Cannabis aphid, Phorodon cannabis
All aphids are small insects that have six legs, a soft body, and long antennae. These insects have piercing-sucking mouthparts which they use to extract phloem from the plant. Aphids give live birth rather than lay eggs like most other insects, and all are females that can be winged or wingless. Winged forms (alates) are produced when host plants become overcrowded so that a new host can be found. Aphids also tend to produce alates before moving to their overwintering sites.
Cannabis aphids can be found on the leaves and stems of hemp plants. While the color of an aphid is not indicative of its species, cannabis aphids range in color from pale to light green or light brown. They can be winged or wingless, with dark spotting present on the winged forms only. Wingless forms often have pale stripes on the top of the abdomen.
- The cannabis aphid is a pest of hemp grown indoors and outdoors.
- Many aphids are generalists that feed on a variety of plants; however, the cannabis aphid is a specialist herbivore that only feeds on hemp plants.
- Cannabis aphids vary in color from yellow to light green and are similar in color to many other aphid species.
Cannabis aphid adult. The cannabis aphid is a specialist herbivore that only feeds on hemp plants. Aphids can reproduce asexually and can produce multiple overlapping generations each year. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Winged cannabis aphid. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Signs of cannabis aphid infestation. Note the presence of cast skins (exuviae) on the hemp leaf. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Cannabis aphid eggs on a hemp leaf. Adults, nymphs, and cast skins are also visible. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Sampling for cannabis aphids on immature hemp flowers. Image credit: Melissa Schreiner, Colorado State University, Bugwood.org
Life history and habits
Cannabis aphids normally reproduce asexually and give birth to live young, which often results in multiple overlapping generations each year. Their densities can be very high on a single plant. As aphids grow, they molt and leave behind their old exoskeleton (cast skins or exuviae). These skins can be used to detect an infestation of these aphids. In outdoor systems, cannabis aphids switch to sexual reproduction in the late summer or early autumn. During this time, winged males are produced and mate with females. After mating, the eggs are laid externally on hemp leaves where they overwinter. Initially the eggs are yellow and green but change to black.
Cannabis aphids cause minimal injury to plant tissues. Over time, however, high densities of aphids can slow plant growth and cause wilting and yellowing of leaves. In addition, aphids excrete a sugary fluid called honeydew that drops onto leaf surfaces below and can lead to sooty mold growth, which reduces plant health.
Honeydew appears as small shiny spots on the upper surface of leaves and is an excellent indicator that aphids are present. The presence of exuviae on the upper surface of leaves is another sign of cannabis aphid activity.
To sample for cannabis aphids, plants that are upright can be shaken above beating sheets, white trays, or half-gallon ice cream buckets. In the case of hemp varieties with long and bushy shoots, scouting can be difficult. In such instances, individually inspecting leaves can be useful. Placing aphids in a vial with ethanol can help preserve the specimens for identification. The deployment of yellow sticky traps can be useful for detecting winged adults.
The use of row covers from planting through the onset of bloom can provide plants with a physical barrier against aphids. Before planting, all weeds and volunteer crops should be removed. Nitrogen levels should be monitored carefully, as high inputs of nitrogen can increase aphid reproduction. Lastly, the use of reflective mulches can repel winged aphids when they migrate.
There are many natural enemies that feed on aphids, including pirate bugs, bigeyed bugs, lady beetles, lacewings, hoverflies, and parasitoid wasps. Therefore, practices that encourage natural enemies are recommended. For example, it is not uncommon for ants to protect aphids from natural enemies, and when possible ants should be prevented from accessing plants. This may be difficult to do in an outdoor setting, but ant baits can be effective in enclosed growing spaces.
When aphid populations are established, the release of natural enemies can be an effective control measure. Among the most frequently used are green lacewings (Chrysoperla spp.), a species of predatory midge (Aphidoletes aphidimyza), and certain species of ladybug such as Hippodamia convergens.
Insecticides should only be used when needed and applicators should rotate chemicals to prevent resistance from developing in the aphid population. Organic options include insecticidal soaps and oils that work by killing aphids on contact. Consult the Pacific Northwest Handbook of Pest Management for more information on chemical control of cannabis aphids.
Colorado State University. 2018. Cannabis Aphid. Available https://webdoc.agsci.colostate.edu/hempinsects/PDFs/Cannabis%20aphid%20October%202018%20revision%20(1).pdf
NC State. 2020. Cannabis Aphid in Industrial Hemp. NC State: Extension. Available https://content.ces.ncsu.edu/cannabis-aphid-in-industrial-hemp
Oregon Department of Agriculture. 2017. Pest Alert: Cannabis or bhang aphid. Available https://www.oregon.gov/oda/shared/Documents/Publications/IPPM/CannabisAphidAlert.pdf
Oregon State University. (n.d.). Hemp-Aphids. Pacific Northwest Pest Management Handbooks. Available https://pnwhandbooks.org/insect/agronomic/hemp/hemp-aphids
Utah State University. (n.d.). Cannabis Aphids. Utah State University: Extension. Available https://extension.usu.edu/pests/ipm/notes_ag/veg-cannabis-aphid