Rosemary Collier

Host‐plant selection by insects – a theory based on ‘appropriate/inappropriate landings’ by pest insects of cruciferous plants

Seven hypotheses, including the ‘Resource Concentration Hypothesis’ and the ‘Enemies Hypothesis’, have been put forward to explain why fewer specialist insects are found on host plants growing in diverse backgrounds than on similar plants growing in bare soil. All seven hypotheses are discussed and discounted, primarily because no one has used any of them to produce a general theory of host plant selection, they still remain as hypotheses. However, we have developed a general theory based on detailed observations of insect behaviour. Our theory is based on the fact that during host plant finding the searching insects land indiscriminately on green objects such as the leaves of host plants (appropriate landings) and non‐host plants (inappropriate landings), but avoid landing on brown surfaces, such as soil. The complete system of host plant selection involves a three‐link chain of events in which the first link is governed by cues from volatile plant chemicals, the central link by visual stimuli, and the final link by cues from non‐volatile plant chemicals. The previously ‘missing’ central link, which is based on what we have described as ‘appropriate/inappropriate landings’, is governed by visual stimuli. Our theory explains why attempts to show that olfaction is the crucial component in the central link of host plant selection proved intractable. The ‘appropriate/inappropriate landings’ theory is discussed to indicate the type of work needed in future studies to improve our understanding of how intercropping, undersowing and companion planting can be used to optimum effect in crop protection. The new theory is used also to suggest how insect biotypes could develop and to describe why pest insects do not decimate wild host plants growing in ‘natural’ situations.

VISUAL DETECTION OF HOST PLANTS BY CABBAGE ROOT FLIES

Using mimics of real plants, we found that female cabbage root flies, Delia radicum (L.), detected plants within host patches at least partly on the basis of leaf colour and area, with leaf pattern playing an insignificant role in the case of multi‐leaved plants. Composition of the background, whether bare soil or green grass, did not affect female ability to distinguish between leaf mimics of different colour. The contribution of visual stimuli in eliciting female landings on individual plants within a patch decreased as distance between the plants was increased.

Companion planting – do aromatic plants disrupt host‐plant finding by the cabbage root fly and the onion fly more effectively than non‐aromatic plants?

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.

Matching control options to a pest complex: the integrated pest management of aphids in sequentially-planted crops of outdoor lettuce

Abstract Sequentially planted short-season vegetable crops grown in temperate climates offer the opportunity to use known variations in pest phenology through the season to develop a strategic way of matching control options on different plantings to predicted levels of pest risk. To test this approach in the UK, five field experiments were done over two years to test integrated pest management (IPM) programmes for four aphid pest species ( Nasonovia ribisnigri, Macrosiphum euphorbiae, Pemphigus bursarius and Myzus persicae ) on outdoor lettuce crops. Crops were planted to coincide with different periods of forecast aphid risk. The results suggested that acceptable levels of aphid control could be achieved, provided a full range of treatment options (resistant cultivars, selective insecticides, biocontrol agents and validated pest forecasts) could be utilised. Commercial and technical constraints to the commercial adoption of this approach are discussed.

The influence of host and non-host companion plants on the behaviour of pest insects in field crops

Companion plants grown as ‘trap crops’ or ‘intercrops’ can be used to reduce insect infestations in field crops. The ways in which such reductions are achieved are being described currently using either a chemical approach, based on the ‘push‐pull strategy’, or a biological approach, based on the ‘appropriate/inappropriate landing theory’. The chemical approach suggests that insect numbers are reduced by chemicals from the intercrop ‘repelling’ insects from the main crop, and by chemicals from the trap‐crop ‘attracting’ insects away from the main crop. This approach is based on the assumptions that (1) plants release detectable amounts of volatile chemicals, and (2) insects ‘respond’ while still some distance away from the emitting plant. We discuss whether the above assumptions can be justified using the ‘appropriate/inappropriate landing theory’. Our tenet is that specialist insects respond only to the volatile chemicals released by their host plants and that these are released in such small quantities that, even with a heightened response to such chemicals, specialist insects can only detect them when a few metres from the emitting plant. We can find no robust evidence in the literature that plant chemicals ‘attract’ insects from more than 5 m and believe that ‘trap crops’ function simply as ‘interception barriers’. We can also find no evidence that insects are ‘repelled’ from landing on non‐host plants. Instead, we believe that ‘intercrops’ disrupt host‐plant finding by providing insects with a choice of host (appropriate) and non‐host (inappropriate) plant leaves on which to land, as our research has shown that, for intercropping to be effective, insects must land on the non‐host plants. Work is needed to determine whether non‐host plants are repellent (chemical approach) or ‘non‐stimulating’ (biological approach) to insects.

Thinking about ‘food security’: engaging with UK consumers

‘Food security’ has recently gained policy salience in the UK and internationally. Definitions vary, but the term is generally used by policy makers to imply sustained access by all consumers to sufficient food that is affordable, safe, nutritious and appropriate for an active and healthy life. Recent attention partly reflects anxiety over possible resource and environmental instabilities within the food system and the effects of economic recession. Food prices are often used to signal potential food insecurity; prices have risen recently in Britain as elsewhere, along with increased fuel costs and significant financial and job insecurities. All these factors are likely to have differential effects on food management in households living in different social and economic circumstances. Recent research using a mixed methods approach explored some of these complexities by engaging with UK consumers to examine peoples reactions to increasing food prices and their views on responsibility for ‘food security’. Well aware of increased food costs, most could identify key commodities and many cited increased oil and input prices as causes; some made links to the larger financial crisis. Few knew the term ‘food security’; though most initially interpreted it as food safety and quality, the idea that affordable, healthy food should be available and accessible for all was widely recognised. Many saw this as increasingly difficult for themselves and others in current circumstances and, while acknowledging commercial realities, look to government primarily to secure nutrition and food security for all.

Laboratory studies on aestivation in the cabbage root fly (Delia radicum)

Laboratory experiments at 16°–30°C showed that the cabbage root fly can only be induced into aestivation during the early part of the pupal stage. The relationship between the percentage of pupae entering aestivation (y) and the average daily temperature (T) was the same under both constant and alternating temperatures. The percentage of pupae entering aestivation at a particular temperature was given by y = 13 T — 255. An additional 13% of the pupae entered aestivation for each degree rise between 20° and 27° C. There was no constant period of arrested development following the induction of aestivation. Aestivating pupae started to develop into flies as soon as the temperature fell below 20°C. The reinduction of aestivation occurred more readily than the initial induction. Insect survival was not reduced after a month of aestivation. The effects of aestivation on forecasting the time of the second (summer) generation of flies are discussed.

COMPLETION OF DIAPAUSE IN FIELD POPULATIONS OF THE CABBAGE ROOT FLY (DELIA RADICUM)

The various diapause and post‐diapause stages entered by cabbage root fly pupae during the overwintering period are shown schematically. Although diapause induction started in mid‐Aug., the early‐pupating insects did not develop further but were maintained in diapause by the warm autumn temperatures. Therefore, diapause development was simultaneous in all Wellesbourne pupae, whether of second or third generation origin. Diapause development started only in mid‐Oct., when mean soil temperatures fell below 10°. In the field, 90% of the overwintering population of cabbage root fly pupae had completed diapause by 5 March 1980, 17 Feb. 1981 and 18 Feb. 1982. This was equivalent to a duration of 19 weeks from mid‐Oct. onwards, during the winters of 1979–80, 1980–81 and 1981–82 respectively. A further break between the completion of diapause and the warm conditions required to start post‐diapause development also helps to condense the emergence of flies in the spring. Hence, an accurate forecast of the time of spring attack by populations of flies similar to those at Wellesbourne should be possible.

Companion planting – behaviour of the cabbage root fly on host plants and non-host plants

Six‐hundred individual female cabbage root flies (Delia radicum L.) (Diptera: Anthomyiidae) were each observed for 20 min under laboratory conditions to record how they behaved after landing on a host or a non‐host plant. Fly movements were recorded on host plants [cabbage –Brassica oleracea var. capitata (Cruciferae)] and non‐host plants [clover –Trifolium subterraneum L. (Papilionaceae)] surrounded by bare soil and on cabbage surrounded by clover. The most frequently observed behaviours made by the flies were (1) hops/spiral flights and (2) walks/runs. In the bare soil situation, the 50 individual flies observed in each treatment made 66 hops/spiral flights on the cabbage and 94 on the clover. When the two plants were tested together the movements were not additive as, instead of the expected 160 hops/spiral flights in the mixed plant treatment, the flies made 210 hops/spiral flights when they landed initially on cabbage but only 130 when they landed initially on clover. Few of the flies that landed initially on clover moved onto the host plant, even though the host plant was only a few centimetres away. The duration of the individual walks and runs made by the cabbage root flies were similar on both the host and non‐host plants. The only differences were the numbers of walks/runs made and the time the flies remained inactive. On the host plants, the females made four walks/runs, each of about 12 s duration, interspersed by rest periods that totalled 1.5 min. In contrast, on the non‐host plants the females made 10 walks/runs, each of about 9 s duration, interspersed by rest periods that totalled 7 min. Therefore, after landing on a plant, the flies, on average, left the host plant after 2.25 min and the non‐host plant after 8.5 min. Our conclusion is that the protracted time spent on the non‐host plants is the mechanism that disrupts insects from finding host plants in diverse plantings. Hence, the flies were arrested by non‐host plants rather than being repelled or deterred as suggested in earlier studies.

Effects of the angle of inclination of traps on the numbers of large Diptera caught on sticky boards in certain vegetable crops

The angle of inclination of the surface on which flies prefer to land in vegetable crops was studied in field plots and in field‐cages using one‐sided sticky traps aligned in one plane but orientated in eight directions. The four Delia species studied, D. antiqua, D. floralis, D. platura and D. radicum, preferred to land on horizontal surfaces, indicating that they are likely to be trapped in largest numbers on traps with a horizontal trapping surface, such as water traps. This trend was even more pronounced with the Syrphidae. In contrast, greatest numbers of the carrot fly, Psila rosae, were caught on the lower surface of traps inclined at 45° to the vertical. The advantages of using traps inclined in this way for trapping P. rosae are that more flies are caught on such surfaces, the sticky trapping compound is protected from the adverse effects of rain, and the traps are highly selective.