Photo by Mel Frank

Cannabis has exploded in popularity among consumers, but people aren’t the only species who seem to enjoy the plant—a number of unique pests and mites also feed on cannabis.

A closer look at these pests will help cultivators identify when a problem is beginning, so they can take corrective steps sooner. Here are tips to help growers identify pests.

Identifying Cannabis Aphids

In our research program at North Carolina State University, we isolate our mother stock from other plants. Any new plant material should be quarantined by placing the plants in a separate facility for a few weeks to ensure they are free of pests and diseases. Unfortunately, we didn’t notice cannabis aphids (Phorodon cannabis) on new cultivars we acquired, so we spent the summer getting the infestation under control.

Along the way, we acquired information on this cannabis-specific pest.

1. In general, cannabis aphids are smaller than the green peach aphid (Myzus persicae, adults of which vary from 1.8 mm to 2.1 mm) and are most often found on the underside of leaves or along stems (Fig.1).

2. Many aphid species exist and have similar characteristics, so we suggest sending a sample to a diagnostic clinic to obtain a proper identification.

Figure 1. Cannabis aphids (Phorodon cannabis) are most often found on the underside of the leaves or along the stems.

Treating for Root Aphids

In 2019 we also learned about root aphids on cannabis. Cannabis Business Times contributor Dr. Raymond Cloyd of Kansas State University spoke about them during his presentation at the 2019 Cannabis Conference in Las Vegas. Root aphids are not common with commercial floriculture production, so we found the occurrence of them interesting.

The rice root aphid (Rhopalosiphum rufiabdominale) has been reported in many other states but remains uncommon in North Carolina. The pest will feed on a wide array of plant species, but with cannabis it is primarily a pest of indoor production. It thrives in moist environments, which occur with rockwool root cubes and around the pot edges with peat-based substrates. The dark brown to black coloration of these aphids allows them to easily camouflage with a peat-based substrate, so they may not be noticed (Fig. 2).

For us, discovering winged adults crawling up the stem, onto leaves and flying to other plants was our first sign of trouble.

3. We eliminated our infestation by soaking the root balls and containers in water for 10 minutes, rotating to new mother stock, and placing our new cuttings (clones) in our water rehydration bucket for 10 minutes to drown any aphids.

4. We commonly add 1 milliliter of dish soap per 1 liter of water to help clean the leaves. This also helps eliminate the surface tension and air pockets around the cuttings during the rehydration step.

Figure 2. (left) Root aphids blend into the root substrate, which can make them difficult to detect.' Figure 3. (right) The initial symptoms of a broad and cyclamen mite infestation is a slight upward leaf curl (plant on the left).

Identifying Broad and Cyclamen Mites

Curled and distorted leaf growth, especially on the new and emerging leaves, is a typical symptom of a broad (Polyphagotarsonemus latus) and/or cyclamen (Phytonemus pallidus) mite infestation. The initial symptoms are a slight upward leaf curling (Fig. 3), which manifests into distorted growth (Fig. 4). These tarsonemid mites are smaller than the typical two-spotted spider mite (Tetranychus urticae) that observers can see with the naked eye.

5. An 80X to 100X magnification is needed to adequately view these mites. Populations are highest on the leaf underside and can quickly increase with indoor production.

6. Rain, wind and beneficial insect feeding help limit population explosions from occurring with outdoor culture, although significant damage has been observed in arid climates.

Figure 4. Broad and cyclamen mites both cause similar distorted new growth of cannabis. Severe distortion occurs with major populations.; Figure 5. Downward leaf curl is the initial symptom of a hemp russet mite infestation. It takes an 80X to 100X microscope to view them.

Identifying and Preventing Hemp Russet Mites

Another pest that can become problematic in cannabis is the hemp russet mite (Aculops cannibicola). These are also extremely small mites that can only be observed with 80X to 100X magnification. Hemp russet mites are pale green in color and have elongated bodies (<1 mm long).

The initial symptom is a downward curling of the new leaves (Fig. 5). Advance damage to leaves and stems appear as an overall browning or russeting, which lends the name to the pest (Fig. 6). Damage is usually only noticed when populations explode on a crop late in the production season. Due to their small size and lack of plant symptomology with low populations, hemp russet mites are often not noticed.

7. Inspect the lower leaf surfaces with a high magnification microscope to determine if the plants are infected. Hemp russet mites can spread from clothing, infected plants and wind currents.

Some cultivars infected with russet mites will display a bear claw like morphological change under extremely high population densities.

8. Because cannabis has such variability in leaf morphology, look specifically for any stunting or distorting pattern of the leaves such as curling, cupping, bending, wrinkling, etc.

Figure 6. Hemp russet mites can cause a browning of the foliage when heavy population infestations occur.

Other insect pests also can attack cannabis, such as western flower thrips (Frankliniella occidentalis), two-spotted spider mites (Tetranychus urticae), whiteflies and numerous caterpillar species. These pests are larger in size and are easy to see with the naked eye.

9. Weeds growing in or around the greenhouse can harbor many insect and mite pests, so be sure destroy those. In our NC State greenhouses, we are always removing any germinated weeds. Much to our surprise, many of these weeds also contain a stray aphid, whitefly, thrips or spider mite despite the low weed population within the greenhouse.

Whenever possible, prevention is the best pest management practice for cannabis. Cultivators should know the pests they’re facing, no matter how small.

10. Starting with clean, insect and disease-free quality cuttings is ideal. Propagating and growing clones from pest-free plants will go a long way in preventing problems.

Brian Whipker1,2; Paul Cockson1; Patrick Veazie1; David Logan1; Matthew Davis1; W. Garrett Owen3 ; 1 Department of Horticultural Science, North Carolina State University, Raleigh, NC; 2 Floriculture Extension and Research; 3Department of Horticulture, Michigan State University