Distribution. This species is of European origin. It was detected in the eastern United States and eastern Canada during the early 1970s, and in the northwestern United States and British Columbia shortly thereafter. It has become a problem in California but remains absent from the Great Plains region. Lettuce aphid also is established in South America. Like many aphids, it is favored by cool weather, and seems likely to remain a pest mostly in the northern areas of North America.
Host Plants. Lettuce aphid alternates between its primary, or winter, hosts and secondary, or summer, hosts. The winter hosts are Ribes spp., principally gooseberry but also red and black currant. Summer hosts are more varied, consisting of numerous plants in the family Compositae, including hawksbeard, Crepis spp.; hawkweed, Hieracium spp.; sowthistle, Sonchus sp.; and nipplewort, Lampsana communis. Some members of the family Scrophulariaceae (Euphrasia spp., Veronica spp.) and Solanaceae (Nicotiana spp., Petunia spp.) are also colonized. Among the vegetable crops injured are chicory, raddicio, and lettuce.
Natural Enemies. Few natural enemies of lettuce aphid are documented, though the parasitoid Mono-ctonus crepidus (Haliday) (Hymenoptera: Aphidiidae) is known to attack lettuce aphid in Canada. It seems likely that common aphid predators such as lacewings (Neuroptera: Chrysopidae and Hemerobiidae), flower flies (Diptera: Syrphidae), and lady beetles (Coleoptera: Coccinellidae) are important natural mortality agents.
Life Cycle and Description. Following egg hatch, parthenogenetically reproducing females and offspring are found on currant and gooseberry plants in British Columbia from March or April-May. Migration to secondary hosts occurs during late May and June, and many generations develop during the summer months. Migration back to primary hosts occurs in September and October, followed by mating, ovipo-sition and overwintering in the egg stage. The eggs are deposited on buds, twigs, and foliage of overwintering hosts. Under mild-winter conditions, some aphids can remain on lettuce through the winter months.
These aphids are pale-green to apple-green when fed on Ribes. Winged dispersants (alatae) bear extensive black or dark pigmentation, often including dark head, thoracic regions and cornicles, dark patches or bars on the abdomen, and dark bands on the legs. The wings lack pigmentation. The wingless forms (apterae) developing on lettuce are pale-yellow to green, and sometimes reddish; they also are marked with darker patches, though the head and thorax are not uniformly dark. The antennae and cornicles are long in this species. Alatae measure 1.3-3.2 mm long and apterae 1.5-3.1 mm. The sexual forms are similar, averaging slightly smaller.
Lettuce aphid and its damage were described by Forbes and Mackenzie (1982). Description and keys were found in Heie (1979) and Blackman and Eastop (1984).
Lettuce aphid can be a serious pest because, unlike other lettuce-infesting aphid species, it can colonize
the interior portions of developing lettuce heads, making detection and removal difficult. Both field- and greenhouse-grown lettuce may be affected, and though the interior leaves are infested, exterior leaves are more heavily populated. In addition to contamination of lettuce heads, lettuce aphids cause injury by transmitting such lettuce-infecting viruses as cucumber mosaic. The early season aphid colonies occurring on Ribes, though small, cause leaf curl and retardation of shoot growth. On gooseberry, lettuce aphids transmit gooseberry veinbanding virus. Kennedy et al. (1962) reported that several other viruses also were transmitted by lettuce aphid.
Cultural Practices. If head interiors become infested it is important to destroy the crop by tillage rather than to simply abandon the field, as this reduces the potential to spread to other fields. Weeds can also be important sources of aphids.
Host-Plant Resistance. Host-plant resistance has been examined extensively in Europe. Although partial resistance to lettuce aphid is not uncommon, varieties resistant to lettuce aphid often are not equally resistant to other aphids infesting lettuce, such as potato aphid, Macrosiphum euphorbiae (Thomas), and lettuce root aphid, Pemphigus bursarius (Linnaeus). The exception seems to be iceburg lettuce, which displays considerable resistance to all three aphid species (Dunn and Kempton, 1980b). Some resistance among Ribes cultivars also has been observed (Keep and Briggs, 1971).
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