Table of Contents
The three most important insect pests of carrots in Ontario are: (a) the carrot rust fly, Psila rosae, (Fab.); (b) the carrot weevil, Listronotus oregonensis (Le Conte); and (c) the aster (formerly six-spotted) leafhopper, Macrosteles quadrilineatus (Forbes). Larvae of the carrot rust fly and carrot weevil create tunnels in the roots of carrots, making them unmarketable. The aster leafhopper carries and transmits the mycoplasma-like organism causing aster yellows (AY-MLO), a disease that affects carrots as well as a wide range of crops.
These pests are of most concern in the Holland and neighbouring marshes, where more than 80 per cent of Ontario's carrots are grown. All three pests are present to varying degrees in much of southern Ontario. Timing of seasonal events and importance of pest damage varies in the province according to climate and soil type. For example, in south-western Ontario, the aster leafhopper-aster yellows complex is the major pest.
Although carrot weevil and carrot rust fly damage may appear similar in the early stages, normally they are distinguished easily on mature carrots. Carrot weevil feeding tunnels tend to occur in the upper one-third of the root, with a conspicuous, darkened, partly open tunnel in the crown after the larva has matured and left the carrot (Figure 1). The larva is a legless grub with a reddish-brown head.
Carrot rust fly tunnels (Figure 2), found mainly in the lower two-thirds of the root, are narrower and more winding than those of the carrot weevil. The larvae are legless and headless maggots. In severe early-season attacks by either insect, some young plants may wilt or be killed, although that type of damage generally is not economically important in commercial fields. The nature of first-generation carrot rust fly injury depends upon the development of the plant at the time of the attack. There may be tunnelling or destruction of the extremity of the tap root only, or, if the root has thickened, tunnels similar to those caused by the second generation may occur. Both pests can also attack celery, parsnip, wild carrot and other Umbelliferae. In severe attacks, young celery plants can be destroyed.
Figure 1. Feeding tunnels of the carrot weevil, formed mainly in the upper one-third of the root.
Figure 2. Feeding tunnels of the carrot rust fly, found mainly in the lower two-thirds of the root.
Aster leafhoppers are inconspicuous and evidence of their
attack shows up much later in the form of aster yellows. Infected carrots
have excessive yellow, twisted, fernlike foliage, with hairy roots
The adult carrot rust fly is a slender, shiny, black fly, about 6 mm long, with a small but characteristic reddish head and long yellow legs (Figure 4). The insect overwinters in the soil in the pupal stage in a small, seedlike puparium (Figure 5).
Figure 3. Aster yellows of carrot,with excessive yellow, twisted foliage and hairy roots.
Figure 4. Adult carrot rust fly.
Adults begin to emerge in late May, after about 250 degree-days above 5 °C, and may be present until near the end of June. They lay eggs in the soil around young carrots. The young larvae feed at first on root hairs and small rootlets and after their third moult, enter the carrots to complete their development. When mature (Figure 5), they pupate in the soil.
Near the end of July, after about 1150 degree-days above 5 °C, another generation of adults emerges that may last for nearly two months. The larvae of the second-generation pupae spend the winter in the soil, although in some years a few adults may emerge in October. Normally it is too late for them to cause any extensive damage.
Carrot rust fly adults can be monitored with orange/yellow sticky traps (Figure 6) placed in the carrot field or in nearby trees where the adults rest.
Figure 5. Pupa of carrot rust fly, left; larva, right.
Figure 6. Rust fly on yellow sticky trap in carrot field.
Such monitoring indicates if the rust fly is present at levels high enough to warrant an insecticide application. Treatments can be properly timed based on monitoring information. Consult your local Horticultural Crops Advisor for specific recommendations as well as OMAFRA Publication 363, Vegetable Production Recommendations. Also consult OMAFRA publication, Integrated Pest Management for Onions, Carrots, Celery and Lettuce in Ontario, Order No. 700, for monitoring thresholds and treatment guidelines. Home gardeners may consult the most recent issue of OMAFRA publication 64, Insect and Disease Control in the Home Garden. Insecticides are rarely recommended for small gardens.
Monitoring of individual fields or farms could enable some growers to reduce the number of sprays applied. Because of the shelter-seeking habits of the flies, fields located in open areas often escape with little damage and need fewer sprays than those in more sheltered locations. Similarly, isolated carrot fields in other parts of the province may be free from the insect and not require sprays.
Dates of planting and harvest also affect the need to use insecticides. In the Holland Marsh, carrots seeded after mid-May normally are not seriously injured by the first generation larvae and do not require spraying for that generation. Spraying within one month before harvest is not required, because at least one month is required after egg deposition until larvae enter the carrots.
The sheltering behaviour of the carrot rust fly often results in a gradient of damage in the field if damage is severe near the ends of the rows but declines rapidly with distance from the shelter. This can create problems when harvesting mechanically, because carrots with different levels of infestation can become mixed in the pallet boxes. By leaving badly infested areas unharvested, however, the number of overwintering pupae in the soil will be greater in the following year.
As indicated above, the period of attack by second generation larvae continues from early September until the end of harvest, during which time the number of infested carrots will increase. By examining samples of the crop periodically, beginning in mid-September, an indication of the ultimate level of carrot rust fly injury can be obtained. The crop could then be harvested earlier to minimize the proportion of damaged carrots.
The carrot weevil adult is a dark-brown snout beetle about 6 mm long (Figure 7). It overwinters in plant debris in and about carrot fields that were infested the previous year.
In spring, adults become active and mate after a few warm days. They are capable of laying eggs by mid- to late May. However, they do not attack the new crop of carrots until the first true leaf stage. Adult females chew small cavities in the petioles or crown of the carrots and deposit an average of two to three eggs in each (Figure 8), sealing the cavity with a black exudate. Eggs hatch after one to two weeks and the young larvae tunnel down into the root or leave the stalk and enter the roots from the soil. Some young plants may wilt and die as the slender root is tunnelled by the developing larva. The damage is not otherwise conspicuous until the larvae (Figure 9) are nearly mature.
Figure 7. Adult carrot weevil.
Figure 8. Cavity in carrot root containing weevil eggs.
After feeding for at least three weeks, larvae leave the carrot and pupate (Figure 9) in the soil. After one to two weeks, adults emerge. If warm weather or an early carrot crop has permitted adults to mature early enough in the summer, some second-generation eggs and larvae may occur.
Normally, however, there is only one generation in the Holland Marsh. In warmer areas of the continent, second-generation injury is more common. Behaviour of the summer adults after they emerge is not well understood, but they do overwinter as described above.
Monitoring the activity of carrot weevil adults is an effective means of determining the level of infestation and forecasting the need for insecticide treatments. Wooden-plate traps (Figure 10) or carrot-root sections placed in the soil (Figure 10), can be used to monitor adult activity, starting at the time of seeding.
Figure 9. Pupa of carrot weevil, left; larva, right.
Figure 10. Carrot weevil monitoring methods: wooden-plate trap, left; carrot-root sections, right.
Consult OMAF publication, Integrated Pest Management for Onions, Carrots, Celery and Lettuce in Ontario, Order No. 700, for monitoring and threshold guidelines. Treatment thresholds are based on a cumulative weevil per trap count. Between zero and two applications may be necessary, depending on the level of weevil activity in the field. In some cases, border applications may provide adequate control of adults. Parasitism by an egg parasitoid, Anaphes sp. is a significant control factor and should be encouraged. Border areas with weeds offer a suitable habitat for the parasitoid.
Carrot weevil adults rarely fly and therefore the insect does not spread rapidly. Its presence in a particular location should be evident for a season or two before it builds up to serious levels. Development of a serious infestation can be delayed by not planting carrots on or adjacent to sites that were infested the preceding year.
Aster leafhopper adults are inconspicuous, active insects, about 3 mm long (Figure 11), that feed and breed upon a wide range of cultivated and weed hosts.
In Ontario, the insect overwinters in the egg stage on winter grain. The immature stages, or nymphs, hatch in early May and become adults about two weeks later. The adults move from winter grain to various hosts such as spring grain, grasses, vegetables and weeds. During the summer, two to five generations of leafhoppers develop on the various hosts, some of which will harbour AY-MLO. Adults moving to vegetables then introduce the organism and later transmit it within the crop.
In central Ontario, aster leafhoppers first appear on carrots in late May or early June and occur throughout the balance of the season. In southwestern Ontario and the Niagara Peninsula, they usually appear in early May and often in very high numbers.
Figure 11. Adult aster leafhopper.
Reduction of weed hosts through the use of herbicides has reduced the number of aster yellows in the Holland and neighbouring marshes. However, in many parts of southern Ontario, the potential for disease is serious, although it varies from year to year. Leafhopper adults can be monitored with sticky traps or with a sweep net. Currently, there is no way of forecasting what level of aster yellows infectivity may be present in the leafhoppers, however, this capability should be available in the near future.
Some of the insecticides used in carrot rust fly control are also effective against leafhoppers. Therefore, special sprays for leafhopper control will not likely be required unless there is a large increase in leafhopper numbers between generations of the rust fly. In southern Ontario, leafhopper populations may be heavy, and if sprays for carrot rust fly are not required, it may still be necessary to spray for leafhopper control. (See OMAF publication 363, Vegetable Production Recommendations).
It is important to eliminate potential wild hosts of leafhoppers or aster yellows in the vicinity of carrot fields and to remove any infected carrot plants that are detected. Among the weeds that are hosts for aster yellows are thistles, fleabane, wild lettuce, sow-thistle, chicory, wild carrot, galinsoga, dandelion, plantain, cinquefoil and others. Crops such as lettuce, endive, parsnip and celery may also be affected by aster yellows, as well as numerous florist crops in the aster family.
For additional information, consult OMAFRA Factsheet Order No. 98-057, The Aster Leafhopper & Aster Yellows, and OMAFRA publication, Integrated Pest Management for Onions, Carrots, Celery and Lettuce in Ontario, Order No. 700.
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