Relation of Aphid Populations to Field Spread of Lettuce Mosaic Virus in New York1

Aphid species migrating to and colonizing lettuce plantings were sampled to assess their importance in field spread of lettuce mosaic virus (LMV) under conditions existing in New York State. The studies were carried out in small-plot experiments and included an evaluation of various insecticides for regulation of field spread of LMV through reduction of numbers of aphids colonizing lettuce. The potato aphid, Macrosiphum euphorbiae (Thomas) was the predominant species colonizing lettuce, but high-level populations throughout the season were not correlated with significant field spread of LMV. Conversely, LMV infection of more than 90% was noted when plants were treated with oxydemetoumethyl and aphid numbers were very low. Large numbers of LMV-infected plants were detected only in late August and September, and this corresponded closely with peak numbers of alatae aphid populations captured in water-filled yellow pans (Möericke traps). A close correlation was found between incidence of LMV-infected plants and peak numbers of alatae of green peach aphid, Myzus persicae (Sulzer). Infected plants appeared in an apparent random pattern. Healthy plants protected from wandering apterae by sticky enclosures became infected with LMV as readily as plants not protected. These observations indicated that LMV was spread chiefly by the alatae. Oxydemetonmethyl was the most effective insecticide for reducing and maintaining low numbers of aphids on lettuce, but plants treated with a combination of DDT and malathion had more aphids than untreated plants. None of the insecticides was effective for preventing the spread of LMV.