for corn earworm in hemp, according to Colorado State, Utah State and the Pacific Northwest handbook. For European corn borer, University of Vermont Extension suggests a working threshold of 15% of plants with larvae or fresh feeding. For other caterpillars, growers act on what they find while scouting, combined with [pheromone trap](https://genetixbay.com/knowledge/glossary/pheromone-trap.md) counts that show moth flights.

**Short answer.** If a cola has a brown, dead patch with dark crumbs (frass) inside, a caterpillar bored in and bud rot followed; outdoors in North America it is usually the corn earworm. Stem borers (European corn borer, Eurasian hemp borer) wilt and swell stems; loopers and armyworms chew leaves. All of them start as eggs laid by night-flying moths, all run on a degree-day clock, and all are only reachable for a few days after hatch. Trap the moths, scout for eggs and hatchlings, net the plants where you can, and use Bt on small larvae.

The main bud-boring species has its own deep article: [Budworms: corn earworm and tobacco budworm](https://genetixbay.com/knowledge/pests/budworms-corn-earworm.md).

## Which caterpillars attack cannabis?

Five groups of caterpillars matter on cannabis and hemp in North America, and they split by where they feed. Bud borers ruin flowers; stem borers weaken and wilt shoots; leaf chewers mostly cost leaf area.

| Species | Family | Feeds on | Importance on cannabis | Source |
|---|---|---|---|---|
| Corn earworm, *Helicoverpa zea* | Noctuidae | Flowers, inside colas; seed heads | Key outdoor pest in the US; bud rot follows | [1][2][3] |
| Tobacco budworm, *Chloridea (Heliothis) virescens* | Noctuidae | Flowers | Listed for hemp; mixed with corn earworm in the Southeast | [6] |
| European corn borer, *Ostrinia nubilalis* | Crambidae | Stems, petioles, buds | Stem breakage and wilt, worst near corn; Northeast and Midwest | [8] |
| Eurasian hemp borer, *Grapholita delineana* | Tortricidae | Stems and flower buds | Specialist on hemp; eastern Colorado and elsewhere; late-summer generation most damaging | [7] |
| Beet armyworm, *Spodoptera exigua* | Noctuidae | Leaves, sometimes flowers | Leaf chewer of warm regions (no hemp-specific damage data found) | [10][12] |
| Cabbage looper, *Trichoplusia ni*; alfalfa looper | Noctuidae | Leaves | Usually minor on mature hemp; generally no control needed | [6] |
| Others: bertha armyworm, spotted cutworm, false corn earworm | Noctuidae | Leaves, flower buds | False corn earworm recently found on buds in southern Oregon | [6] |

Indoors, caterpillars are uncommon. They arrive as moths through unscreened intakes, roof vents or doors left open at dusk, or as larvae on branches brought in from outside.

The glossary entry is [caterpillar](https://genetixbay.com/knowledge/glossary/caterpillar.md); for the main budworm see [corn earworm](https://genetixbay.com/knowledge/glossary/corn-earworm.md).

## How do I recognise caterpillar damage on cannabis?

Caterpillar damage on cannabis shows as chewed holes, frass (dark, granular droppings) and dying tissue, and the pattern tells you which group you have.

- **Bud borers (corn earworm, tobacco budworm).** Brown, wilted or dead sections of a cola, sometimes a single dead sugar leaf poking out of a bud; dark frass inside the bud and on leaves below; larvae up to 25 mm, mostly green on hemp, striped along the body [1][3].
- **European corn borer.** Bored holes and stem wounds, sawdust-like frass at the hole, wilting above the entry; larvae light grey to faint pink with a dark head and dark spots, up to about 25 mm [8].
- **Eurasian hemp borer.** Stem swelling, wilting, dieback and protruding frass; late larvae red-orange and small, 6-8 mm when grown [7].
- **Loopers and armyworms.** Ragged holes and edges in leaves [6]; loopers walk with a looping arch. Leaf feeding rarely reaches buds.

Differential: frass and tunnels mean caterpillar. Rot without frass is plain [bud rot](https://genetixbay.com/knowledge/diseases/botrytis-bud-rot.md). Wilting with no hole may be a root or vascular problem; see [How to diagnose any problem](https://genetixbay.com/knowledge/diagnosis/how-to-diagnose-any-problem.md).

## Caterpillar life cycles in numbers

Every caterpillar species on cannabis has a lower developmental threshold between about 10 and 14 °C and needs roughly 340-550 Celsius degree-days to go from egg to adult. Below are the best-documented values. None were measured on cannabis.

| Parameter | Corn earworm | European corn borer | Beet armyworm | Cabbage looper | Eurasian hemp borer |
|---|---|---|---|---|---|
| Lower threshold | 12.6 °C [9] | 10 °C (50 °F) [8][19] | 12.2 °C (field model) [10]; 13.1 °C (r_m model) [12] | 10.0 °C [11] | no published value found |
| Upper threshold | 33.3 °C [9] | no value verified | 32.2 °C (r_m model) [12] | 32.2 °C [11] | no published value found |
| Egg to adult | 422.3 °C-DD [9] | ~556 °C-DD egg-to-egg (derived, see below) [8] | 490 °C-DD females, 543 males [10] | 439.7 °C-DD [11] | no published value found |
| Egg to adult at 25 °C | ~34 days (model) | no value verified | 27.2 days (lab, sugar beet) [12] | ~29 days (model) | no published value found |
| Egg to adult at 30 °C | ~24 days (model) | no value verified | 16.9 days (lab) [12] | ~22 days (model) | no published value found |
| Eggs per female | up to 25-35/day; ~1,500 lifetime [1][6] | 200-500 over 2-3 weeks [8] | R0 377.7 female offspring at 25 °C [12] | no value verified | several hundred over a couple of weeks [7] |
| r_m (/day) | no published value for *H. zea*; *H. armigera* 0.147 at 27.5 °C [13] | no value verified | 0.149 at 20 °C; 0.276 at 30 °C [12] | no value verified | no published value found |
| Mean generation time T | no value verified | no value verified | 35.9 days at 20 °C; 15.1 days at 33 °C [12] | no value verified | no published value found |
| Generations per year | 2-4 (Utah) [3] | 1-2 (Northeast), 3rd possible in Kentucky [8][19] | no value verified | no value verified | multiple, most damaging in late summer (Colorado) [7] |
| Overwinters as | pupa in soil [1] | larva in crop debris [8] | no value verified | no value verified | grown larva in stems and seed-head leaf folds [7] |

Sources and conditions: corn earworm from the USDA MOTHZV-2 field model (Texas) [9]; beet armyworm from a Californian cotton field model [10] and a lab life table on sugar beet leaves (Iran) [12]; cabbage looper from a lab artificial-diet model (Riverside, California) [11]; European corn borer from Vermont field degree-days [8].

The European corn borer egg-to-egg figure is derived, not measured: first eggs at about 450 °F-DD and second-generation first eggs at about 1,450 °F-DD (base 50 °F) give ~1,000 °F-DD, or ~556 °C-DD, per generation in Vermont [8].

Beet armyworm's two thresholds are not a contradiction: 12.2 °C is the developmental base of a field flight model [10]; 13.1 °C is where the fitted population growth rate reaches zero [12].

## How do degree-day models time caterpillar flights?

A degree-day model sums daily heat above a species' lower threshold, and stage transitions happen at fixed totals. For European corn borer in Vermont (base 50 °F, accumulated from early spring), first moths fly at about 374 °F-DD, egg laying begins at about 450 and peak flight is about 631; the second flight starts around 1,400, with eggs at 1,450 and hatch at 1,550 [8].

Corn earworm stage requirements above 12.6 °C are 40.5 °C-DD for eggs, 81.7 for small larvae ([instars](https://genetixbay.com/knowledge/glossary/instar.md) 1-3), 120.6 for large larvae, 179.5 for pupae and 62.6 for pre-oviposition adults [9]. Add up to the stage you care about. The useful one for spraying is egg plus small larvae: about 122 °C-DD, roughly 10 days at 25 °C.

Calculation method matters. UC IPM uses the single-sine method with stated cutoffs; Kentucky's corn borer model counts degrees above 50 °F per 24 h from 1 January [9][19]. Use the method the model was built with, from a weather station near your field.

Caveats: models were built on corn, cotton, beet or artificial diet; UC IPM advises testing any model for a season or more locally before relying on it [9]. Corn borer distributions are patchy, so regional data should not replace your own trap [8]. Migrant species (corn earworm in the north) arrive on the wind, so a degree-day clock started on 1 January predicts the local generation after they arrive, not the arrival itself.

See [degree-day](https://genetixbay.com/knowledge/glossary/degree-day.md) and [developmental threshold](https://genetixbay.com/knowledge/glossary/developmental-threshold.md).

## Why do temperature and humidity matter for caterpillars?

Temperature controls how fast caterpillars cycle and how long the vulnerable surface-feeding window lasts. Beet armyworm on sugar beet leaves took 27.2 days from egg to adult at 25 °C and 16.9 days at 30 °C; its intrinsic rate of increase rose from 0.149/day at 20 °C to 0.276/day at 30 °C; no development occurred at 12 or 36 °C [12].

Heat speeds things up only to a point. For *Helicoverpa armigera*, the closest well-studied relative of corn earworm, r_m peaked at 27.5 °C (0.147/day), constant temperatures outside 17.5-32.5 °C prevented development to adult, and adult life fell from 34.4 days at 15 °C to 7.6 days at 35 °C [13]. Alternating day-night temperatures widened the survivable range to 10-35 °C, which is why field populations tolerate hot afternoons [13].

Humidity mostly works through the plant. Larval frass and tunnels inside dense colas, plus wet, cool autumn nights, turn caterpillar wounds into Botrytis bud rot [2][5]. Indoor flower set-points (22-26 °C, 45-55% RH, 40-50% in late flower) are comfortable for caterpillars, so indoors you exclude them rather than climate them out.

Diapause decides who survives winter. Corn earworm overwinters as a pupa in soil only where winters are mild (central and southern Utah, not northern) [3]; European corn borer and hemp borer overwinter as larvae in plant residue [7][8]. That is why residue removal works on the borers and not on migrant earworms.

## How fast can a caterpillar population grow?

A caterpillar population on cannabis grows mainly by moths flying in, not by local generations, because one generation takes 17-57 days depending on species and temperature [9][12]. Still, life-table r_m values show why warm late summers are dangerous.

Method: N(t) = N0 × e^(r_m × t), [doubling time](https://genetixbay.com/knowledge/glossary/doubling-time.md) = ln 2 / r_m. Start with 10 beet armyworm eggs on sugar beet leaves (lab, constant temperature) [12].

| Temperature | r_m (/day) | Doubling time | After 21 days | After 42 days |
|---|---|---|---|---|
| 20 °C (68 °F) | 0.149 | 4.7 days | ~229 | ~5,200 |
| 30 °C (86 °F) | 0.276 | 2.5 days | ~3,300 | ~1,080,000 |
| 27.5 °C (*H. armigera* proxy for corn earworm) | 0.147 [13] | 4.7 days | ~219 | ~4,800 |

This is the unchecked exponential upper bound: no predators, parasitoids or disease, unlimited food, and a population already at stable age distribution. Real outdoor numbers are far lower.

The practical point: 10 °C of extra warmth roughly halves beet armyworm doubling time. In flower, what counts is not the 42-day projection but how many eggs hatch into larvae that reach the inside of a bud before you find them.

## Where do caterpillars come from and how do they get in?

Caterpillars on cannabis come from eggs laid by moths that fly in at night, from corn, weeds, vegetable crops and wild or feral hemp. Corn earworm migrates north each year and leaves maturing corn in large flights; Virginia catches peaked around mid-August with moths emerging from cornfields [3][20]. European corn borer may shift to other hosts when corn planting is delayed, and crops near corn are at higher risk [8].

Eurasian hemp borer is a weak flier, so heavy infestations appear near wild hemp or fields grown to hemp in previous years, and infestations usually begin at field edges [7]. Drying sheds can act as overwintering sites; CSU advises siting them at least half a mile from crops [7].

Clones are not the usual vector, but harvested or transplanted material can be. Larvae ride on branches into [drying rooms](https://genetixbay.com/knowledge/glossary/drying-room.md) and keep feeding (Grower practice). Hemp borer larvae overwinter in stems and seed-head leaf folds, so moving residue moves them [7].

Indoors and in greenhouses, the entry points are unscreened vents and doors during late-summer flights. Lit greenhouses at night attract moths (Grower practice).

## How do I scout for caterpillars?

Scout cannabis for caterpillars weekly from early flower to harvest, and twice weekly during moth flights. Check the upper third of colas, bracts and pistils for single pale eggs, 2 mm hatchlings, small holes and frass [1][3]. Check stems for swelling, entry holes and sawdust-like frass, and leaf top and underside on low and high parts of the plant for borer eggs [7][8]. A 10x lens shows eggs; larvae are visible by eye.

### How do pheromone traps help with caterpillars?

Pheromone traps catch male moths of one species and tell you when a flight has started, so you know when to scout harder and when to time Bt. They do not control anything [1][8].

- **Corn earworm:** one Heliothis-type net trap with a corn earworm lure at the field border by late July or early August; check twice weekly; change the lure every two weeks; keep it to harvest [1]. Utah State starts in July and checks every 3-5 days [3].
- **European corn borer:** two Heliothis-type net traps at least 50 ft apart along the field edge, the trap bottom above grassy weeds and no more than about 4 in above the vegetation, not over bare ground. Bait one with each strain's lure (New York and Iowa strains occur in New England). Check once or twice a week and change lures every other week [8].
- **Eurasian hemp borer:** CSU does not list a trap; it suggests scouting for moths, sweep netting, and black lights [7].

Light traps (black-light traps) catch adults of many moth species at once. They are useful for monitoring and confirming activity but costly and unselective [7]; they are not a control.

### What are the action thresholds for caterpillars?

There is no established treatment threshold for corn earworm in hemp [3][6]. For European corn borer, University of Vermont Extension suggests treating if 15% or more of plants have larvae or fresh feeding damage, and treating again if infestation is still above 15% after 4-7 days [8]. That figure is a working extension threshold, not a cannabis economic threshold. For loopers and armyworms on mature plants, the PNW handbook says control is generally not needed [6].

See [action threshold](https://genetixbay.com/knowledge/glossary/action-threshold.md), [scouting](https://genetixbay.com/knowledge/glossary/scouting.md) and [sticky trap](https://genetixbay.com/knowledge/glossary/sticky-trap.md).

## How do I control caterpillars at each life stage?

Caterpillar control is a race to the hatchling: eggs and first instars on the plant surface are reachable, larvae inside stems or colas mostly are not [3][7][8]. Match the tactic to the stage.

| Stage | What works | Notes |
|---|---|---|
| Egg | *Trichogramma* egg parasitoids; generalist predators; hand removal of egg masses (corn borer) | Species-matched wasps only [15][17] |
| Newly hatched larva on surface | Bt (IRAC 11A), NPV for Helicoverpa (IRAC 31), spinosyns (IRAC 5) where registered | Must be eaten; full bud and stem coverage [2][14] |
| Larva inside stem or bud | Cut out and bag; hand-pick bud borers at dusk or dawn | Sprays largely fail once inside [7][8] |
| Overwintering larva/pupa | Remove and destroy residue; tillage where allowed | Strong for borers, weak for migrant earworm [7][8] |
| Moth | Exclusion netting, screened vents; traps for timing | [1] |

### Why does Bt work best on young larvae?

Bt (*Bacillus thuringiensis*) is a stomach poison: the larva must eat a crystal protein toxin; IRAC classes it as a microbial disruptor of insect midgut membranes [18]. It therefore works best on young caterpillars that feed on treated surfaces, and poorly on large larvae that feed inside tissue [3][14]. Utah State advises Bt only on corn earworm under about 12 mm (0.5 in) [3].

The instar window is short. For corn earworm, first to third instars take about 82 °C-DD above 12.6 °C, about 6.6 days at 25 °C [9]. European corn borer larvae feed on leaves for about a week before boring into stems [8]. Eurasian hemp borer larvae are exposed only briefly after hatching, so CSU says applications must be timed to egg hatch [7].

Strain matters. In Virginia lab bioassays on hemp, kurstaki-strain Bt killed 32.5-35% of field-collected third-instar corn earworm in four days and an aizawai-plus-kurstaki product 67.5%; the authors blame widespread Cry1Ab resistance in field populations [14]. For corn earworm see the [budworm article](https://genetixbay.com/knowledge/pests/budworms-corn-earworm.md).

Residual is short: Bt often lasts under seven days on plants in sunlight [1]. Virginia Tech advises reapplying every 2-3 days, early morning or late evening, during heavy pressure [2]. Glossary: [Bacillus thuringiensis](https://genetixbay.com/knowledge/glossary/bacillus-thuringiensis.md).

### How often should caterpillar treatments be repeated?

Repeat caterpillar treatments at an interval shorter than egg hatch time plus the surface-feeding window, and within the product's residual, for as long as moths are flying. For corn earworm at 25 °C that is egg about 3.3 days plus small larvae about 6.6 days [9], against Bt residual under seven days [1]: twice weekly in a flight, as Oregon State suggests [1].

For European corn borer, Vermont's 15% re-check after 4-7 days [8] works because larvae spend about a week feeding on leaves before boring in.

At cooler late-season temperatures, development slows (corn earworm egg plus small larvae is about 16 days at 20 °C) [9], so product breakdown, not hatching, sets the interval.

### Which product classes are used, and how do I rotate?

Rotate caterpillar controls by IRAC mode-of-action group, never repeat one group through a whole flight, and use only what is registered on cannabis or hemp where you grow. Follow the label and local law. No brands or rates are given here [18].

| Class | IRAC group | Notes on cannabis |
|---|---|---|
| *Bacillus thuringiensis* (kurstaki, aizawai) | 11A | Most widely permitted biological on hemp; short residual; strain choice matters [2][14] |
| Baculoviruses (Helicoverpa NPV) | 31 | Species-specific; slow kill on large larvae [14] |
| Spinosyns | 5 | Best kill in Virginia tests but not legal on hemp in Virginia at the time [2][14] |
| Diamides | 28 | Major field-crop class against caterpillars; no cannabis registration data found |
| Pyrethrins / pyrethroids | 3A | Resistance widespread in corn earworm in parts of the US (see budworm article) |
| Azadirachtin (neem extract) | UN | Unknown or uncertain mode of action per IRAC [18] |

The *[Beauveria bassiana](https://genetixbay.com/knowledge/glossary/beauveria-bassiana.md)* fungus was tested on hemp in Virginia and did not give effective control of corn earworm alone [2].

Resistance in caterpillars is inherited from field-crop selection on huge, mobile populations. A home or craft grower cannot create much of it, but can waste a season using a class the local population already shrugs off. Glossary: [mode of action](https://genetixbay.com/knowledge/glossary/mode-of-action.md), [pesticide resistance](https://genetixbay.com/knowledge/glossary/pesticide-resistance.md).

## Which natural enemies control caterpillars?

Caterpillars on outdoor cannabis are attacked by big-eyed bugs, damsel bugs and lady beetles (eggs and small larvae), *Trichogramma* egg parasitoids, and larval parasitoids such as the tachinid fly *Archytas marmoratus* and the braconid wasp *Microplitis croceipes*; their role on hemp is not fully investigated [6]. Spiders, lacewings and birds add to it. Broad-spectrum sprays remove all of them.

### How do Trichogramma releases work?

*Trichogramma* wasps lay their eggs inside moth eggs, preventing them from hatching [8]. They are bought as parasitized host eggs on cards and released when moths begin laying, within a week of the start of the flight [8]. They do not overwinter well and must be released each season [15].

For European corn borer, *Trichogramma ostriniae* has the best data. Cornell lists 30,000-60,000 wasps per acre over three weekly releases in sweet corn and 100,000-120,000 per acre in peppers and potatoes; there are no established rates for hops or hemp [15]. New York trials (UConn summary) gave 80-100% parasitism in most inundatively released fields at 120,000 per acre, and 65% average egg-mass parasitism from a single [inoculative release](https://genetixbay.com/knowledge/glossary/preventive-release.md) of 30,000 per acre [16].

Dispersal and temperature: highest parasitism within about 100 ft of release points, so release points about 100 ft apart; ideal development 82-91 °F (28-33 °C); reduced parasitism in prolonged cool, rainy weather [15]. Wasps moved up to 340 ft and persisted up to 30 days in New York trials [16].

Species matter. *T. ostriniae* gives little or no substantial parasitism of corn earworm eggs [15]. For corn earworm, three releases of 100,000 *T. pretiosum* per hectare in Brazilian corn raised egg parasitism up to fivefold and cut ear damage by 26% [17].

Compatibility: Bt and NPV are compatible with *Trichogramma* (Grower practice; supplier technical sheets agree). Broad-spectrum [contact insecticides](https://genetixbay.com/knowledge/glossary/contact-vs-systemic.md) are not. Glossary: [parasitoid](https://genetixbay.com/knowledge/glossary/parasitoid.md), [biological control](https://genetixbay.com/knowledge/glossary/biological-control.md).

## Can I use the environment against caterpillars?

The most powerful environmental tool against caterpillars is a physical barrier: fine insect netting outdoors, or screened vents and doors on greenhouses and light-dep houses, keeps egg-laying moths off the plants entirely (Grower practice; strong in principle, little cannabis research). Fit it before flowering and before local flights.

Timing: plant date, transplant timing and light-deprivation scheduling can move flowering away from the local peak flight. Transplanting early lets seedlings harden before pests arrive [8]. Your own trap records are the best guide [3].

Site and sanitation: plant away from corn and from last year's hemp ground; remove and destroy stems and stalks after harvest to kill overwintering hemp borer and corn borer larvae; remove wild hemp nearby; work residue into soil where allowed [7][8]. Keep weeds down, since they host first-generation borers and leaf-feeders [8].

Climate indoors: caterpillars tolerate any set-point cannabis tolerates. Indoors, the levers are screens, door discipline and quarantine of outdoor material, not temperature or humidity.

Trade-off: netting slightly raises humidity and reduces airflow around colas, which matters for Botrytis in wet autumns. Keep netting off the plant surface and open it on dry, sunny days if pressure allows.

## What should I not do for caterpillars in flower?

Do not use broad-spectrum synthetic insecticides on flowering cannabis; most are not registered on cannabis and residues fail testing. Follow the label and local law.

Do not spray once and assume protection; Bt breaks down within days in sunlight [1].

Do not spray Bt at large larvae inside colas or stems and expect a kill; it must be eaten [3].

Do not spray late in the day in a way that leaves buds wet overnight in cool weather; that feeds bud rot [5].

Do not leave damaged bud sections on the plant; cut a margin of clean tissue around them and bag them [5].

Do not leave infested stalks in or near the field over winter; that is where borers wait for spring [7][8].

## What are the common myths about caterpillars on cannabis?

- **"Bt kills caterpillars on contact."** *Myth.* Bt must be eaten [3].
- **"Caterpillars are only an outdoor problem."** *Myth.* Greenhouses with open vents and indoor rooms with unscreened intakes get them (Grower practice).
- **"Pheromone traps control moths."** *Myth.* They attract males only and serve for timing [1][8].
- **"Leaf holes in veg mean a ruined harvest."** *Myth.* Looper and armyworm feeding on mature hemp usually needs no control [6].
- **"Removing crop debris reduces next year's borers."** *Established.* Hemp borer and corn borer overwinter as larvae in residue [7][8].
- **"Trichogramma works on every caterpillar."** *Myth.* Species matching matters; *T. ostriniae* barely touches corn earworm [15].

## Common mistakes

- Waiting until buds rot to start scouting.
- Spraying Bt on large larvae already inside buds or stems.
- Buying the corn borer *Trichogramma* species for a corn earworm problem.
- Running a degree-day model from a distant weather station or with the wrong calculation method.
- Leaving last year's hemp stalks at the field edge.
- Ignoring the rot that follows larvae and letting it spread in the drying room.

## Evidence

- Species list and hemp damage: *Established* from multiple extension sources [1][3][6][7][8].
- Degree-day thresholds and requirements: *Established* on other crops and diets; *not validated on cannabis* [9][10][11].
- Life-table r_m values: *Supported*, lab data on sugar beet and diet [12][13].
- Bt, NPV and spinosyn efficacy on hemp: *Supported* by Virginia trials and bioassays [2][14].
- European corn borer 15% threshold: *Grower practice / extension guidance*, not a cannabis economic threshold [8].
- *Trichogramma*: *Established* for corn borer in sweet corn [15][16]; *Supported* for corn earworm in corn [17]; *untested* in cannabis.
- Exclusion netting: *Grower practice*; little cannabis-specific research.

## Glossary terms used here

- Glossary: [degree-day](https://genetixbay.com/knowledge/glossary/degree-day.md), [intrinsic rate of increase](https://genetixbay.com/knowledge/glossary/intrinsic-rate-of-increase.md), [doubling time](https://genetixbay.com/knowledge/glossary/doubling-time.md), [diapause](https://genetixbay.com/knowledge/glossary/diapause.md), [parasitoid](https://genetixbay.com/knowledge/glossary/parasitoid.md), [IPM](https://genetixbay.com/knowledge/glossary/ipm.md)

## Quick answers

- [What is eating holes in my leaves?](https://genetixbay.com/knowledge/answers/holes-in-leaves.md): Holes in cannabis leaves are usually chewing pests, and the clue left behind names them.

## Sources

1. Shrestha, G. 2022. Corn earworm management in hemp (EM 9626). Oregon State University Extension. https://extension.oregonstate.edu/catalog/em-9626-corn-earworm-management-hemp
2. Kuhar, T., Britt, K., Doughty, H. 2019. Corn earworm pest problems on hemp and results of recent insecticide tests. Virginia Tech. https://ag-pest-advisory.vaes.vt.edu/corn-earworm-pest-problems-on-hemp-and-results-of-recent-insecticide-tests/
3. Utah State University Extension. Corn Earworm (hemp). https://extension.usu.edu/planthealth/ipm/notes_ag/hemp-corn-earworm
4. Cranshaw, W. 2020. Hemp pests (CSU talk). https://webdoc.agsci.colostate.edu/bspm/InsectInformation/Talks2020/HempPests.pdf
5. Utah State University Extension. Hemp bud rot. https://extension.usu.edu/planthealth/ipm/notes_ag/hemp-bud-rot
6. Pacific Northwest Insect Management Handbook. Hemp: caterpillars. https://pnwhandbooks.org/insect/agronomic/hemp/hemp-caterpillars
7. Colorado State University. Eurasian hemp borer, Grapholita delineana. https://agsci.colostate.edu/agbio/ipm-pests/eurasian-hemp-borer
8. University of Vermont Extension (SARE LNE16-348, ONE19-333). The European corn borer in hops and hemp. https://projects.sare.org/media/pdf/E/u/r/European-Corn-Borer-Hemp-Hops-factsheet-FINAL.pdf
9. UC IPM. Corn earworm phenology models. https://ipm.ucanr.edu/weather/phenology-models-description/corn-earworm-cotton-bollworm-tomato-fruitworm/
10. UC IPM. Beet armyworm phenology model (Hogg & Gutierrez 1980). https://ipm.ucanr.edu/PHENOLOGY/ma-beet_armyworm.html
11. UC IPM. Cabbage looper phenology models (Toba et al. 1973; Butler et al. 1975). https://ipm.ucanr.edu/weather/phenology-models-description/cabbage-looper/
12. Karimi-Malati, A., Fathipour, Y., Talebi, A.A., Bazoubandi, M. 2014. Life table parameters and survivorship of Spodoptera exigua at constant temperatures. Environmental Entomology 43(3): 795-803. https://doi.org/10.1603/EN11272
13. Mironidis, G.K., Savopoulou-Soultani, M. 2008. Development, survivorship, and reproduction of Helicoverpa armigera under constant and alternating temperatures. Environmental Entomology 37(1): 16-28. https://academic.oup.com/ee/article/37/1/16/547157
14. Britt, K.E., Kuhar, T.P. 2020. Laboratory bioassays of biological/organic insecticides to control corn earworm on hemp in Virginia, 2019. Arthropod Management Tests 45(1): tsaa102. https://doi.org/10.1093/amt/tsaa102
15. Cornell IPM. Trichogramma ostriniae biocontrol agent fact sheet. https://cals.cornell.edu/integrated-pest-management/outreach-education/fact-sheets/trichogramma-ostriniae-biocontrol-agent-factsheet
16. Seaman, A. 1999 (rev. 2012). Using Trichogramma and Bt for European corn borer control. UConn IPM. https://ipm.cahnr.uconn.edu/using-trichogramma-and-bt-for-european-corn-borer-control
17. Nogueira de Sá, L.A., Parra, J.R.P. 1993. Releases of Trichogramma pretiosum against Helicoverpa zea in corn. Scientia Agricola 50(3): 355-359. https://revistas.usp.br/sa/article/view/20130
18. IRAC. Mode of Action Classification (online). https://irac-online.org/mode-of-action/classification-online/
19. Bessin, R. Predicting European corn borer development (ENTFACT-106). University of Kentucky. https://entomology.mgcafe.uky.edu/ef106
20. Kuhar, T., Britt, K. 2020. Corn earworm moth trap catch around Blacksburg, VA. Virginia Tech. https://ag-pest-advisory.vaes.vt.edu/?p=5110