Insect Control

Aster Leafhopper (Macrosteles quadralineatis)

Aster leafhopper is a minor pest in New England but a major pest in the Midwest. Although it inflicts very little direct feeding injury to carrots or parsnips, it is important because it vectors aster yellows. This mycoplasma-like pathogen causes distortion and discoloration of leaves and stunted, hairy, and bitter roots in both carrots and parsnips. Lettuce, celery, celeriac, parsley, corn, and potato are also susceptible. Aster leafhoppers also feed on cereal grains, especially oats, wheat, and barley, clover, and various weeds. The adults are small, less than 4 mm long, light green with grey wings, and have 6 pairs of black spots on the top and front of the head. Among vegetables, lettuce is the primary crop that is suitable for leafhopper reproduction. Eggs are laid in plant tissues, and the yellowish nymphs feed and develop into adults in 3-4 weeks. There are 3 generations per year in northern states. Aster leafhoppers migrate north annually from the southern US, and can arrive as early as May, sometimes already infected with the aster yellows pathogen. In northern states, they can also overwinter in the egg stage, on weeds or winter grains.

Unlike many insect-vectored viruses, adults or nymphs must feed for at least 2 hours to become infected with the aster yellows mycoplasma. This could occur on infected crops in a southern state before migration or on a local crop or weed. Weeds that may be infected include thistle, fleabane, wild lettuce, sow thistle, chicory, wild carrot, galinsoga, dandelion, plantain, and cinquefoil. There is an incubation period of 2-3 weeks inside the leafhopper; thereafter, it can transmit the pathogen for its lifetime. Transmission from the leafhopper to a non-infected plant also requires at least 2 hours of feeding. It takes 10-15 days for infected plants to show symptoms.

If aster yellows disease becomes a problem on your farm, plant tolerant or resistant varieties, which are available for carrot and lettuce. Control weed hosts and avoid growing susceptible crops in fields close to winter grains. Reflective or light-colored straw mulch effectively reduces aster yellows infection, and row cover prevents infection by keeping out leafhoppers. Cool, wet weather limits leafhopper activity and disease transmission. Monitor nearby grain fields or carrot crops in July and August using sweep nets. Thresholds widely used in the Midwest are based on the aster yellows index, which is the product of the percent infection of the leafhopper population (determined by testing) and the number of leafhoppers per 100 sweeps. At 2% infection, the threshold in carrots is 25 leafhoppers per 100 sweeps in susceptible varieties and 37 leafhoppers per 100 sweeps in tolerant varieties. In the absence of test results for % infection, assume 2%. It is important to control leafhoppers before infection takes place. Because several hours of feeding are required for the aster leafhopper to transmit aster yellows to a plant, disease suppression can be achieved by killing the vector before inoculation occurs.

IRAC Group 1:

• carbaryl (Sevin XLR Plus): 1 to 2 qt/A; PHI 7d, REI 12h, Bee: H, Group 1A. Do not apply to crops or weeds in bloom. 
• methomyl (Lannate* LV): 1.5 to 3 pt/A; PHI 1d, REI 48h, Bee: H, Group 1A. Carrot only.

IRAC Group 3:

• alpha-cypermethrin (Fastac* EC): 1.8 to 3.8 oz/A; PHI 1d, REI 12h, Bee: H.
• beta-cyfluthrin (Baythroid*XL): 1.6 to 2.8 oz/A; PHI 0d, REI 12h, Bee: H. Carrot only.
• deltamethrin (Delta Gold*): 1.5 to 2.4 oz/A; PHI 3d, REI 12h, Bee: H.
• esfenvalerate (Asana* XL): 5.8 to 9.6 oz/A; PHI 7d, REI 12h, Bee: H. Carrot only.
• pyrethrin (PyGanic EC5.0^OG): 4.5 to 15.61 oz/A; 0.25 to 0.50 oz/gal, 3 gal/1000 sq ft in greenhouse for backpack sprayers; PHI 0d, REI 12h, Bee: M.
• zeta-cypermethrin (Mustang* Maxx): 1.76 to 4 oz/A; PHI 1d, REI 12h, Bee: H.

IRAC Group 4:

• flupyradifurone (Sivanto): 7 to 14 oz/A; PHI 7d, REI 4h, Bee: L, Group 4D. 
• imidacloprid (Admire Pro): 4.4 to 10.5 oz/A soil, 1.2 oz/A foliar; PHI 21d soil, 7d foliar, REI 12h, Bee: H, Group 4A.
• sulfoxaflor (Transform WG): 1.5 to 2.75 oz/A; PHI 7d, REI 24h, Bees: H, Group 4C. 
• thiamethoxam (Actara): 1.5 to 3 oz/A; PHI 7d, REI 12h, Bee: H, Group 4A.
• thiamethoxam (Platinum 75SG): 1.7 to 4.01 oz/A; REI 12h, Bee: H, Group 4A. Systemic insecticide used at planting.

IRAC Group 8:

• sodium tetraborohydrate decahydrate (Prev-AM): 100 oz/100 gal; REI 24h, Bee: L, Group 8D. Do not apply in midday sun or mix with copper.

IRAC Group UN:

• kaolin (Surround WP^OG): 25 to 50 lb/A, 0.25 to 0.5 lb/gal; PHI 0d, REI 4h, Bee: L. Suppression only. Supplemental controls may be needed for complete control. 

Carrot Rust Fly (Psila rosae)

While principally considered a pest of carrots, carrot rust fly can also damage parsnips, celery, celeriac, parsley, and dill as well as umbelliferous weeds. Adults are slender flies, 4-5 mm long, with slightly iridescent wings, yellowish-brown head and legs, dark red eyes, and shiny black thorax and abdomen. Adult flies enter fields to oviposit and return to field edges daily. Clusters of 1-3 eggs are laid in the soil near the base of host plants. Larvae are milky-white to yellowish, without legs, tapered at the head with dark mouth hooks, and 6-9 mm when fully grown. Hatching larvae feed on roots, which may kill young plants or cause forking, stunting, or fibrous growth in established plants. Larvae burrow into the main root as they grow larger, tunnel down, and exit through the bottom third of carrot roots or top portion of parsnip roots to form an oval brown pupa up to 10 cm deep in the soil. Larvae sometimes overwinter in fall carrot roots, but pupae usually overwinter in the soil, and adults emerge in May and June. Cool, moist conditions favor adult emergence. Early-season carrots are susceptible to attack by this first flight, especially the earliest successions, as flies tend to select larger carrots to lay eggs. The summer generation of adult flies emerges in August and is active through September, causing damage to late or long-season carrots. Fall damage may increase in later harvests. Larval mining renders roots unmarketable, leaving blotchy areas that develop a rusty color. Wounds provide entry to secondary plant pathogens. The foliage may become red or yellow. In carrots, the larval mines are mostly in the lower portion of the root, but in parsnips, they are in the upper portion. In celery, the larva may tunnel upward into the crown and stalks. Damage is often worst near sheltered field edges, with damage decreasing toward the center of the field.

Avoid leaving crops in the field over winter, where they support overwintering larvae or attract spring egg-laying. Because adults are weak fliers and are limited to the Apiaceae crop family, crop rotation to a separate field is effective; plant carrots in open fields where wind protects them from adult flights if possible. The crop is most vulnerable around the edge of sheltered fields surrounded by woods. Row covers protect the crop from egg-laying. Intercropping with onion has been shown to reduce damage by carrot rust flies. Some varieties show partial resistance to rust flies. Stagger plantings to distribute risk, or if possible, time plantings to avoid the first and second flight periods. In the fall, harvest edges first, as these may have the most damage. Monitor flight with yellow sticky traps placed upright on a stake just above the canopy and several feet into the field; count flies twice weekly. Use multiple traps per field, especially along field edges that are sheltered from wind. Traps reflect population levels and indicate the beginning, peak and end of flight periods, and are used to time insecticides or determine when it is safe to remove row cover. In Quebec and Ontario, insecticides are recommended at thresholds of 0.1-0.2 flies per trap per day, although no registered products are available in New England. Where active periods coincide, sprays for carrot weevil may help control carrot rust flies.

Carrot Weevil (Listronotus oregonensis)

Carrot weevil attacks crops and weeds in the Apiaceae family and can cause severe damage to parsley, dill, carrot, celery, and parsnip. Adult beetles are brown, less than 6 mm long, with the typical weevil ‘snout-like’ mouthparts. They overwinter in soil or plant debris near previous host crops. Although able to fly, they travel and invade fields mostly by walking. In spring, females lay eggs into holes that they gouge in petioles or tops of roots but only oviposit in plants that are older than the 4-leaf stage. Egg-laying starts at 234 growing degree days (GDD) using a base temperature of 44.6°F. Young larvae tunnel downward through stalks or roots and may kill young plants. Tunnels are very pronounced and may be secondarily invaded by fungi. Unlike carrot rust fly, feeding damage by weevil larvae is limited to the upper third of the root. Pupation occurs in the soil, and new adults emerge after 1-2 weeks. There is generally 1 generation per year in New England. Weevils tend to be worse in organic soils. To prevent damage, rotate carrot and parsnip crops to new fields to escape overwintering adults. Delay planting until after eggs have been deposited (90% of oviposition is expected to be completed by 820 GDD). Carrot-baited traps (Mason jar, Boivin, or modified Boivin traps) deployed at field edges can detect incoming adults. Sampling roots is an effective way to estimate the population of larvae. Insecticide must be applied before egg-laying begins, so timing is important; GDD can be useful. Sprays targeting the adult beetles should be applied once or twice, 10-14 days apart, from the 1- to 3-leaf stage.

IRAC Group 1:

• malathion (Malathion 57 EC): 1 to 2 pt/A; PHI 7d, REI 24h, Bee: H, Group 1B. Parsnips only.
• oxamyl (Vydate* L): 2 to 4 pt/A soil directed spray; PHI 14d, REI 48h, Bee: H, Group 1A. Must be incorporated into soil by water or mechanical means to depth of at least 2 inches. Carrot only.

IRAC Group 3:

• beta-cyfluthrin (Baythroid*XL): 2.8 oz/A; PHI 0d, REI 12h, Bee: H. Carrot only.
• esfenvalerate (Asana*XL): 9.6 oz/A; PHI 7d, REI 12h, Bee: H. Begin treatment when weevils become active. Carrot only.
• pyrethrin (PyGanic EC5.0^OG): 4.5 to 15.61 oz/A; 0.25 to 0.50 oz/gal, 3 gal/1000 sq ft in greenhouse for backpack sprayers; PHI 0d, REI 12h, Bee: M.

IRAC Group UN:

• Isaria fumosorosea Apopka Strain 97 (PFR-97 20% WDG^OG): 1 to 2 lb/A soil drench; PHI 0d, REI 4h, Bee: M.

Cutworms

In carrots, cutworms feed on petioles, cutting them near the ground. One cutworm can destroy several plants in a single night. See cutworms in the Pepper and Tomato (Outdoor) sections for more information on black and variegated cutworms. Use spot treatments in affected areas.

IRAC Group 1:

• carbaryl (Sevin XLR Plus): 1 to 2 qt/A; PHI 7d, REI 12h, Bee: H, Group 1A. Do not apply to crops or weeds in bloom. 
• methomyl (Lannate* LV): 0.75 to 1.5 pt/A; PHI 1d, REI 48h, Bee: H, Group 1A. For variegated cutworm on carrot only. 

IRAC Group 3:

• alpha-cypermethrin (Fastac* EC): 1.3 to 3.8 oz/A; PHI 1d, REI 12h, Bee: H.
• beta-cyfluthrin (Baythroid* XL): 1.6 to 2.8 oz/A; PHI 0d, REI 12h, Bee: H. Carrot only.
• bifenthrin (Brigade* 2EC): 5.1 to 6.4 oz/A; PHI 21d, REI 12h, Bee: H.
• esfenvalerate (Asana* XL): 5.8 to 9.6 oz/A; PHI 7d, REI 12h, Bee: H. Carrot only.
• zeta-cypermethrin (Mustang* Maxx): 1.28 to 4 oz/A; PHI 1d, REI 12h, Bee: H.

IRAC Group 5:

• spinosad (Seduce^OG): 20 to 44 lb/A, 0.5 to1 lb/1000 sq ft.; PHI 3d, REI 4h, Bee: M. Spread bait on soil around plants.

IRAC Group 11:

• Bacillus thuringiensis aizawai (XenTari^OG): 0.5 to 1.5 lb/A; PHI 0d, REI 4h, Bee: L. Must be ingested. For resistance management, may be rotated with Bt kurstaki products (Dipel).
• Bacillus thuringiensis kurstaki (Dipel DF^OG): 0.5 to 2 lb/A; PHI 0d, REI 4h, Bee: L. Must be ingested. For resistance management, may be rotated with Bt aizawai products (XenTari).

IRAC Group 18:

• methoxyfenozide (Intrepid 2F): 8 to 16 oz/A; PHI 1d, REI 4h, Bee: L. Suppression only.

Wireworms

See the Potato section for more information on wireworms. These root pests are favored by low, heavy soils that have been in sod or pasture. Delay planting susceptible crops, such as corn, potatoes or sweet potatoes, and other root crops, on such land for at least 2 years after the sod has been broken.