(This is the 10th installment in a series focused on cultivation planning for marijuana and hemp growers. The previous installment is available here.)
It’s no secret that many insects love cannabis, but research shows that four main pests do the majority of damage to outdoor cultivation.
According to Whitney Cranshaw, an entomology professor and extension specialist at Colorado State University, researchers identified 150 genera of arthropods – insects, mites and spiders – in hemp fields during field sampling during the 2016, 2017 and 2018 growing seasons in Colorado.
The overwhelming majority of those insects were not directly damaging those crops, Cranshaw said during an Advancing Hemp online event in May.
But four key pests stood out as the most damaging in outdoor cannabis production:
Researchers are still restricted from studying marijuana because of its federally illegal status. But because marijuana and hemp are so similar, many of the findings from hemp research can often be applied to marijuana crops.
Cannabis aphids extract fluids from the plant through “precision feeding.” In high populations, they can cause a serious problem for crops.
They are also known to secrete a sticky fluid called honeydew, which can cause fungal contamination, Cranshaw said.
One big question is how cannabis aphids can survive between seasons in areas of the country with hard-freezing winters.
Aphids typically need to feed on live plants to reproduce asexually.
Late in the season – around the first week of September in Colorado, with declining daylengths – different sexual forms show up, male and female, to mate, produce eggs and lay eggs on the flowers and leaves. Those eggs stay dormant through the winter, Cranshaw said.
Cannabis aphids can survive outdoors on the debris of feral, hemp, wild volunteers, which produce seed. When the seed germinates the next spring, the newly hatched insects feed on it, Cranshaw said.
Culling volunteers will prevent eggs from hatching and reduce the chances of overwintering a cannabis aphid population, according to Cranshaw.
In indoor production, where cannabis aphids are particularly common, they can survive all year, as long as the daylength is long enough, he said.
Hemp russet mites are so small, they make a spider mite look like an elephant, Cranshaw said.
Because little is known about hemp russet mites, the insect is a current focus of research for Colorado State University’s entomology program.
Sometimes hemp varieties will curl at the base of the leaves when infested with high populations of russet mites, but other plants don’t have the same symptoms – and further, there are some cultivars that will curl more than others, Cranshaw said.
Cultivators can’t tell if they have russet mites just by looking for curling – they must look for the mites with hand tools and microscopes.
Because they’re so small, russet mites feed on the epidermal cells to kill the first top layer of the plant, but they can’t go down any further than that.
But even that can cause damage, Cranshaw said.
More commonly, plants can take on a grayish tone from russet-mite damage, which cultivators can look for in addition to the curling and rolling leaves.
Russet mites will continue to move on to the newer growth and, ultimately, onto the buds, damaging the developing flower parts.
In a trial last year on the hemp cultivar Unicorn, Cranshaw’s research team used sulfur as a treatment.
Researchers saw a shocking 46% increase in dry-weight yield with sulfur, but the treatment is currently experimental and has not been registered yet.
Eurasian hemp borers
The Eurasian hemp borer, which attacks cannabis flowers, has several generations in a season. It can be found in the stems in the first generation, which might be completed very early in the season by June and another generation completed by July.
Cutting the stem near wilting leaves might reveal the insects’ tunnels within the plant, Cranshaw said.
Later in the season, when flowers are in production, that tunneling might extend into the bud, which creates dead areas and damage, and can leave the plants vulnerable to other insects.
Eurasian hemp borers could also be a problem on hemp grown for seed. A study of the insect on wild hemp produced a 28% reduction in seed production, Cranshaw said.
These insects cause the most damage, which is a problem for hemp or other outdoor cannabis produced for flower.
A well-known pest, corn earworms attack everything from corn and cotton to peppers and tomatoes.
Corn earworms lay eggs at night in sweet corn and field corn on the emerging tassels with green silks. Marijuana and hemp are most attractive to corn earworms when the plants start budding flowers.
Corn earworms don’t lay eggs on plant leaves until there are flowers, and once flowers have been harvested, they’ll leave the plants alone, Cranshaw said.
The young caterpillars don’t do much damage but once they molt and get bigger, the later stage insects can ream out the bud and bore through multiple buds during their lifespan.
Corn earworms will tunnel into the buds from the side, chew them up and often will defecate in the buds, which can cause disease.
The Colorado State Hemp Insect Website has developed a management plan for this pest, which is adopted from managing corn earworms in organic sweet corn.
Some products that are federally registered through the U.S. Environmental Protection Agency have listed hemp on the application label and might help with these key pests.
But because cannabis has been illegal for so long, many states developed their own policies on legal and prohibited pest-control products, both for state-legal marijuana crops and hemp.
Any pesticides allowed are typically food-crop-tolerance exempt and most are organic-certified, Cranshaw said.
But some states are stricter than others, so cultivators must confirm their market’s pesticide policy before purchase and use.
Laura Drotleff can be reached at firstname.lastname@example.org.