Pemphigus bursarius Linnaeus Homoptera Aphididae

Natural History

Distribution. This species is likely of European origin, but is now found widely around the world, including the Middle East, Central Asia, Siberia, North and South Africa, and Australia in addition to North America. It thrives in temperate environments, and is found in southern Canada and in northern regions of the United States, and in California.

Host Plants. The primary, or winter, host of lettuce root aphid is poplar, Populus spp. The Lombardy poplar, Populus nigra var. italica is the most common host, but other varieties of P. nigra and of P. deltoides also are recorded as hosts. The secondary, or summer hosts of lettuce root aphid are normally plants in the family Compositae such as wild lettuce, Lactuca seriola; sow thistle, Sonchus arvensis; and dandelion; Taraxacum officinale. Plants from other families such as redroot pigweed, Amaranthus retroflexus (Amaran-thacea); shepherdspurse, Capsella bursa-pastoris (Cruciferae); lambsquarter, Chenopodium album (Che-nopodiacea); and white campion, Lychnis alba (Caryo-philaceae) have been reported to support small populations of these aphids (Alleyne and Morrison, 1977b), but these records are suspect. Vegetable crops damaged by lettuce root aphid include chicory, endive, and lettuce.

Natural Enemies. Several natural enemies are known from Quebec (Alleyne and Morrisson, 1977c) and from Europe (Dunn, 1960). The natural enemy complex in Quebec varies considerably depending upon the host plant and location of the aphids on the host. It is difficult for large predators to gain access to aphids feeding within intact galls, but galls are sometimes damaged by birds, and small predators can gain entry through the gall exit hole. The pirate bugs Antho-corus spp. (Hemiptera: Anthocoridae), the flower flies Syrphus ribesii (L.) and Metasyrphus musculus (Say) (both Diptera: Syrphidae), the chaemaemyiid Leucopis pemphigae Malloch (Diptera: Chamaemyiidae), and the lacewings Hemerobius and Chrysopa spp. (Neuroptera: Hemerobiidae and Chrysopidae) feed on aphids within galls. Aphids on the surfaces of poplar and lettuce leaves also are subject to predation by such insects as Chrysopa sp. lacewings; spined assassin bug, Sinea diadema (Fabricius) (Hemiptera: Reduviidae); the damsel bug Nabis ferus (Linnaeus) (Hemiptera: Nabidae); and such lady beetles as Adalia sp., Coccinella spp., Coleomegilla maculata (De Geer), and Hippodamia spp. (all Coleoptera: Coccinellidae). Aphids feeding on roots are less exposed to predation, but Thaumatomyia glabra (Meigen) and T. bistriata (Meigen) (both Diptera: Chloropidae), and Sphaerophoria menthastri (Linnaeus) (Diptera: Syrphidae) were observed in Quebec. An aphid fungal disease readily infects aphids below-ground, and protozoa and a nematode were occasionally observed. In England, similar natural enemies were observed, though the species are different. In addition, however, rove beetles (Coleoptera: Staphyli-nidae) and ground beetles (Coleoptera: Carabidae) are aphid predators in England, and further study in North America likely would demonstrate the importance of these predators.

Life Cycle and Description. Lettuce root aphid normally overwinters in the egg stage on poplar trees, hatching in March-April. The females hatching from these eggs are large, rotund, and highly fecund. They reproduce parthenogenetically, and after two generations on poplar, winged forms (alatae) disperse to summer hosts. Dispersal of alatae usually occurs in June-July, and on summer hosts the aphids produce about three generations of parthenogenetic wingless aphids (apterae). Beginning in about August, winged aphids are again produced, with dispersants migrating to the overwintering hosts. On poplar, small non-feeding male and egg-producing female aphids are produced, mating occurs, and eggs are produced. However, some aphids can also remain on lettuce or weed roots throughout the winter, colonizing nearby plants in the spring.

  1. The egg is elongate oval, initially greenish-white but eventually orange, and enveloped in slender strands of waxy material. The egg measures about 0.48 mm long and 0.23 mm wide. They are deposited in cracks and crevices of poplar bark at various heights up to at least 9 m on the tree.
  2. Nymphs infesting foliage are green, and initially are found within poplar leaf petiole galls caused by feeding of the first generation adult. The nymphs have five instars, and may require about 5060 days in the spring to complete their development. Nymphs also are present on summer hosts, where their development time is much shorter, and where they respond to increasing density and declining host quality by producing winged progeny that disperse to poplar trees (Judge, 1968; Dunn, 1974). Nymphs on roots of summer hosts are not green; rather, they are light-gray and covered with bluish wax. Nymphs are not capable of burrowing through soil, but they move freely through cracks and over the surface of the soil, sometimes moving from plant to plant in this manner. Nymphs of all instars can overwinter below-ground on summer hosts, often occurring at depths of 10-20 cm (Alleyne and Morrison, 1978a).
  3. The adults of this species lack elongate cornicles; they are pore-like or absent. Adults possess relatively short antennae and legs. The adult female resulting from overwintering eggs is enveloped in bluish-white wax. It is greenish, with dark appendages, and measures 2.0-2.2 mm long. The feeding of this adult induces formation of a gall on the leaf petiole of poplar. The gall measures 6-18 mm in diameter and can eventually contain about 100 aphids, or more. The adults emerging from the gall are winged (alate), disperse to summer hosts, and immediately begin reproduction. The new females produce 15-25 nymphs within the first hour, which give rise to wingless (apterous) adults. Wingless adult females produced during the remainder of the year are oval and yellowish-white in color, and measure 1.6-2.5 mm long. The short antennae are grayish to dark with black head. The dorsal surface of the body bears six longitudinal rows of circular wax glands, which produce waxy threads over the body. The entire aphid also is dusted with a fine white powder. The population of aphids peaks in August, at which time some nymphs move closer to the surface and molt into winged adults. Winged adult females (alatae) are gray, green, or brownish, with darker head, antennae, thorax and markings on the abdomen. The wing veins are dusky brown. The alatae also produce a waxy secretion, and measure 1.7-2.2 mm long. The reproductive forms produced late in the season on poplar are orange and lack functional mouthparts. The sexual forms are quite small, the males measuring 0.65 and the females 0.75 mm long. The abdominal cavity of the female is filled with a single egg that is deposited in a bark crevice.

The biology of this aphid was described by Dunn (1959a,b) and Alleyne and Morrison (1977b). Cottier (1953) provided a good morphological description. A

Adult female lettuce root aphid, wingless form.

Adult female lettuce root aphid, wingless form.

method of rearing was presented by McLean and Kin-sey (1961). Lettuce root aphid was included in the keys of Palmer (1952) and Blackman and Eastop (1984).

Damage

Feeding by lettuce root aphid results in wilting, yellowing and stunting of leaves. Rootlets may turn brown and die, and roots are covered with masses of bluish white, woolly material if aphids are present. Heavy infestations can cause softening and collapse of the lettuce head, and the death of the plant.

Management

Insecticides. Insecticides may be applied to the soil for protection of lettuce varieties that are susceptible to root aphid attack. Contact and systemic materials, liquid and granular formulations, and plantingtime as well as side-dressed applications can be made, but effectiveness varies among products, and most insecticides work better as a preventative (Gentile and Vaughan, 1974; Alleyne and Morrisson, 1977a).

Cultural Practices. Destruction of old lettuce plants, particularly root stumps, and alternate hosts is often recommended to decrease the likelihood that aphids will be present when future crops are planted. Although the practice of sanitation always has considerable merit, it is most effective if soil temperatures are warm. This is because at cool temperatures lettuce root aphids can survive for months without food (Dunn, 1959a; Alleyne and Morrison, 1978a). Also, the presence of Lombardy poplars near lettuce fields is detrimental to aphid management, and planting of Lombardy poplars in lettuce production areas should be avoided, but few advocate destruction of these short-lived trees if they already exist.

Soil water management can be important in minimizing aphid damage. Adequate and even levels of soil moisture promote rapid growth of lettuce, enabling the plants to mature quickly and to escape the eventual population increase of the aphids. Plants also are more tolerant of injury if they be provided with adequate moisture. Constant soil moisture also limits cracking of the soil, limiting access to the roots by aphids and interplant movement.

Host-Plant Resistance. Alleyne and Morrison (1978b) evaluated lettuce varieties for susceptibility to aphid attack in Quebec. The varieties varied considerably in their susceptibility to damage, but some commercial romaine (cos) and crisphead varieties displayed some resistance. Dunn and Kempton (1980b) identified additional resistant varieties.

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