Cavariella aegopodii Scopoli Homoptera Aphididae

Natural History

Distribution. Willow-carrot aphid is widespread in the temperate areas of the world. In North America it occurs mostly in the northern United States and probably in southern Canada. It is a pest principally along the west coast, from Washington to California. Its origin seems to be Asia.

Host Plants. Willow-carrot aphid alternates between primary hosts, which are fed upon during the autumn and spring months, and secondary hosts, which are fed upon during the summer. The primary host is willow, Salix spp. (Dunn, 1965; Kundu and Dixon, 1994). Willow-carrot aphid feeds on Umbelli-ferae during the summer months. Vegetables attacked include carrot, celeriac, celery, chervil, coriander, fennel, parsnip, parsley, and parsnip. Also infested are the herbs anise, dill, and caraway, and numerous umbelliferous weeds such as wild carrot, Daucus carota; cow parsnip Heracleum lanatum; angelica, Angelica sylvestris; lovage, Ligusticum spp.; and water hemlock, Circuta spp. (Palmer, 1952; Dunn and Kirkley, 1966).

Natural Enemies. The enemies of willow-carrot aphid are similar to those affecting most other aphids. Lady beetles (Coleoptera: Coccinellidae), flower flies (Diptera: Syrphidae), and anthocorid bugs (Hemiptera: Anthocoridae) were the dominant predators observed in England. An unspecified fungal disease and parasi-toids were also observed.

Weather. Probably the most important factor affecting aphid populations in England is weather. Precipitation, either at normal levels or above normal, suppresses aphid abundance, especially when it occurs during the period of dispersal by winged forms (ala-tae). Severe winter weather has the potential to kill aphids overwintering on secondary hosts, both cultivated and wild Umbelliferae; this greatly reduces the population size in the subsequent crop season (Dunn, 1965).

Life Cycle and Description. The life cycle of this insect has been thoroughly studied only in England, but it is likely very similar in the temperate areas of North America. Overwintering occurs on willow, with egg hatch as early as February. In the spring, several generations may occur on willow, followed by dispersal to summer hosts in the plant family Umbelliferae. During autumn, aphids migrate back to willows and oviposit. In England, some individuals seem to overwinter successfully in the adult and immature stages on umbellifers that remain in the soil and do not die back completely during the winter months (Gratwick, 1992).

  1. Females of willow-carrot aphid apparently do not discriminate among the species of willows when they select oviposition sites in the autumn. The overwintering eggs are shiny and black, and are found around the bud axils.
  2. Young aphids feed through the thin bark of new shoots if the buds are not yet emerged from willow, later moving to the leaves and catkins. The form developing in the spring is rusty brown or red, and measures 1.8-2.0 mm long. Nymphs of summer populations developing on umbellifer foliage are green and elongate, and are very difficult to discern.
  3. Parthenogenetic reproduction occurs on willow, and starting in about May alatae (winged forms) are produced. Apterae (wingless forms) are green or yellowish-green, and measure 1.8-2.2 mm long. They are also greenish, but have black or dark coloration on the head, thorax, and abdomen, with the abdominal markings in the form of irregular bars. The aphids measure 1.2-2.0 mm long. The legs and cornicles of both apterous and alate forms are pale or dusky, and darker distally. Most alatae migrate to, and colonize, Umbelliferae. However, some disperse to, and colonize, other willows. Concurrently, some aph-ids apparently overwinter on umbellifers, probably carrot, and migrate to willow in the spring. The number of aphids on willow, regardless of the source, dissipates through the summer months, and essentially is eliminated by autumn.

Winged aphids arrive on, and infest, secondary hosts over a period of several weeks, with peak inva sion occurring about early June. Peak numbers occur about three weeks after the June influx owing to reproduction, and then are decreased due to emigration and the activity of predators and parasitoids. Aphid numbers increase on secondary hosts again in the autumn, immediately preceding migration to primary hosts. Males are produced before migration. Not all aphids depart, of course, some remain on the secondary hosts for the winter. In the autumn, after dispersal back to willow, oviparous sexual forms are produced and mating occurs. Females then deposit overwintering eggs on willows. The egg-producing form is unlike the summer forms, but resembles the early spring aphids and is rusty brown and black.

The work by Dunn (1965) and Dunn and Kirkley (1966) presented the most complete information on willow-carrot aphid biology. Cottier (1953) provided a good description. Keys for identification of willow-carrot aphid, and many other common aphids, were provided by Palmer (1952) and Blackman and Eastop (1984).

Damage

There are two forms of damage caused by willow-carrot aphid—direct feeding injury and disease transmission. Direct feeding causes development of reddish or yellow foliage, and distortion of foliar tissue. Hon-eydew also accumulates on foliage. For parsley, contamination of the foliage with aphids makes the crop unmarketable. These are considered to be relatively minor problems.

The importance of willow-carrot aphid is related to its ability to transmit a circulative virus complex, known as carrot motley dwarf, to carrots. Virus infection causes carrot foliage to turn red or yellow and to be stunted. Reservoirs of this virus, in addition to carrot, cilantro, and chervil, are the many other secondary umbelliferous hosts fed upon by willow-carrot aphid during the summer months. Of lesser importance is the ability of C. aegopodii to transmit parsnip yellow fleck virus, anthriscus yellow virus (Elnagar and Murant, 1976), and many other stylet-borne viruses (Kennedy et al, 1962).

Management

Insecticides. Insecticides may be applied in the spring as aphids move from primary to secondary hosts and 2-3 applications are adequate. Either systemic or contact insecticides may be used. In the absence of high insect and disease pressure, which is the case in North America if crops are managed properly, such treatments should not be necessary.

Cultural Practices. The most critical element of cultural control is sanitation (see section on disease transmission, below). Overwintering sources of aphids and disease must be eliminated.

Other aspects of cultural control are helpful, but less important. Attempts to identify carrot cultivars with resistance to willow-carrot aphid have not been very successful (Dunn, 1970). However, susceptibility of carrots to the carrot motley dwarf virus complex varies considerably among commercially available cultivars (Watson and Falk, 1994). Chemical attractant and repellent research (Chapman et al., 1981) has demonstrated that willow-carrot aphid is sensitive to odors, but this information has not yet been put to practical use. In research conducted in England, intercropping of carrots with onions did not affect invasion of carrots by willow-carrot aphid (Uvah and Coaker, 1984), but plant density was important. As the distance between carrot rows decreased, there was a corresponding diminution of aphid numbers.

Disease Transmission. Although carrot motley dwarf virus can be very damaging, in some areas it can be managed effectively. Carrot cultivars vary widely in their susceptibility to infection by carrot motley dwarf virus, so cultivar selection is important. In California, the proximity of overwintering carrots is directly related to the severity of the disease. Weeds are not important sources of aphids or disease in this production area. Overwintering carrots should be eliminated, isolated from new production sites, or treated with insecticide to reduce the number of vectors available to transmit the virus (Watson and Falk, 1994).

In Washington, secondary spread of the virus within carrot fields is especially important (Howell and Mink, 1977). Virus was introduced into carrot fields in the spring by C. aegopodii, as was the case in California. However, there can be high levels of secondary transmission within the carrot fields during the summer months. Howell and Mink (1977) suggested that green peach aphid, Myzus persicae (Sulzer), was implicated in secondary spread though this seems unlikely because it is not a known vector of this disease.

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