The carrot weevil (Listronotus oregonensis) is a pest of carrots as well as parsley and celery. A new profile in the Journal of Integrated Pest Management gathers a variety of IPM methods for fending off carrot weevil and highlights where research is needed to better understand its biology and behavior. (Image originally published in Justus and Long 2019, Journal of Integrated Pest Management)

By Edward Ricciuti

Carrots, parsley, and celery crops draw the destructive carrot weevil (Listronotus oregonensis) like health nuts to a new juice bar, with most of the damage done by its white, brown-headed larvae, only two millimeters long at hatching. Infestations are worst through much of the eastern United States and southeastern Canada. According to Ontario’s Ministry of Agriculture, the weevil is the most damaging insect pest to many carrot growers in that province.

Ed Ricciuti

Scientists are unclear why the carrot weevil is a major pest in some places but not in others. Perhaps because it is not as widespread as many other pest insects, understanding of carrot weevil behavior and biology is sketchy, and integrated pest management (IPM) methods to control it fall short. This is the conclusion of two researchers at the Ohio State University Department of Entomology, who have reviewed research on the carrot weevil and singled out gaps that need to be filled for more effective management of this important but somewhat discounted pest. One major issue, they found, as actor Paul Newman said in the film Cool Hand Luke, is a “failure to communicate.” Researchers need to network more, sharing new information on strategies to control the weevil, suggest the authors.

Several avenues of cultural, biological, and chemical control, notably crop rotation and foliar insecticides, have been explored as ways to control damage by the weevil, but with limited success, write researchers Emily J. Justus and Elizabeth Y. Long, Ph.D., in a report published this month in the Journal of Integrated Pest Management. Moreover, they say, management strategies that have been tried and explored focus mainly on carrots, neglecting its cultivated cousins, celery and parsley. All belong to the family Apiaceae, which includes wild plants such as Queen Anne’s lace (Daucus carota), pineapple weed (Matricaria discoidea), plantain (Plantago major and Plantago lanceolata), and wild turnip (Brassica rapa), also host for the weevil. To date, not much is known about the carrot weevil’s eggs and larval development in wild plants.

Mated carrot weevil adults, only a sixth of an inch long, overwinter in the top layer of soil where crops have been grown and, if the same crop is replanted, return the following year. If not, they must migrate to a more suitable area, usually walking even though they are winged. They favor organic and mineral-rich soils, moving away from sandy soil as quickly as they can.

Emily Justus

When crops emerge, adult carrot weevils nibble on foliage, but the damage they cause is negligible compared to that from the larvae, which hatch from eggs laid in cavities chewed and then sealed by the female into the petiole or crown of the plant. As the larvae tunnel down through the crown toward the root through four instars, they cause their damage, killing young plants and often rendering survivors inedible.

“Carrot weevil management has largely focused on strategies to monitor adult activity in the field, by scouting for egg scars or using passive traps to synchronize the application of foliar insecticides with peak adult activity,” according to the paper. Because they overwinter, crop rotation also has been a control mainstay. Most research to date focuses on protecting carrot crops from adult weevils, with much less attention given to exploiting key aspects of the insect’s behavioral ecology to protect crops against it.

Adult carrot weevils (Listronotus oregonensis) nibble on foliage, but the damage they cause is negligible compared to that from the larvae, which tunnel down through the crown and into the root. The damage they cause can kill young plants and often renders survivors inedible. (Photo credit: Whitney Cranshaw, Colorado State University, Bugwood.org)

When it comes to monitoring adults, difficult to eyeball because of their small size and tough to catch by sweep net or suction because they stay close to the ground, passive pan and pitfall traps have been commonly used. Pan traps, however, are more suited to actively flying insects, while pitfall traps capture too few weevils to be meaningful. So-called “Boivin traps,” cage-like arrangements placed over a carrot bait, work a little better but far from well. Monitoring scars in surveillance plants has been tried with parsley, but schlepping around a field checking plants is tedious work—plus, by the time the scars are visible, the damage to crops already has been done. What’s more, the method has not been checked out on other crops.

Besides crop rotation, cultural controls such as floating row covers, barrier plantings, and cleaning up debris have been tried on carrots with some success. Delaying planting carrots until the weevils already have laid eggs works but can severely reduce crop yields.

Foliar insecticides are the predominant tool used against adult carrot weevil in all cropping systems, the authors note, adding that chemical control of carrot weevils was dealt a blow in 2012 when the U.S. Environmental Protection Agency clamped down on the main carrot weevil pesticide Guthion (azinphos-methyl), due to human health and environmental concerns. Removal of the most effective tool used by growers forced some to turn to other insecticides, such as phosmet, malathion, diazinon, methomyl, and pyrethroids, but they lack the punch of guthion. By and large, insecticides now recommended for use against carrot weevil adults and young larvae are hampered by the fact that larvae are shielded from exposure inside plant tissue or soil and adults by growing foliage. Adults, moreover, seem resistant to the rates of application allowed in the field. “The behavioral or physiological mechanisms underlying carrot weevil tolerance to insecticides are not well understood,” say the authors.

Biological controls using natural enemies against carrot weevils have drawn considerable attention from scientists and has been studied extensively in carrot crops, but not celery and parsley. Some approaches have promise while others seem iffy. Entomopathogenic (i.e., insect killing) fungi (EPF) have been observed infecting carrot weevils in the field. Even so, however, commercially available EPF, Beauveria bassiana, which causes white muscadine disease, had no impact on weevil damage when treated and nitrated carrot plots were experimentally compared.

Some insects, which are predators or parasitoids of the weevil, seem likely candidates for biological control but, as with fungi, their practicality is unproven, and nothing has been published on use with celery and parsley. Ground beetles, which forage along the soil surface for prey, in theory, could provide biological control for all carrot weevil life stages, say the authors. Under laboratory conditions, smaller carabid beetles were the best consumers of carrot weevil eggs, while Pterostichus melanarius, a large carabid, was the only species to consume newly emerged and overwintered adults, and all sizes of carabids consumed fourth instar larvae. “However,” say the authors, “these promising laboratory results may not translate to outcomes in the field, as the majority of the carrot weevil life cycle occurs in the plant or soil, rather than along the soil surface.”

Four species of parasitoid wasps, Anaphes victus, Anaphes listronoti, Anaphes cotei, and Anagrus spp., all in the family Mymaridae, parasitize carrot weevil eggs. A study shows a mortality rate of from 49 percent to 68 percent. One hitch to using these parasitoids, though, is their low numbers in commercial fields, limiting their effectiveness. Both predators and parasitoids are inhibited by application of foliar insecticides, which kill natural enemies as well as intended targets.

Of all candidates as biological controls, entomopathogenic nematodes (EPNs) show the most promise because they inhabit soil, where they can attack several life stages of the weevil. After entering natural openings in the weevil’s body, the nematode releases deadly bacteria that kill the insect. Nematodes that have been shown effective against the weevil include Steinernema carpocapsae, S. riobrave, S. feltiae, Heterorhabditis megidis, and H. bacteriophora. In lab studies focusing on the adult carrot weevil life stage, S. carpocapsae was the most effective EPN in sandy soil, causing 88 percent cumulative mortality over an 8-day period, while H. bacteriophora was most effective in muck soil, causing 81 percent mortality cumulative mortality of adult carrot weevils.

As with other natural enemies, EPNs have thus far not been widely used in the real world. The method is costly and, since a control must protect an extremely high percentage of the crop, has not seen much field use. EPNs move only short distances through the soil and thus far have been applied as a foliar spray or by drenching soil around the root zones of plants. “EPNs do not reduce carrot weevil populations quickly and are more promising as long-term biological control agents,” says lead author Justus, a Ph.D. Candidate in Long’s insect ecology laboratory. The authors suggest that reducing tillage and planting cover crops such as barley and clover might increase the survival of EPN in the soil.

Holding back the development of novel strategies for controlling carrot weevils is limited understanding of their behavior when it comes to finding mates and hosts. Virtually no research has targeted finding attractants, repellents, or pheromones that might be used in management of the weevil.

Among other holes that need to be filled in understanding of carrot weevil is a grasp of how they find mates, whether they prefer cultivars or weeds, and their patterns of movement and dispersal between overwintering sites and new fields to conquer.

“Further research on carrot weevil biology, particularly behavioral ecology, is needed to develop sustainable and effective IPM strategies for this insect in the future,” advise the authors.

Ed Ricciuti is a journalist, author, and naturalist who has been writing for more than a half century. His latest book is called Bears in the Backyard: Big Animals, Sprawling Suburbs, and the New Urban Jungle (Countryman Press, June 2014). His assignments have taken him around the world. He specializes in nature, science, conservation issues, and law enforcement. A former curator at the New York Zoological Society, and now at the Wildlife Conservation Society, he may be the only man ever bitten by a coatimundi on Manhattan’s 57th Street.