Cold resistance and overwintering survival of the cabbage root fly, Delia radicum (Anthomyiidae), and its parasitoid, Trybliographa rapae (Cynipidae), in England

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Abstract
Overwintering Delia radicum (L.) in a field of swedes (Brassica napus L.) near Ascot, Berks., England, were exposed to soil temperatures below 10°C on 176 days from 21 October 1983 to 22 April 1984, but no temperatures below 0°C were recorded. Collections of D. radicum taken at monthly intervals from 1 November 1983 to 30 April 1984 showed that parasitism by the cynipid Trybliographa rapae (Westw.) and by the staphylinid Aleochara bilineata Gyll. was the main source of mortality. A substantial increase in parasitism by A. bilineata occurred during November, but much of the increase was by superparasitism of pupae previously parasitized by T. rapae. Mortality from causes other than parasitism was greater in the November and December collections (c. 22%) and in the spring (c. 12%) than during the winter (c. 3%) and could not be attributed to low temperatures. In all collections, most of the adult D. radicum (c. 90%) emerged within 230°D5.6. The mean number of °D5.6 to eclosion did not change from 1 November to 5 March but decreased significantly by 2 April. Late-emerging adults remired 259 to 992°D5.6 to eclosion and no changes in the nean number of °D occurred among collections. Among unparasitized D. radicum, individual supercooling points showed a strong peak at c.-23°C. A significant proportion of pupae with supercooling points above-20°C were found only in the 1 November 1983 and the 30 April 1984 collections. The ‘high’ supercooling points in the November collection may have comprised apparently healthy but moribund individuals, whereas in the April collection they may have included individuals that had initiated postdiapause development. The mean supercooling point of individuals in the ‘low’ category (supercooling points ≤20°C) did not vary among monthly samples. Parasitism by T. rapae increased the variability in supercooling points, resulting in a few individuals with lower, and many more with higher supercooling points than among unparasitized individuals. The mean supercooling point increasing from the collections of November and December to those of January to April. Puparia containing unparasitized D. radicum pupae were heavier and contained more water than those with parasitized pupae, but neither group showed significant ranges over winter. Supercooling points were positively correlated with puparial live weight among unparasitized but not among parasitized pupae. Supercooling points were not correlated with water content for either group. Parasitism did not affect the occurrence or concentration of sugars and polyhydric alcohols (allD. radicum can be considered to be over-protected from lethal freezing in the pupal stage and its high supercooling capacity in England may persist because it is conferred by the structural properties of the dipteran puparium and pupa within it and therefore is not subject to selection pressures.