Study on inheritance of resistance to root aphid, Pemphigus bursarius, in lettuce

The inheritance of resistance to lettuce root aphid, Pemphigus bursarius, was studied in lettuce using the Wellesbourne cultivars Avondefiance and Avoncrisp as resistant parents and Borough Wonder and Webb's Wonderful as aphid-susceptible parents. All four cultivars were crossed in all possible combinations including reciprocals and the response to root aphid of plants in the P₁F₁F₂ and BC generations was assessed using apterae of P. bursarius from the lettuce cv. Iceberg. Resistance to attack was clearly inherited and the parents appeared to be homozygous for their resistance or susceptibility. In the F₁ generation, however, in all crosses between resistant and susceptible parents, segregation into susceptible, resistant and some slightly less resistant plants occurred. This and the highly significant differences in segregation between pairs of reciprocal crosses in the F₁ and other generations indicate that the inheritance of resistance to root aphid is controlled by extra-nuclear factors. Modifying genes might also be involved but there appears to be no linkage of root aphid resistance with resistance to downy mildew, for which the Wellesbourne lettuces were bred.     

Seasonal changes in aphid populations on Brussels sprouts

Populations of Brevicoryne brassicae (L.) and Myzus persicae (Sulz.) were assessed weekly on Brussels sprouts at Wellesbourne, from July onwards in each of 3 years. A peak of B. brassicae numbers occurred every year about the end of July, and in 1968 and 1970 this was larger than the September peak. Some alates were produced in the generation born to the June immigrants and the dispersal of those produced at the end of July consistently brought a decline in B. brassicae populations in early August. In 1968 and 1970, however, outbreaks of entomogenous fungi which followed rain, added considerably to this decline. Records of predators and parasites indicated that these natural enemies had only a minor effect on aphid numbers. M. persicae colonized sprouts slightly earlier than B. brassicae; in 1969 and 1970 its numbers in July were low and some persisted after the first week of August in 1968 only. In each year the first eggs of B. brassicae were laid at the beginning of October, but viviparae were still numerous for a further 7 to 10 weeks.     

THE EFFECT OF TEMPERATURE ON REPRODUCTION BY THE ALATAE OF PEMPHIGUS BURSARIUS (L.)

The reproductive rate of the winged forms of Pemphigus bursarius was studied at temperatures between 38 and 82° F. The fundatrigeniae gave the greatest total reproduction at about 60° F. but the highest reproductive rate at 70–81° F. Corresponding temperatures for the sexuparae were 55 and 67° F. approximately. The average outdoor air temperatures prevailing at the time when the respective winged forms are most abundant correspond closely to the temperatures required by the alatae for maximum reproduction. On a basis of temperature, adult sexuparae trapped underground can expect to survive about 12 days in early September and about 30 days at the end of October.     

WING COLORATION AS A MEANS OF DETERMINING THE AGE OF THE COLORADO BEETLE (LEPTINOTARSA DECEMLINEATA SAY)

The age of Colorado beetles can be determined, for at least 20–23 days after emergence, by means of the colour of the membranous hind-wings. The colour changes are described and a series of colour reproductions illustrate the appearance of a selection of wings during the first 25 days after emergence from the soil.