One of the most destructive pests in Cannabis, especially outdoor cultivation, are the various budworm moths in the genus Helicoverpa and closely related. Long maligned for their annual global agricultural damage of billions of dollars, there are three main species with functionally similar physiology and damage: the Corn Earworm (Helicoverpa zea), the Cotton Bollworm (Helicoverpa armigera), and the Tobacco Budworm (Chloridea virescens).
In all cases, these three species deposit eggs on the floral tissue of a host plant—like Cannabis—from which larvae quickly hatch and burrow. This tunneling wounds the plant’s flower structure and fecal pellets from the larva can fester, fouling the final product and exacerbating the total damage inflicted on the plant.
Due to the delicate nature of floral Cannabis maintenance, any measures that even mildly contact trichomes can be a contentious prospect, even costly. Yet yield losses are often massive if the moths are left unchallenged. Fortunately there are strategies for prevention and treatment that can minimize damage.
Cotton Bollworm Larva (Helicoverpa armigera)
Paolo Mazzei, Bugwood.org
old world bollworm, cotton bollworm, Helicoverpa armigera
Adult
Paolo Mazzei, Bugwood.org
Detection
Female budworm moths have been documented laying multiple hundreds of eggs, and their emergence in warmer months can be followed by multiple generations that burrow into flowers, feed, burrow into the ground, pupate, and then emerge as adults. These generations extend into late summer or autumn depending on one’s location and to what degree it stays warm. In North America, overwintering as immobile pupae generally occurs south of the N40° latitudinal line.
Historically, more northern regions do not support these pupae and many die, though changes in climate may increase habitable zones. Adult moths have been documented to travel more than 750 kilometers distance at altitudes of 30 meters, using wind currents to guide their dispersal.
To gauge when moths are first active and most active in a particular setting, pheromone traps can be used that are specific to the species desired, such as the Corn Earworm, which attracts males that can then be counted at regular intervals. Presence can tell the cultivator that populations are active to some degree, and it may be prudent to apply treatments very soon, and possibly earlier next season in anticipation of the first emergence.
tobacco budworm, Chloridea virescens, Adult
Johnny N. Dell, Bugwood.org
Eggs are small, globular and white, usually laid singly or in small clusters along the floral structure of Cannabis. It is not easy to see these moth eggs, in many cases it is impossible to confirm their identity with certainty, but given other contextual cues like the season and presence in the field treating such eggs as budworm eggs can be an effective preventative measure.
Larvae do not spend very long outside the flower; at a few millimeters in length, they make very small tunnels which expand as they feed, growing to around 20 millimeters in length after a week or two.
Usually brown, necrotic tissue as well as fungal growth accompanies flower damaged in this way, and the structure often folds into itself after extensive loss of integrity. Sometimes, budworms will travel down the interior of the stem, girdling the terminal end of the plant and contributing to greater damage as the top becomes unable to receive water or nutrients. Since most signs are most apparent after several days of feeding, recognition of larval damage should be followed with vigilant appraisal of other flowers. This may also include culling damaged flowers before they rot or the caterpillars spread.
corn earworm, tomato fruitworm, Helicoverpa zea Larva
Clemson University – USDA Cooperative Extension Slide Series , Bugwood.org
corn earworm, tomato fruitworm ,Helicoverpa zea, Damage
Whitney Cranshaw, Colorado State University, Bugwood.org
It should be noted that budworms are consummate generalists that feed on hundreds of disparate plant species, meaning not only that they are well-adapted to plant defenses, but also a population may develop or overwinter within adult flight range. Suitable plants and the nearby ground could support them, so it may be worth checking non-crop plants on or near the property for activity during the first warm months.
The adults of all three species are relatively nondescript for harrowing agricultural pests: brown or light-yellow wings, at rest in a triangular position. Like other owlet moths in the family Noctuidae, these moths are moderately sized and active almost exclusively at night.
Treatment
Tunneling shelters the budworm from environmental stressors and contact treatments, most popularly the biopesticidal microbes such as Bacillus thuringiensis strains kurstaki and aizawi as well as a natural virus called Helicoverpa armigera Nucleopolyhedrovirus. Even if the larva receives a lethal dose of the microbial pathogen, the body may still rot in the flower, though if it remains small, the subsequent damage can be significantly reduced and may be culled by subsequent scouts or cultivators. To that end coverage is incredibly important and applications, particularly outdoor, must be regularly re-applied to achieve appropriate effects.
An impassable physical barrier is an excellent preventative measure. Different contexts will require different applications but insect netting that separates any moths from the floral tissue and spaced away from the trichomes removes the ability for the life cycle to start in the first place. Greenhouses and indoor cultivation generally limits moth and other pest movement significantly, but the greater the exposure, the more likely the incidence.
Unlike certain other moth larvae, budworms do not move between plants over the ground under most circumstances, so there is minimal chance that budworms will seek this route. Fine netting like pores meant to filter out thrips can decrease airflow to some degree. This can increase humidity and should be accounted for.
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