What Are Carrot Weevils: Tips On Carrot Weevil Management In Gardens

What Are Carrot Weevils: Tips On Carrot Weevil Management In Gardens

By: Jackie Carroll

Carrot weevils are tiny beetles with big appetites for carrots and related plants. Once they’re established, these insects can devastate your carrot, celery, and parsley crops. Read on to find out about carrot weevil management.

What are Carrot Weevils?

Only about one-sixth of an inch (4 mm.) long, carrot weevils are snout beetles that love to dine on members of the carrot family. They feed during the warm months and then spend the winter hiding in the top layer of soil and in weeds, grass, or debris left in the garden. If you have them one year, you can count on their return the following year.

Since they overwinter in the location where carrots grew the previous year, crop rotation is an important part of the strategy for controlling carrot weevils. Move your carrot patch every year and wait at least three years before growing them in the same location. At the same time, keep the garden clean and weed free to eliminate some of their favorite hiding places.

The adult beetles feed on plant foliage. Females lay eggs in the carrot roots through a small puncture wound. If you see a small dark spot on a carrot, rub it and look for a wound underneath. If you see a puncture wound, you can be fairly certain that there are carrot weevil larvae tunneling through the root. The larvae are white, C-shaped grubs with brown heads. Their feeding activity can weaken and kill a carrot. Carrot weevil damage leaves the roots inedible.

Controlling Carrot Weevil Organically

There are plenty of organic strategies for managing carrot weevils, so you will probably never need to spray toxic chemical insecticides to get rid of them. Traps are effective in catching the larvae. You can buy them at a garden center or make your own from mason jars and paper cups.

Place a few slices of carrot in the bottom of a mason jar to serve as bait. Poke holes in the bottom of a plastic coated paper cup and fit it into the opening in the jar. The larvae can fall though the holes but can’t crawl out. Alternatively, sink a baited container in the garden soil so that the opening is level with the soil surface. Add soapy water to the container. The carrot weevil larvae will drown when they fall in.

Milky spore and Bacillus thuringiensis are organisms that kill grubs like carrot weevil larvae without harming people, the environment, or animals. These completely safe products are very effective when you apply them early, but they won’t kill older larvae. You may continue to see larvae for a while because they don’t die immediately. Use neem-based sprays on older larvae.

Keeping your garden clean and weed free, rotating the carrot crop, using traps, and beneficial organisms should be enough to control carrot weevils. If you’re still having trouble, check your garden center for insecticides labeled for use against the pest. Keep in mind that systemic chemical insecticides also kill beneficial insects and may cause more problems than they solve.

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What Are These Bugs Doing in My Dry Goods and How Do I Get Rid of Them?

Learn why the weevil is the scourge of every kitchen.

So you followed pandemic advice and outfitted your pantry for the long haul. Or maybe you belong to a warehouse club, have a big family to feed, or appreciate BOGO deals. At any rate, you stocked up on flour for baking. You purchased a good amount of pasta or rice, figuring that starches never really go bad anyway. Then you might have also invested in big-bulk cereals, pancake mixes, oatmeal, and instant mashed potatoes.

And now, you have 8- to 10-millimeter-long, black- or brown-bodied bugs with elongated, bent snouts crawling through all of it. Elsewhere, you're finding piles of them in the corners of your cabinets, some dead, some barely alive. They seem to be everywhere, even in unopened boxes of mac-and-cheese. What are they? How did they get there? And most importantly, how can you get rid of them?


Pests of Carrots

Since carrots are root crops, soil-inhabiting pests such as wireworms and vegetable weevils have the most direct effect on produce quality. Armyworms, however, may cause indirect injury to the taproot by cutting stems and/or consuming foliage above ground. Few other insect problems are common in North Carolina.

A. Chewing insects that cut holes or entire leaves

  1. Caterpillars with three pairs of legs and five pairs of prolegs
    1. Armyworm – This pale green to yellowish or brownish-green, smooth-bodied caterpillar up to 35 mm long has three dark longitudinal stripes and a tan or greenish-brown head mottled with darker brown (Figure 1). It feeds primarily at night on foliage and succulent stems.
    2. Parsleyworm – This yellowish-green caterpillar up to 40 mm long has transverse black bands and deep yellow or orange spots. The head is greenish-yellow with black stripes (Figure 2). When disturbed, a pair of horn-like scent organs protrudes from behind the head. They feed during the day.
    3. Yellow woollybear – This caterpillar is white to yellow or brown or red and has dense white hairs covering its body (Figure 3). Young larvae feed in colonies on the underside of leaves older larvae disperse and feed anywhere.
  2. Vegetable leafminer – These bright yellow maggots grow up to 3.0 mm long and make S-shaped leaf mines which are often enlarged at one end (Figure 4). Heavily infested leaves sometimes turn brown.
  3. Vegetable weevil and larva – Dull grayish-brown weevil with a short, stout snout and light V-shaped marks on wing covers, feeding primarily at night on buds and foliage (Figure 5A). Both weevils and grubs are about 6.4 mm long. The pale green legless larvae grow up to 10 mm long and have dark mottled heads (Figure 5B).

B. Insect with needle-like mouthparts that cause foliage to be yellowed or distorted

  1. Aster leafhopper – Yellowish-green aster leafhoppers are up to 5 mm long and have six black spots on front of head (Figure 6). Nymphs are sometimes light brown instead of yellow or green.
  2. Tarnished plant bug – Oval-shaped brown bugs are up to 6.4 mm long and have long legs, long antennae, and a white triangle between its "shoulders" (Figure 7A). Nymphs are yellowish-green to green with black spots on its back (Figure 7B).

C. Insects that feed on underground plant parts

  1. Vegetable weevil and larva – Dull grayish-brown weevil with a short, stout snout and a light V-shaped marks on wing covers, feeding primarily at night on buds and foliage (Figure 8A). Both weevils and grubs are about 6.4 mm long. The pale green legless larvae grow up to 10 mm long and have dark mottled heads (Figure 8B). Vegetable weevils feed at night, often attacking large taproots.
  2. Southern potato wireworm – These slender, wire-like, cylindrical larvae have three pairs of short legs and a pair of fleshy anal prolegs (Figure 9). The white, cream, or yellow-gray larvae have red-orange heads and grow to 17 mm long. The last abdominal segment has a closed oval notch. These wireworms form irregular holes in infested taproots.

Figure 1. The armyworm is pale green to yellowish or brownish-green, smooth-bodied, up to 35 mm long, and has three dark longitudinal stripes and a tan or greenish-brown head mottled with darker brown.


Solutions

  • Weevil larvae can be controlled by using LawnPro Protectsprinkled around the roots of susceptible plants and watered well in, including where Argentine stem weevil are damaging lawns.
  • Identify the weevil species and check all the plants in your garden that are commonly eaten by that species. When weevil adults are causing damage to susceptible plant foliage spray with PLANThealth Spectrum .
  • It is also recommended to remove weeds that weevils will feed on, and to cultivate soil to control weevil larvae.

Control of Weevils in Trees, Shrubs and Ornamentals

  • Apply PLANThealth Insect Hit Granules or LawnPro Protect on the the soil around the roots of affected plants and spray with PLANThealth Spectrum.

Note: The above retail product PLANThealth Insect Hit Granules has been discontinued. However, if you have a significant problem a commercial option is still available, please contact Kiwicare directly for more information.


Monitoring Carrot Weevils Using Wooden Boivin Traps

The carrot weevil, Listronotus oregonensis, is a major pest of parsley, celery and carrots in the Great Lakes region of the United States and Canada. Adult carrot weevils are small (0.2-0.6 centimeters long), mottled-brown beetles with a distinctive snout that is typical of weevils (Figure 1). These weevils rarely fly thus, they colonize fields primarily by walking from overwintering sites and will feign death when disturbed.

Figure 1. Adult carrot weevils.
Photo by Elizabeth Long.
Figure 2. A parsley field damaged by carrot weevil larvae (left). A parsley plant killed by carrot weevil larvae (right). Photo by Elizabeth Long.

Adult carrot weevils feed on the foliage of plants in the carrot family, including parsley. The female lays eggs in the petiole or crown of the plant when it reaches the four-leaf stage, while the larvae cause the most severe damage by feeding and tunneling through the roots (Figure 2). Tunneling often leads to plant death, and in some cases, up to 100 percent loss has been reported in parsley. Monitoring carrot weevil populations is especially challenging because the adults are well camouflaged, the eggs remain hidden within the plant, and currently, no synthetic lures exist to attract and trap the adults.

Monitoring Adult Carrot Weevils

Figure 3. An assembled wooden Boivin trap (A), the top and bottom pieces of the trap (B), visible teeth of the trap (C), and the bottom of trap to scale (D). Photo by Suzanne Blatt.

Scouting for egg-laying scars is one method used for monitoring carrot weevils (Torres and Hoy, 2002). One hundred and fifty plants are inspected for egg-laying scars in an X-shaped pattern across the field. If 1 percent of plants have egg-laying scars, action is recommended. However, this scouting technique is time-consuming and requires previous knowledge of what egg-laying scars look like on the host plant.

Another option for monitoring carrot weevil populations is to use a wooden Boivin trap baited with a single carrot. This trap is roughly a foot long and is hollowed out in the center with “teeth” along the edge (Figure 3). The channel in the center of the trap houses the carrot bait and the “teeth” provide a tight space for the weevils to hide as they feed on the carrot. The action threshold for Boivin traps is 1.5 weevils per trap (Boivin and Brodeur, 1992).

How to Use Wooden Boivin Traps

Place wooden Boivin traps along the edge of fields where the focal crop will be planted. The key is to place traps early, before the crop emerges. The tooth-side of the trap should be placed down on the soil to make it easy for carrot weevils to enter. Secure the bottom and top of the trap together by placing a rubber band over each end. Check traps every three to four days to look for weevils and replace the carrot. As the carrot rots, it will become less effective as bait.

Figure 4. Number of adult carrot weevils collected from wooden Boivin traps from April 11 to June 12, 2017.

Hundreds of weevils can be collected in these traps early in the season before the crop emerges (Figure 4). However, as the season progresses, the carrot bait may be outcompeted by the surrounding crop, such that significantly fewer adults are attracted. More importantly, use of these traps early in the season still provides an early warning of carrot weevil activity when the crop is young and most vulnerable.

Tips for Using Wooden Boivin Traps

  1. When checking a trap, first carefully turn it over and inspect the outside bottom while it is still closed. Then, remove the plywood bottom and check all sides for adult weevils. Finally, inspect the inside by running a stick or small paintbrush through the teeth to expose any weevils that may be hiding.
  2. Bring a jar or Ziploc bag to collect weevils, and dispose of your catch by placing the sealed container in the freezer and then the trash.
  3. Mark the trap with a flag for easy location.

How to Make Wooden Boivin Traps

Figure 5. Construction of a wooden Boivin trap. Red lines symbolize cuts black lines symbolize previous cuts and gouges. (Top) Post with cuts at every 12 inches. (Middle left) 12-inch piece of post with single longitudinal cut. (Middle right) Cuts necessary to make the bait channel. (Bottom Left) Cuts necessary to make teeth. (Bottom right) Finished base with canal and teeth gouged out. Schematics not drawn to scale.

  • 8 feet of 4 x 4-inch post (pine or cedar)
  • A single 4 x 8 piece of ½-inch exterior plywood
  • 10-inch table saw blade width 1/8 inch or larger
  • Protective eye wear

  1. Set the table saw so the saw is all the way up.
  2. Using the table saw, cut each 8-foot post into 12-inch pieces (Figure 5).
  3. Cut each piece of post in half to get 4 x 2 x 12-inch pieces.
  4. Set the table saw so it is between ¼ and ½ inch.
  5. To make the center channel of the trap, cut 1 inch in from the long sides of the trap, then make cuts close together down the length so the middle can be removed.
  6. To make the “teeth” cut 2 inches in from each short side. Working interior to these cuts, create the “teeth” by making a cut every ½ to ¾ of an inch.
  7. Once the “teeth” have been created use a chisel to remove the cut wood, revealing the channel and the teeth.
  8. Repeat steps 3 through 5 on every 4 x 2 x 12-inch piece of post.
  9. Cut the 4 x 8-foot piece of plywood into 4 x 12-inch plates.
  10. Secure the plywood plate to the trap, covering the channel, by placing a rubber band at each end.

This process should yield 16 traps. Always wear proper personal protective equipment.


Methods for Monitoring Carrot Weevil Populations

Use of Passive Traps to Monitor Adults

Several techniques have been evaluated to monitor carrot weevil populations in crop fields. Given their small size and cryptic behavior, actively scouting fields for adult carrot weevils is not recommended. Rather, passive sampling methods, including pan traps and pitfall traps, have been tested as indicators of adult activity in crop fields ( Perron 1971, Ryser 1975) however, these methods are unreliable. Pan traps are most useful in capturing flying insects and pollinators ( Disney et al. 1982, Vrdoljak and Samways 2012), while pitfall traps capture too few individuals to accurately reflect activity in the field ( Boivin 1985). Blacklight traps placed within 0.25 miles of crop fields have also been used to monitor adult carrot weevils however, this method does not accurately measure activity or level of infestation because they target flying insects attracted to lights, and carrot weevils rarely fly ( Perron 1971, Ryser 1975, Boivin 1985). Likewise, active sampling techniques for adults, like suction-sampling and sweep-netting, do not provide reliable indication of adult activity because these methods sample the foliage rather than the base of plants or the soil surface, where adult carrot weevils tend to be found ( Ryser 1975).

The most reliable monitoring tool developed to date is known as the ‘Boivin trap.’ A true Boivin trap, or radiator trap, consists of 22 wooden plates, separated by metal washers placed over a carrot ( Boivin 1985). A ‘modified Boivin trap’ has also been developed and is more easily constructed and used ( Ghidiu and VanVranken 1995). This trap consists of a wooden base with ‘teeth’ that surround a canal where a whole carrot is placed to serve as bait ( Fig. 7). These traps are the most successful in attracting adult carrot weevils, although numbers may still be low. The utility of these traps decreases once crop hosts have emerged ( Boivin 1985), possibly because the carrot bait within is unable to compete with volatiles emanating from crop hosts. Thus, growers must rely on other monitoring methods to inform their management strategies once traps become ineffective.

A modified Boivin trap baited with carrot. View of an open trap from the top (A), and closed trap viewed from the side (B), showing ‘teeth’ of the trap.

A modified Boivin trap baited with carrot. View of an open trap from the top (A), and closed trap viewed from the side (B), showing ‘teeth’ of the trap.

Scouting for Egg-Laying Scars

Detecting egg-laying scars has been explored as a potential monitoring strategy for the carrot weevil. Ryser (1975) placed sentinel parsley plants in the field and monitored the appearance of egg-laying scars. However, this strategy was unsuccessful, most likely because the number of sentinel plants was insufficient. Currently, scouting egg-laying scars is the recommended strategy for monitoring adult carrot weevil activity in parsley. Torres and Hoy (2002) found that sampling 150 parsley plants, in an x-shaped pattern across the field, is most effective for gaining a reliable measure of carrot weevil activity. Following this method, 10 plants are examined at equal intervals along an x-shaped transect, such that seven groups of 10 plants are evaluated along one ‘line’ of the x shape and eight groups of 10 plants are evaluated along the other ‘line.’ This allows growers to systematically evaluate the presence of egg-laying scars in both the center and edges of a field. Based on this scouting method, action is recommended if 1% of plants exhibit oviposition scars ( Torres 2001). This process is tedious, time consuming, requires correct identification of scars and does not provide advance warning. Once egg-laying scars are seen, it is too late to intervene and prevent damage. Additionally, this method of scouting has only been empirically evaluated in parsley cropping systems and the results of this method may differ on other crops. Even with trap monitoring and scouting, carrot weevil damage can go unnoticed until it is severe ( Torres and Hoy 2002).


Meet the Weevil

Get to know rice and maize weevils, two of more than 60,000 species of herbivorous beetle. They're the scourge of pantries everywhere. Entomologists with Ehrlich Pest Control say that, in particular, "rice weevils have been reported infesting rye, barley, buckwheat, table beans, stored cotton, grapes, cashew nuts, cereals, wheat products, pasta, and bird seeds."

Meanwhile, Dave Lofquist, technical training manager for Arrow Exterminators, notes that adults of both kinds of weevils "can live for many months and are capable of wandering a good distance from the original infested item. Their chewing mouthparts can penetrate plastic and cardboard packaging, which will enable them to spread the infestation."

In other words, as pests, weevils are really high up there on the scale, given that they don't seem to discriminate in their food sources much and easily burrow into closed packaging—or escape from it. In fact, Lofquist says, "Some will also damage vegetable gardens and ornamental plants."

Scott Svenheim, director of training services for Truly Nolen Pest Control, says weevils also eat "furniture, clothing and other fabrics, and decorative items." This is why he recommends complete eradication of any infested food products plus a solid cleaning and vacuuming of cupboards.


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