Budworms on cannabis: corn earworm and tobacco budworm in numbers

Short answer. The "budworm" that hollows out outdoor colas in the United States is mostly the corn earworm, Helicoverpa zea, with the tobacco budworm, Chloridea (formerly Heliothis) virescens, mixed in. A generation needs about 422 Celsius degree-days above 12.6 °C, roughly 34 days at 25 °C. Females lay up to ~1,500 single eggs on flowers. Trap moths from late July, inspect buds weekly in flower, and hit hatchlings with Bt in their first week, before they tunnel in and let bud rot follow.

For the wider caterpillar picture (European corn borer, hemp borer, armyworms, loopers) see Caterpillars on cannabis.

Which budworms attack cannabis?

Two closely related heliothine moths cause nearly all budworm damage on outdoor cannabis and hemp in the United States. The corn earworm (Helicoverpa zea, also called cotton bollworm and tomato fruitworm) is the main one; Virginia Tech surveys found it among the most prevalent and consistent hemp pests in the state ^[2][16]. The tobacco budworm (Chloridea virescens) is listed as a hemp caterpillar in the Pacific Northwest handbook ^[4] and is sometimes mixed into corn earworm populations in the Southeast ^[9].

The two are hard to tell apart as larvae in the field. Both are striped, 25 mm (1 in) when grown, and highly variable in colour; on hemp corn earworm larvae are mostly green ^[1]. Separating them reliably needs a lens on the larval skin or the adult moth, and for cannabis management the difference rarely matters: they lay eggs in the same places and need nearly the same heat to develop ^[5][6].

The practical difference is insecticide resistance in field crops. NC State notes corn earworm is now resistant to pyrethroids in North Carolina, and tobacco budworm has long been resistant to them ^[9]. That matters if you ever rotate classes outdoors.

The glossary entry is at corn earworm; the broader term is caterpillar.

Budworm numbers at a glance

The MOTHZV-2 model (Hartstack et al. 1976, USDA) was built for both species together, which is why its stage values are identical for the two ^[5][6]. UC IPM uses the single-sine method with an intermediate cutoff for it and advises testing any model locally for a season before relying on it ^[5].

A brief often-quoted figure of "about 690 °F degree-days" per generation does not match the models UC IPM lists: egg to adult is 760 °F-DD, and adult to adult 873 °F-DD ^[5].

How long does a budworm generation take at each temperature?

A corn earworm generation takes about 34 days egg to adult at a constant 25 °C (77 °F), and about 39 days adult to adult, using the 422.3 °C-DD and 484.9 °C-DD values above a 12.6 °C threshold ^[5]. Days are degree-days divided by (temperature − 12.6). This is a linear estimate; real development near the threshold is slower and less predictable than the arithmetic suggests.

Computed from the Hartstack et al. (1976) model as listed by UC IPM ^[5]. Not cannabis data: the model was fitted to field data from College Station, Texas.

The 35 °C row is the model's cutoff talking, not biology. In the closest lab study, on the Old World bollworm Helicoverpa armigera, no eggs completed development to adult at constant temperatures outside 17.5-32.5 °C, though alternating day-night regimes widened that to 10-35 °C ^[11]. Hot days with cool nights are survivable; a constant 35 °C is not.

Host plant changes the numbers. Butler (1976) found corn earworm larvae needed 273 °C-DD on corn, 207 on wheat-germ diet and 432 on cotton (with a lower 7.7 °C threshold on cotton) ^[5]. No cannabis-specific development study was found; flowers are a high-quality food, so corn-like values are a reasonable working assumption, not a measured one.

What does the full corn earworm life cycle look like on cannabis?

Corn earworm passes egg, five (sometimes six) larval instars, a pupa in the soil and a night-flying moth ^[1][4]. Eggs are laid singly, about 0.5 mm across, white to pale green, turning darker with ridges as they mature ^[10]. Extension sources give egg hatch as 2-4 days in summer ^[4] to "about a week" ^[1]; the degree-day model gives 3.3 days at 25 °C and 5.5 days at 20 °C ^[5].

Young larvae (first and second instars) are about 2 mm long, dark, with black bristles, and cluster near flowers ^[4][10]. Larvae reach full size, about 25 mm, in two to three weeks ^[1][4]. Mature larvae drop to the soil and pupate; summer moths emerge about two weeks later ^[1].

Corn earworm overwinters as a pupa in the soil where winters allow. Utah State reports pupae survive winter in central and southern Utah but not in northern Utah ^[3]. Northern populations are rebuilt each year by moths migrating north ^[3][10].

In North Carolina, overwintered pupae produce moths in April and May; the first generation develops on whorl-stage corn and wild hosts, the second on corn ears, and second-generation moths leave cornfields from mid-July into early to mid-August, looking for soybean, cotton, peanut and other hosts ^[9]. That mass exodus from maturing corn is the flight that meets early-flowering outdoor cannabis.

Virginia trap data show the same pattern: catches near Blacksburg peaked around mid-August with moths emerging from cornfields and eased by late August ^[8].

Why do temperature and humidity matter for budworms?

Temperature sets how fast budworms cycle: at 30 °C a corn earworm generation takes about 24 days, at 20 °C about 57 days ^[5]. Warm Augusts therefore mean more overlapping larvae in the crop and less time between egg hatch and larvae boring in. At 25 °C the surface-feeding window (egg hatch to end of third instar) is about 6.6 days; at 20 °C it is about 11 days ^[5].

Above roughly 33 °C development slows (the model's upper threshold) ^[5], and for the related H. armigera the intrinsic rate of increase peaked at 27.5 °C (0.147/day) and adult life fell from 34.4 days at 15 °C to 7.6 days at 35 °C ^[11]. Mid-summer heat waves therefore do not make budworms unstoppable; mild, warm late summers are the danger.

Humidity matters mainly through the plant, not the insect. Larval tunnels and frass inside dense colas plus cool, humid autumn nights are what turns a caterpillar wound into Botrytis bud rot ^[2][17]. Outdoors you cannot set humidity, but you can avoid making it worse: no late-day sprays that leave colas wet overnight.

Diapause: corn earworm overwinters as a pupa in the soil ^[1][9], and pupae do not survive northern Utah winters ^[3]. The exact day-length and temperature triggers for hemp-feeding populations have not been studied (no published value found). In practice, warm regions can still see larvae in October while northern plots go quiet once nights cool (Grower practice).

Indoor set-points (flower ~22-26 °C, 45-55% RH) are well inside the comfortable range for budworms. Indoors the defence is exclusion, not climate.

How fast can a budworm problem grow?

A budworm problem grows mostly by immigration, not by local breeding: moths fly in from corn and wild hosts every night of a flight, and each female can lay up to ~1,500 eggs ^[1][9]. A single generation in the crop takes 24-57 days depending on temperature ^[5], so most outdoor cannabis sees one, at most two, local generations between first flowers and harvest.

No intrinsic rate of increase (r_m) has been published for H. zea that we could verify. The closest life-table data are for Helicoverpa armigera: r_m 0.147/day at 27.5 °C constant (lab, artificial diet) ^[11]. Used only as a proxy, the unchecked exponential N(t) = N0 × e^(r_m × t) gives:

Proxy species and lab conditions; not cannabis data ^[11]. This is an upper bound with no predators, no parasitoids and unlimited food.

The real-world lesson is different from mites. Outdoors, one moth flight can put dozens of eggs on a single plant in a week regardless of what bred there last month. What you control is how many eggs hatch into larvae that reach the inside of a cola.

Where do budworms come from and how do they get in?

Budworm moths fly in at night from corn, cotton, soybean, tomato and wild hosts, and from overwintering pupae in nearby soil ^[1][9][10]. Northern regions are repopulated each year by migrating moths ^[3]. Cannabis is a preferred egg-laying site once it flowers: in a Maryland grower trial, moths laid more eggs on hemp than on sweet corn in a two-choice lab test ^[12].

Distance from corn reduces pressure but does not remove it. In the same Maryland trial, hemp rows next to a sweet corn border had the most larvae, and numbers fell from 12 to 60 ft away ^[12]. Virginia trap catches peaked as moths left maturing cornfields ^[8].

Clones are not a vector; eggs and larvae arrive on flowering plants. Indoors and in greenhouses, moths come in through unscreened intakes, roof vents and doors left open at dusk. Light-deprivation greenhouses that flower in August are as exposed as open fields if vents are unscreened (Grower practice).

Moving cut plants is a vector in one sense: harvested branches can carry live larvae into a drying room, where they keep feeding until the bud dries. Shake or inspect branches before hanging (Grower practice).

How do I scout for budworms on cannabis?

Scout outdoor cannabis for budworms weekly from the first pistils to harvest, and twice weekly during a moth flight: inspect the upper third of the colas, bracts and pistils for single pale eggs, 2 mm larvae, small holes and dark frass ^[1][3][10]. A 10x hand lens is enough to see eggs; larvae are visible to the naked eye once past about 5 mm.

Pheromone traps tell you when to look harder. Oregon State's protocol: one Heliothis-type net trap with a corn earworm lure at the field border by late July or early August, maintained through harvest, checked about twice a week (for example Tuesday and Friday), lure changed every two weeks ^[1]. Utah State starts traps in July and checks every 3-5 days ^[3]. Lures attract males only ^[10].

There is no trap-count threshold for hemp ^[3][4][10]. Utah State suggests building your own: record catches each year and use them to identify high-risk weeks ^[3]. Regional networks such as Virginia's county-level corn earworm trap reports can warn you a flight is under way before your own trap catches it ^[18].

Signs inside the bud: a single dead sugar leaf poking out of a cola, a brown or wilted section, webbing-free tunnels with granular frass, or frass dropping onto leaves below. Open suspect buds; damage hides inside ^[1][3].

Action threshold: none published for cannabis or hemp ^[3][4][10]. Grower practice: treat when the trap shows a flight and you find eggs or first-instar larvae on buds, because that is the only stage you can still reach.

Moths are active only at night, so without a trap you will rarely see them ^[1]. Black-light traps catch them too, but also catch many other species.

How do I control budworms at each life stage?

Budworm control works only on eggs and small surface-feeding larvae; once a larva is inside a cola, sprays rarely reach it ^[1][3]. At 25 °C that window is about 3 days of egg plus about 6.6 days of first to third instars ^[5]. Time everything to the flight and the hatch.

How often should budworm treatments be repeated?

Repeat budworm treatments at an interval shorter than the egg stage plus the surface-feeding window, and shorter than the residual of the product, for as long as moths are flying. At 25 °C, eggs hatch in about 3.3 days and larvae stay small for about 6.6 days ^[5], while Bt residue often lasts under seven days in sunlight ^[1].

That arithmetic explains the extension advice. Oregon State suggests spraying likely twice a week during outbreaks ^[1]; Virginia Tech advises reapplying Bt every 2-3 days, early morning or late evening, and notes dead larvae may not show until 48 hours after the first application ^[2]. A weekly interval leaves a gap in which eggs laid just after a spray can hatch and bore in before the next.

At cooler late-season temperatures the window widens: at a 20 °C mean, egg plus instars 1-3 is about 16.5 days ^[5], so the limiting factor becomes product residual, not development.

Which product classes work, and what about resistance?

Rotate budworm controls by mode-of-action class, follow the label and local law, and check what is registered on cannabis or hemp where you grow; registrations vary by state and country and most residues are tested for on cannabis ^[15]. No brands or rates are given here.

• Bt (IRAC 11A). Two subspecies are used: kurstaki and aizawai ^[2]. In Britt and Kuhar's 2019 lab bioassays on hemp seed heads, kurstaki-strain products killed 32.5-35% of field-collected third-instar corn earworm in four days, and an aizawai-plus-kurstaki product 67.5% ^[7]. With lab-reared larvae, kurstaki products killed 60-67.5%. The authors attribute the gap to widespread Cry1Ab resistance in Virginia field populations ^[7].
• Helicoverpa nucleopolyhedrovirus (NPV, IRAC 31). Specific to heliothine larvae. Slow: in the same bioassay it killed 10% within 96 hours, and the authors note it often takes longer than 96 hours on larger larvae ^[7]. Oregon trials found it useful ^[1].
• Spinosyns (IRAC 5). The spinosad product was the only one giving effective control in Virginia's 2019 field trial and killed 95-97.5% in bioassays ^[2][7], but it was not legal on hemp in Virginia at the time ^[2]. Legality is the deciding factor, not efficacy.
• Diamides (IRAC 28). A major field-crop class against heliothines; NC State asked growers to limit its use in soybean to protect its value in cotton ^[9]. We found no cannabis registration data; check locally.
• Pyrethrins and pyrethroids (IRAC 3A). Corn earworm is now pyrethroid-resistant in North Carolina, and tobacco budworm has long been ^[9]. Virginia authors expected pyrethrins to underperform on field larvae for the same reason ^[7].
• Beauveria bassiana (fungal biological). Tested in Virginia field trials; did not give effective control alone ^[2].

Resistance in budworms comes from huge, mobile, multi-generation populations exposed to Bt crops and insecticides across the corn, cotton and soybean belt, not from your garden. You inherit it. The rotation rule still applies: do not lean on one class for an entire flight.

Which natural enemies attack budworms?

Budworms have a large natural-enemy community outdoors: big-eyed bugs, damsel bugs and lady beetles eat eggs and small larvae; Trichogramma wasps parasitize eggs; and the tachinid fly Archytas marmoratus and the braconid wasp Microplitis croceipes parasitize larvae ^[4]. Their role on hemp has not been fully studied ^[4]. Conservation (no broad-spectrum sprays, flowering borders) is the cheapest biological control you have.

Do Trichogramma releases work against corn earworm?

Trichogramma egg-parasitoid releases against corn earworm are supported in corn but unproven in cannabis, and the species matters. In Brazilian corn, three releases of 100,000 Trichogramma pretiosum adults per hectare (about 40,000 per acre) raised parasitism of H. zea eggs up to fivefold and cut ear damage by 26% ^[13].

The species sold for European corn borer, Trichogramma ostriniae, is not a substitute. Cornell reports little or no substantial parasitism of corn earworm eggs by it, an ideal development range of 82-91 °F (28-33 °C), highest parasitism within about 100 ft of release points, and no established rates for hemp ^[14].

Biocontrol supplier technical sheets for Trichogramma generally advise releases timed to the start of egg laying, repeated weekly through the flight (supplier technical sheet, cited generically). Parasitism reduces hatch; it does not stop eggs laid between releases. Wasps are compatible with Bt and NPV, which do not harm adults on contact, but not with broad-spectrum contact insecticides (Grower practice).

Parasitoid and biological control are defined in the glossary.

Can I use the environment against budworms?

Outdoors the environment is mostly out of your hands, so the "environment" levers for budworms are exclusion and timing, not set-points. Exclusion is the strongest tactic: fine insect netting over flowering plants, or screened vents and doors in greenhouses, keeps moths off the buds entirely (Grower practice; strong in principle, little cannabis research).

Timing: Virginia catches peaked in mid-August ^[8] and NC second-generation moths leave corn mid-July to mid-August ^[9]. Light-deprivation scheduling or cultivar choice can move flowering earlier or later than your local peak (Grower practice; check your own trap history, as Utah State advises ^[3]). Earlier-finishing plants also spend fewer late, humid nights with larvae inside.

Site: keep plantings away from corn and from last year's infested ground where possible; larval numbers fell with distance from corn in the Maryland trial ^[12]. Controlling weeds before planting reduces overwintering pupae and alternative food ^[10].

Cultivar: Virginia Tech notes damage depends on the variety grown ^[16]. Looser, airier colas give larvae fewer protected tunnels and dry faster after rain (Grower practice).

Trade-off: netting raises humidity a little and cuts airflow round colas, which matters for Botrytis in a wet autumn. Use the coarsest mesh that still stops moths, and leave the netting off the plant surface.

What should I not do for budworms in flower?

Do not spray broad-spectrum synthetic insecticides on flowering cannabis; most are not registered on cannabis, residues fail testing, and corn earworm is already resistant to pyrethroids in parts of the US ^[9][15]. Follow the label and local law.

Do not spray once at first flower and assume protection; Bt often lasts under a week in sunlight and moths keep arriving for weeks ^[1].

Do not spray Bt on large larvae inside colas and expect a result; it must be eaten, and kurstaki-strain Bt killed only about a third of field-collected third instars in Virginia tests ^[7].

Do not apply sprays late in the day that leave dense colas wet overnight in cool weather; that feeds bud rot ^[17].

Do not leave damaged sections on the plant. Cut out the brown section with a margin of clean tissue and bag it, since larvae and Botrytis spores travel with it ^[17].

Do not hang unchecked branches in the drying room; larvae keep feeding and rot keeps spreading (Grower practice).

What are the common myths about budworms?

• "Bt kills any caterpillar in the bud." Myth. Bt must be eaten; large larvae inside colas eat little treated tissue, and Cry1Ab resistance cuts kurstaki-strain efficacy in Virginia field populations ^[3][7].
• "One spray at the start of flower protects the crop." Myth. Residual is under a week in sunlight and flights last weeks ^[1][2].
• "Pheromone traps control budworms." Myth. Lures catch males only and serve to time scouting ^[1][10].
• "Budworms only hit plants next to corn." Myth. Distance helps, but moths preferred hemp to sweet corn in a Maryland trial and migrants reach isolated plots ^[3][12].
• "Sweet corn borders protect hemp." Contested to unsupported. The only hemp trial found was a grower trial, and it did not support the idea ^[12].
• "Corn earworm and tobacco budworm need different tactics on cannabis." Supported that they do not. Same egg-on-flower habit and near-identical degree-day needs ^[5][6]; resistance differences matter only for synthetic classes ^[9].

Common mistakes

• Starting scouting after brown patches appear; by then larvae are inside.
• Hanging a trap and never recording counts, so next year's timing is guesswork.
• Using one Bt strain all season against populations known to carry Cry1Ab resistance ^[7].
• Spacing treatments a week apart during a peak flight at 25 °C, longer than egg plus small-larva window ^[5].
• Ignoring the rot that follows larvae, and drying infested branches with healthy ones.

Evidence

• Corn earworm biology, fecundity and lack of hemp thresholds: Established by multiple extension services ^{[1][3][4][10]}.
• Degree-day thresholds and stage requirements: Established for cotton, corn and diet; not validated on cannabis ^[5][6].
• Bt, NPV and spinosyn efficacy on hemp: Supported by Virginia field trial and lab bioassays ^[2][7].
• Cry1Ab resistance reducing Bt kill: Supported, author interpretation in a bioassay report ^[7].
• Trichogramma for corn earworm: Supported in corn ^[13]; untested in cannabis.
• Exclusion netting and light-dep timing: Grower practice.
• Trap cropping with sweet corn: Contested; one negative grower trial ^[12].

Glossary terms used here

• Glossary: degree-day, developmental threshold, Bacillus thuringiensis, parasitoid, mode of action, action threshold