Alate rice root aphids (Rhopalosiphum rufiabdominale) recently emerged from the soil climbing on stem of a Cannabis plant (image rotated). (Photo courtesy of Whitney Cranshaw, Ph.D.)

By Whitney Cranshaw, Ph.D.

Whitney Cranshaw, Ph.D.

In the fairly short period since there has been some state-legalized production of Cannabis sativa crops in the United States, several insects and mites have emerged as significant potential pests in its production. Many of these are generalist species associated with indoor production of many crops, such as twospotted spider mite (Tetranychus urticae), broad mite (Polyphagotarsonemus latus), and onion thrips (Thrips tabaci). Others are specialists on the crop, such as cannabis aphid (Phorodon cannabis) and hemp russet mite (Aculops cannibicola). But perhaps the most surprising of all is the presence of rice root aphid (Rhopalosiphum rufiabdominale) as a common insect found on this crop throughout North America.

Together with co-author Suzanne Wainwright-Evans of BugLady Consulting, we try to describe the status of this insect in the paper “Cannabis sativa as a Host of Rice Root Aphid (Hemiptera: Aphididae) in North America,” published this week in the Journal of Integrated Pest Management.

It was a surprise to me seeing rice root aphid in the very first two indoor cannabis facilities in Colorado that I ever visited in 2010. I had never heard of it ever being found on any crops in our state, but I assumed incorrectly that its presence on cannabis was probably a local issue. What prompted a more formal reporting of this was learning how common and widespread this insect is on the crop. As Suzanne related recently, “I am pretty much being contacted every day with questions from around the United States and Canada about root aphid problems.”

Alate rice root aphids (Rhopalosiphum rufiabdominale) trapped on hairs of a Cannabis leaf. (Photo courtesy of Whitney Cranshaw, Ph.D.)

Rice root aphid is an insect of cosmopolitan distribution and has been known in North America for over a century. It is most often reported infesting grain crops and sedges, but it has been found developing on roots of many other kinds of plants, including cotton, squash, peppers, and certain ornamental plants such as dieffenbachia. From observations and reports from growing facilities throughout the U.S., not only it is able to develop on the roots of Cannabis sativa crops, but populations can thrive in all growing media including soil, rock wool, and coconut coir, as well as in aeroponic production. When present in high populations, rice root aphid can retard plant growth and may result in some yield loss.

Rice root aphids (Rhopalosiphum rufiabdominale) on roots of an aeroponically grown Cannabis plant. (Photo courtesy of Whitney Cranshaw, Ph.D.)

Why this insect has become so widespread in North America is unclear, but several aspects of the culture of the crop are favorable to rice root aphid. Also, its cryptic habit of developing in soil makes detection difficult and can allow for long lead times before growers realize the insect is present within a growing facility.

For years, there has been a lot of internet chatter about “root aphids” on cannabis, most all of which contained serious error. Postings typically used borrowed images and incorrect biology information, typically drawing from

totally different root-infesting species, such as grape phylloxera or various Pemphigus species. Our goal, with this paper, is to correctly set the record of “root aphid” issues on C. sativa so that integrated pest management programs can be appropriately developed.

Whitney Cranshaw, Ph.D., is a professor and extension specialist in the Department of Bioagricultural Sciences and Pest Management at Colorado State University in Fort Collins, Colorado. Email: whitney.cranshaw@colostate.edu.