A method for continuously rearing cabbage root fly, Erioischia brassicae (Bch.) is described.Adult flies fed on sucrose, brewers’ yeast and yeast hydrolysate laid an average of 220 eggs per female when caged together in large numbers and 376 eggs per female when caged singly. Egg viability was 86 per cent. Egg hatch was severely reduced by relative humidities of less than 90 per cent, and the embryo became more susceptible to desiccation as it matured. The larvae were reared on swede, and maximum survival was obtained when the swedes were inoculated with an optimum number of eggs calculated from the ratio of 1 egg: 1.6 g. of swede. Different ratios were obtained from turnip and winter radish indicating that survival was probably related to the nutritional value of the rearing medium as well as to its availability. An average of 68 per cent, of the eggs placed on swede produced pupae that were equivalent in size to wild pupae. The fecundity of the flies was directly related to pupal weight and to the availability of protein in their diet. Adult longevity was reduced more by mutual disturbance than by any other factor.

Brassica and Allium host‐plants were each surrounded by four non‐host plants to determine how background plants affected host‐plant finding by the cabbage root fly (Delia radicum L.) and the onion fly [Delia antiqua (Meig.)] (Diptera: Anthomyiidae), respectively. The 24 non‐host plants tested in field‐cage experiments included garden ‘bedding’ plants, weeds, aromatic plants, companion plants, and one vegetable plant. Of the 20 non‐host plants that disrupted host‐plant finding by the cabbage root fly, fewest eggs (18% of check total) were laid on host plants surrounded by the weed Chenopodium album L., and most (64% of check total) on those surrounded by the weed Fumaria officinalis L. Of the 15 plants that disrupted host‐plant finding in the preliminary tests involving the onion fly, the most disruptive (8% of check total) was a green‐leaved variant of the bedding plant Pelargonium × hortorum L.H. Bail and the least disruptive (57% of check total) was the aromatic plant Mentha piperita × citrata (Ehrh.) Briq. Plant cultivars of Dahlia variabilis (Willd.) Desf. and Pelargonium×hortorum, selected for their reddish foliage, were less disruptive than comparable cultivars with green foliage. The only surrounding plants that did not disrupt oviposition by the cabbage root fly were the low‐growing scrambling plant Sallopia convolvulus L., the grey‐foliage plant Cineraria maritima L., and two plants, Lobularia maritima (L.) Desv. and Lobelia erinus L. which, from their profuse covering of small flowers, appeared to be white and blue, respectively. The leaf on which the fly landed had a considerable effect on subsequent behaviour. Flies that landed on a host plant searched the leaf surface in an excited manner, whereas those that landed on a non‐host plant remained more or less motionless. Before taking off again, the flies stayed 2–5 times as long on the leaf of a non‐host plant as on the leaf of a host plant. Host‐plant finding was affected by the size (weight, leaf area, height) of the surrounding non‐host plants. ‘Companion plants’ and aromatic plants were no more disruptive to either species of fly than the other plants tested. Disruption by all plants resulted from their green leaves, and not from their odours and/or tastes.

Volatile plant chemicals are generally credited with a major role in host plant location by many phytophagous insects. Mixtures of volatiles are produced in cruciferous plants mainly by the hydrolysis of non-volatile glucosinolates to volatile isothiocyanates, thiocyanates and nitriles. The cabbage root fly attacks a wide range of cruciferous plants, with differing odours, implying that several volatile chemicals are probably involved in attracting the flies and in stimulating them to lay. Oviposition by the cabbage root fly was studied, therefore, on a large range of wild and cultivated plants to determine the most preferred species. The volatile chemicals in these species were identified in the hope that some of them may prove more useful as attractants for luring cabbage root flies to field traps. Similar volatile chemicals were released from both intact and macerated plants. It was deduced that any one, or combinations, of eleven chemicals may probably be involved in attraction and host plant selection by this fly. The concentrations of these chemicals change during the life of the plant, however, and an initially attractive chemical can become so concentrated that eventually it repels the insect. Difficulties encountered in experiments on olfaction and possible ways in which they can be overcome are discussed. This paper describes how certain host plant chemicals affect cabbage root fly, Delia brassicae (Wiedemann), behaviour and how they can be used in traps either to monitor or reduce populations of this pest. The cabbage root fly was chosen for the initial study because it can be mass-reared easily in the laboratory (Finch & Coaker, 1969), because its peak of activity can be readily determined in the field, and because its larvae damage the roots of crucifers throughout much of the northern hemisphere, including the whole of the British Isles. This root damage prevents uptake of water and nutrients causing the plants to wilt and, in severe infestations, to die. Methods of host erop findingAgronomic practices associated with vegetable production create large ecological disturbances and the insects that become pests of vegetables are usually well adapted to such disturbances (Southwood, 1962). They tend to be highly mobile and so are capable of finding easily new host crop plantings. Host-finding appears to depend in part on the insect reacting to characteristic chemical odours produced by their host plants.

Laboratory and field‐cage tests were done to determine how undersowing brassica plants (Brassica oleraceae L. and B. rapa L.) (Cruciferae) with subterranean clover (Trifolium subterraneum L.) (Papilionaceae) affected host‐plant selection by eight pest insect species of brassica crops. The pest species tested were Pieris rapae (Lepidoptera: Pieridae) (the small white butterfly), Pieris brassicae (Lepidoptera: Pieridae) (the large white butterfly), Delia radicum (Diptera: Anthomyiidae) (the cabbage root fly), Phaedon cochleariae (Coleoptera: Chrysomelidae) (the mustard beetle), Plutella xylostella (Lepidoptera: Yponomeutidae) (the diamond‐back moth), Evergestis forficalis (Lepidoptera: Pyralidae) (the garden‐pebble moth), Mamestra brassicae (Lepidoptera: Noctuidae) (the cabbage moth) and Brevicoryne brassicae (Hemiptera: Aphididae) (the cabbage aphid). In all tests, except two in which the brassica plants were about three times as high as the clover background, 39%–100% fewer of the pest insect stage monitored were found on host plants presented in clover than on those presented in bare soil. Contrary to claims supporting the ‘enemies hypothesis’, differences in colonization alone appeared sufficient to account for the lower numbers of insects found when host plants are undersown with clover. To be effective in reducing plant colonization, the clover must cover 50%, and preferably more, of the vertical profile of the crop plants. As clover used as an undersown crop often has to be cut to make it less competitive with the main brassica crop, temporal aspects of the condition of the clover during critical periods of pest activity need to be recorded carefully before concluding that undersowing does not produce the effect desired against certain pest species under field conditions. The effective clover barrier is like any other treatment, if it is not present at the appropriate time it cannot be expected to reduce pest insect numbers.

Low temperature was shown to be the major factor regulating diapause development of the cabbage root fly. Diapausing pupae had to be subjected to temperatures of from 0°—6° for 22 weeks for all individuals in the population to complete diapause development. Once this was complete, pupae required a further 14 days at 20° for most of the flies to emerge. Flies that emerged within 14 days at 20°, the criterion for diapause completion, were classed as early‐emerging. The percentage of early‐emerging flies gradually increased as the period of low temperature was extended from 12 to 22 weeks. In the range 0° to 10°, parallel linear relationships were established between the logarithm of the duration of the low temperature and the probit of the percentage of early‐emerging flies. The lower the temperature between 0° and 10°, the more effective it was in terminating diapause. Populations of pupae from nine different localities in England and Wales all took the same time (22 weeks) to complete their low‐temperature diapause development. There was no indication that this rate of development was affected by light, photoperiod, diurnal temperature fluctuations or latitude. RÉSUMÉ Effets de l'intensité et de la durée du froid sur l'évolution de la diapause de la mouche du chou Des pupes de moches du chou élevées en laboratoire et en état de diapause durent ětre soumises à un froid de 4° pendant au moins 12 semaines, puis portées à 20°, avant qu'il ne soit possible à des mouches de sortir dans les 14 jours. On a dit des mouches apparues dans les 14 jours à 20° qu'elles sont apparues tǒt. La proportion de mouches apparues tǒt est devenue graduellement plus importante au fur et à mesure que le traitement froid a été porté de 12 à 22 semaines. Lorsque des pupes en diapause ont été soumises à une température constante de 0 à 10°, l'évolution de la diapause s'est faite plus rapidement aux températures les plus basses. Pour chaque température d'essai on a établi des rapports linéaires entre le logarithme de la durée du traitement froid et le probit du pourcentage de mouches apparues tǒt. Il a été confirmé que 6° est le seuil de morphogenèse de post‐diapause pour les mouches apparues tǒt. Des pupes ont été ramassées en 9 endroits en Angleterre et au Pays de Galles de novembre 1980 à janvier 1981, elles ont été conservées à 4° et toutes avaient terminé leur diapause le 9 mars. Toutes les générations F1 des populations ont réagi aux températures de 0 à 10° de manière analogue aux populations de Wellesbourne, l'évolution de la diapause étant terminée dans 84–91% des populations F1 au bout de 19 semaines à 4°.