Fluctuations in populations of wheat bulb fly (Leptohylemyia coarctata) at Rothamsted

Several field experiments at Rothamsted have rotations including plots fallowed before sowing to winter wheat each year, which provide ideal conditions for maintaining infestations of wheat bulb fly (Leptohylemyia coarctata). In eight consecutive years of one experiment, a mean of 13% of the eggs laid on fallows in these rotations eventually survived to become adults. There was a considerable variation between years, ranging from a maximum survival of 28 % to a minimum of 7%. Most deaths occur in the larval stage, and although previous work showed that survival of the larvae increases with the number of shoots available for infestation, much of the variability in survival rates between seasons was apparently caused by other factors, possibly climatic. Eggs laid in fallows were sampled in most years from 1953 to 1972. They varied from 3.57 to 0.02 million/acre (8.81–0.05 million/ha), depending on the season and site; the mean was 1.08 million/acre (2.67 million/ha). Egg populations varied in synchrony at all the three sites studied; there were cycles lasting several years between peaks of abundance or scarcity but there was no consistent tendency for populations to increase or decrease. Fewest eggs were laid when the weather was cold and wet during July and August, i.e. when adults are active. Weather was more closely correlated with the number of eggs laid expressed as a fraction of those laid in the previous year (net reproductive rate) than with the number of eggs laid/acre.     

Techniques for keeping wheat bulb fly larvae (Leptohylemyia coarctata) in sterile conditions

Three techniques are here described with special reference to investigation of bacterial diseases, as recently reported by Hamid (1966). Wheat seed without chemical dressing is sown in boxes and allowed to grow 10 cm high. Plants are then lifted. After washing off soil and discarding the roots and apical half of the leaves, the central succulent part of the plant is chopped up with scissors into very fine pieces. These pieces are crushed in a basin and after adding sufficient water to give a thick suspension, finally macerated for about 5 minutes (a top-drive macerator, Townson and Mercer Ltd. was used). This thick suspension is then centrifuged for 5 minutes at a speed of 5000 revolutions per minute and filtered through a Seitz filter. In the following techniques the filtrate is referred to as “nutrient solution” and the suspension as “macerated wheat”. Rigid bacteriological standards of sterility are maintained throughout the techniques.     

Use of oviposition trays to estimate numbers of wheat bulb fly Leptohylemyia coarctata eggs

To obtain reliable estimates of wheat bulb fly populations in time to provide advice on seed treatment of the following crop a method of collecting eggs in trays of soil placed in fields has been devised. The trays are examined by 20 August when half of the eggs have been laid.     

Persistence and distribution of insecticidal seed-dressings for control of the wheat bulb fly, Leptohylemyia coarctata

Samples of plants and soil from two experimental sites, one clay loam and one peaty loam, were analysed chemically to try to explain differing results with γ-BHC and dieldrin seed-dressings applied to control wheat bulb fly on different soil types, and to suggest reasons for patchy plant stands and occasional failure to control the pest. Seed dressed with dieldrin yielded more than untreated seed on both sites, whereas seed treated with γ-BHC yielded as much as seed treated with dieldrin on the clay site, but it yielded less than untreated seed on peat. The chemical analyses showed that the poor performance of γ-BHC on the peat site could not be explained by its failure to persist, because the soil still contained considerable quantities of γ-BHC at the time of insect attack. Possible reasons may be sorption of the poison by organic matter making it less available, and deeper sowing, permitting larvae to reach the plants without prolonged contact with the insecticide. Bayer 38156 (O-ethyl S-p-tolyl ethyl phosphonodithioate) and trichloro-nate (O-ethyl O-2,4,5-trichlorophenyl ethyl phosphonothionate) persisted in soil long enough and were sufficiently toxic to wheat bulb fly to suggest that organophosphorus compounds might provide alternatives to chlorinated hydrocarbon insecticides for control of the fly. Analyses made on seeds dressed at the laboratory for the experiment showed that the amounts of insecticide on seeds were smaller than expected and that the amounts on individual seeds differed greatly. Of ten samples of seeds dressed commercially with y-BHC, three carried very little insecticide, and the variations in the other seven samples were greater than with experimentally applied dressings.     

Wheat bulb fly, Leptohylemyia coarctata Fall., and its effect on the growth and yield of wheat

In two experiments done in successive years to compare the growth and yield of Cappelle wheat either protected from or exposed to attacks by larvae of wheat bulb fly, the plots were previously fallowed, but egg laying was prevented on half of each plot by using Polythene soil covers. In the first season wheat was sown at the end of October, November and December and in the second season in late October and in early January, when there were two sowings, one with and one without a spring application of herbicide. The infestation rate was 1·1-1·7 million eggs per acre, typical of a moderate attack. Larvae had little effect on the yield of October-sown wheat as the plants had two shoots each when first attacked and few were killed. On plots sown late, yield was decreased by up to 22%, as plants had only a single shoot when attacked and many were killed. The main effect of wheat bulb fly was to reduce the number of ear-bearing shoots by killing plants and restricting the production of new shoots. Surviving plants partially compensated by producing more ear-bearing shoots with heavier ears and slightly heavier seeds than normal. Killed plants were not distributed uniformly but were often in patches several feet across. Wheat on the attacked plots ripened more slowly and unevenly than on the unattacked plots. Weather affects the growth of the plants and activity of the larvae and thus partly determines percentage shoot survival.     

Host plant choice and location by larvae of the wheat bulb fly (Delia coarctata)

Wheat bulb fly (Delia coarctata Fallen, Diptera: Anthomyiidae) is an important pest of winter wheat in the eastern half of the UK, and in northern and eastern Europe. The larvae must find a host plant and invade a tiller soon after hatching in late January. Chemical controls are costly and weather conditions may reduce their efficacy or prevent their application. Post‐emergence control relies on organophosphate insecticides, which may soon be withdrawn due to concerns about their negative health and environmental effects. Winter wheat (Triticum aestivum L.) is the preferred cereal host, but other winter cereals and related grasses may also be attacked, while oats (Avena spp.) are shunned. In choice test bioassays, neonate larvae chose couch grass (Elytrigia repens (L.) Nevski syn. Elymus repens (L.) Gould, Agropyearon repens (L.) Beauv.) seedlings and exudates over wheat seedlings and exudates, and exhibited geotaxis and negative phototaxis. Analysis of larval trails in choice test bioassays of seedling exudates showed that couch exudates are more attractive than wheat exudates, and that wheat exudates are more arrestant than couch exudates. This suggests that infochemicals isolated from couch, wheat, and oats could be used in wheat bulb fly control; possible delivery mechanisms are discussed. These findings, previous research, and a comparison of the phenologies and geographical distributions of D. coarctata and its hosts suggest that E. repens is the natural host of D. coarctata.     

Cathepsin L-like cysteine proteinase (DcCathL) from Delia coarctata (wheat bulb fly): Basis of insecticidal activity

A cDNA encoding a cathepsin L-like cysteine proteinase (DcCathL) was prepared from gut tissue of larvae of wheat bulb fly (Delia coarctata: Diptera). The predicted protein is a homologue of the product of Drosophila melanogaster gene Cp-1 (CG6692), and is similar to a sub-family of cysteine proteinases found in other insects which have roles in tissue remodelling during development, and moulting. Recombinant DcCathL was produced using the yeast Pichia pastoris as expression host, and showed hydrolytic activity in vitro towards the synthetic substrate Z-Phe-Arg-AMC with a pH optimum of 4.5. DcCathL was insecticidal to lepidopteran larvae when injected into haemolymph, causing mortality that was accompanied by systemic melanisation, suggesting that DcCathL was affecting the immune-related proteolytic activation cascade leading to production of active phenoloxidase. This process is normally negatively regulated by serpins in the haemolymph. Recombinant serpins from cabbage moth (Mamestra brassicae) did not inhibit DcCathL, and were susceptible to degradation by the enzyme in vitro in buffer and extracted haemolymph. When M. brassicae larvae were co-injected with a lethal dose of DcCathL and exogenous recombinant serpins, no mortality or systemic melanisation was observed, suggesting that the insecticidal effects of DcCathL in vivo result from degradation of endogenous serpins.     

The spatial distribution of the wheat-bulb fly,Leptohylemyia coarctata (fall.) (Diptera: Anthomyiidae)

The spatial distribution of the eggs, larvae, pupae and adults of the wheat-bulb fly was investigated by fitting 42 sets of data comprising 1334 samples to the Poisson and negative binomial distributions, and by using the power law (S²=am^b). In general, the tests indicated that all stages were aggregated and fitted the negative binomial model.     

Chemical control of wheat bulb fly (Delia coarctata) attacking winter wheat in Eastern England, 1969–1981

Dry powder, liquid and microencapsulated formulations of organophosphate and synthetic pyrethroid insecticidal seed treatments were tested as possible alternatives to the standard organochlorine seed treatments for autumn-sown wheat in mineral and organic soils heavily infested with wheat bulb fly eggs and (subsequently) larvae. Retention of insecticides on the seed coat varied from 40% to 120% of the target dose; it was usually good when microencapsulated formulations were used. Chlorfenvinphos, fonofos, isofenphos and triazophos, each applied at 2-0 g a.i./kg seed, were phytotoxic, the symptoms varying from a slight delaying effect upon germination to an adverse effect upon grain yield. Chlorfenvinphos at 0–2 to 2-0 g a.i./kg seed was consistently effective against wheat bulb fly larvae in both mineral and organic soils. Athidathion 0–8 g a.i./kg, carbophenothion 1–2 g a.i./kg, ethion 1–7 g a.i./kg and fonofos (microencapsulated formulations) at 1-0 or 2-0 g a.i./kg were also effective in mineral and organic soils. Of the synthetic pyrethroids tested as seed treatments, permethrin gave excellent results in mineral soils at 1-0 g a.i./kg or in synergised formulations at 0–12 or 0–24 g a.i./kg but was disappointing in organic soils. In a single comparison of seed treatments applied to wheat sown early (14 October) and late (20 November), chlorfenvinphos was effective at both sowing dates whereas athidathion, ethion and pirimiphos-ethyl gave better results in late-sown wheat. A single trial compared deep with shallow sowing of treated seed. Most insecticides performed better on shallow-sown wheat, but chlorfenvinphos was equally effective against the pest at both sowing depths. Most insecticides restricted the numbers of larvae entering host plants but had little or no subsequent effect upon larval survival within attacked shoots. Fonofos and isofenphos, and to a lesser extent chlorfenvinphos, seed treatments additionally killed many larvae within plant shoots.     

Development of wheat bulb fly (Della coarctata fall.) larvae and pupae at different temperatures

When eggs of wheat bulb fly were added to young winter wheat plants in pots and kept at 5°, 8°, 10°, 12°, 15° and 20°, only 14%–44% of the eggs added produced larvae that succeeded in invading plants, but when newly hatched larvae were added directly 54%–92% were successful. At temperatures from 5° to 25°, the number of days required for larvae to complete their development in well-grown plants ranged from 11–14 days at 25° to 55–80 days at 5°. This corresponds to an accumulated temperature of approximately 286 day degrees C above the developmental threshold of 0.5°.The daily rate of development of larvae was marginally most rapid between 12° and 18°. When larvae were feeding in small, poorly growing plants, development was delayed and more shoots were needed before pupation than for healthy plants.The pupal stage which develops in the field from the end of April to June needs approximately 400 and 420 day degrees for completion in males and females (threshold 5°). Total day degrees required for the hatching of the egg to adult are about 700 (males) and 720 (females).Records of maximum and minimum temperatures from a meteorological screen 1 m above the ground can be used to calculate the number of day degrees accumulated from 1 May to 12 June, from which the dates of peak emergence of wheat bulb fly can be predicted. If 350 day degrees or more accumulate during this period, the peak emergence occurs near 20 June and with 200 or fewer day degrees it occurs near 11 July.The amount of damage to wheat by wheat bulb fly larvae in 1953, 1954, 1965 and 1966 depended largely on the number of eggs laid, the date of sowing, and also on the rate at which temperature accumulated in the autumn and winter; in all years, late sown crops would have had little opportunity to grow beyond the susceptible stage by the time they were attacked.

---

Résumé Quand des jeunes plants de blé d'hiver en pots sont contaminés avec des oeufs de Delia coarctata et maintenus à des températures respectives de 5°, 8°, 10°, 12°, 15° et 20° C, seulement 14% à 44% de ces oeufs donnent des larves qui parviennent á attaquer les plants, alors que si on utilise directement des larves nouveau-nées on obtient 54 à 92% d'attaques réussies. Aux températures de 5° à 25° le temps nécessaire aux larves pour achever leur développement dans les plants bien développés avant de gagner le sol pour la pupaison, est de 11 à 14 jours à 25° contre 55 à 80 jours à 5°.Une courbe de la vitesse de développement établie à partir de la durée du stade larvaire de l'éclosion à la pupaison aux differentes températures montre que le développement est le plus rapide entre 12° et 18° et que le zéro de développement est aux alentours de 0,5°. Le nombre de degrésjours accumulés par la larve de l'éclosion à la pupaison est d'environ 286. Quand les larves se nourrissent aux dépens de plants petits à croissance faible, leur développement est retardé et exige plus de temps et plus de plantes avant la pupaison que lorsqu'elles se nourrissent sur des plants vigoureux.Le stade pupal qui intervient dans la nature de fin avril à juin, nécessite approximativement 400 à 420 degrés-jours pour le développement du mâle et de la femelle au-dessus d'une supposée température de base de 5°. Le nombre total de degrés-jours de l'oeuf à l'adulte est respectivement de 686 et 706 pour le mâle et la femelle.Les enregistrements des températures maxima et minima dans un abri météorologique à 1 m audessus du sol peuvent être utilisés pour établir le nombre de degrés-jours accumulés du 1 er'mai au 12 juin; à partir de ces données les pics d'émergence de D. coarctata peuvent être prédits. Si 350 degrésjours ou plus sont accumulés durant cette période, le pic d'émergence apparaît vers le 20 juin, mais si 200 degres-jours au moins sont notés le pic sera proche du 11 juillet.L'importance des dégâts occasionnés au bleî d'hiver en Angleterre par les larves de D. coarctata en 1953, 1954, 1965 et 1966 dépend largement du nombre d'oeufs pondus, de la date du semis et de la température accumulée en automne et en hiver; au cours de chacune de ces années les cultures à semis tardif auraient eu peu de possibilité de dépasser le stade sensible avant le moment où elles ont été attaquées.

     

Competition between larvae of wheat bulb fly, Delia coarctata (Anthomyiidae), for winter wheat shoots in an insectary

Newly hatched wheat bulb fly larvae were added to 32 wheat seedlings with one shoot and three to four leaves, in large pots in an outdoor insectary so that the numbers of available shoots per larva were 32, 16, 8, 2, 1 and 1/2. Competition for shoots between larvae, measured by the number of shoots attacked and the emergence of flies, occurred only when there was 1 or 1/2 shoot/larva. Fewer flies also emerged from shoots when the number available was one or less than one shoot/larva. In the field, egg numbers in autumn equal to or less than 5×10⁶/ha (2×10⁶/acre), suggest fewer than the expected carrying capacity of winter wheat growing at the usual plant population except in spots where the density is greater than average because of aggregation.Résumé Les larves de Delia coarctata attaquent les pousses du blé d'hiver en janvier et février. Une larve se nourrit sur une pousse et peut se déplacer sur une autre pour achever son alimentation avant pupaison. Le dispositif expérimental comporte 32 jeunes plants de blé d'hiver, comportant chacun une pousse et 3 à 4 feuilles, dans des pots installés dans un insectarium en plein air; des larves venant d'éclore sont réparties en nombre tel que la compétition varie en parallèle avec le nombre de pousses disponibles par larve: 32, 16, 8, 2, 1 ou 1/2 pousse par larve. La compétition mesurée d'après le nombre de pousses attaquées ne se manifeste que s'il y a seulement 1 ou 1/2 pousse par larve. Peu de mouches éclosent quand le nombre de pousses dont dispose chaque larve est égal ou inférieur à `.Dans les champs, le nombre d'oeufs à l'automne est égal ou inférieur à 5×10⁶/ha, ce qui est en dessous de la capacité d'accueil du blé d'hiver à l'hiver à l'égard de la population de ce parasite, excepté en certains points où le nombre d'oeufs et donc le nombre de larves est plus élevé qu'en moyenne, par suite de cette aggrégation.     

Effects of activated charcoal on attack by larvae of the wheat bulb fly, Delia coarctata

Activated charcoal interferes with normal host-seeking by larvae of the wheat bulb fly, Delia coarctata, by adsorbing arrestant compounds exuded from the plants. Thus, in laboratory tests, activated charcoal in soil surrounding one of two rows of wheat seedlings decreased larval attack on the treated row. Charcoal in soil above the level of the seeds decreased attack but had no effect when below the seeds. However, there was no decrease in attack when both rows were in charcoal. Attempts to develop a practical application of this effect in the field are described.     

The activity and distribution of wheat bulb fly (Delia coarctata) (Diptera:Anthomyiidae) in cereal crops and over fallow land

The flight activity and local distribution of adult D. coarctata were studied on Rothamsted Farm during 1970-5 using several trapping methods. Changes in the activity and distribution of the flies were followed as the population aged, and in females these changes were associated with maturation of their eggs. Most females appeared to stay at their emergence sites in winter wheat until egg-laying started about 1 month later. They then dispersed and both sexes were found in previously uninfested cereals and grass as well as on fallow land where the eggs are laid. Flight occurred chiefly in the late afternoon and evening. The concentration of flies at their emergence sites for several weeks before egg laying could facilitate chemical control, provided the practical difficulties of application can be overcome.     

A comparison of wheat bulb fly (Delia coarctata) egg numbers in ridge and furrow soil samples from potato crops

In soil samples taken with a shovel from 97 potato maincrops in east Scotland during 1978 to 1981, wheat bulb fly (Delia coarctata) eggs were more numerous on ridges than in furrows. The ratio of the number of eggs on the ridge to the number of eggs in the furrow was higher in Lothian Region (3·25) than in Fife Region (1·59) but differed significantly (P < 0·05) from 1·0 in both Regions. To obtain maximum precision of the estimate of the total wheat bulb fly egg population in a potato field, more samples should be taken from the ridge than from the furrow, in approximate proportion to the egg numbers.     

Development of a pest threshold decision support system for minimising damage to winter wheat from wheat bulb fly,Delia coarctata

Wheat bulb fly, Delia coarctata, is an important pest of winter wheat in the UK, causing significant damage of up to 4 t/ha. Accepted population thresholds for D. coarctata are 250 eggs/m² for crops sown up to the end of October and 100 eggs/m² for crops sown from November. Fields with populations of D. coarctata that exceed the thresholds are at higher risk of experiencing economically damaging infestations. In the UK, recent withdrawal of insecticides means that only a seed treatment (Signal 300 ES) is available for chemical control of D. coarctata; however, this is only effective for late-sown crops and accurate estimations of annual population levels are required to ensure a seed treatment is applied if needed. As a result of the lack of postdrilling control strategies, the management of D. coarctata is becoming reliant on nonchemical methods of control. Control strategies that are effective in managing similar stem-boring pests of wheat include sowing earlier and using higher seed rates to produce crops with greater pest tolerance. In this study, we develop two predictive models that can be used for integrated D. coarctata management. The first is an updated pest level prediction model that predicts D. coarctata populations from meteorological parameters with a predictive accuracy of 70%, a significant improvement on previous prediction models. Our second model predicts the maximum number of shoots for a winter wheat crop that would be expected at the terminal spikelet development stage. This shoot number model uses information about the thermal time from plant emergence to terminal spikelet, leaf phyllochron length, plant population and sowing date to predict the degree of tolerance a crop will have against D. coarctata. The shoot number model was calibrated against data collected from five field experiments and tested against data from four experiments. Model testing demonstrated that the shoot number model has a predictive accuracy of 65.7%. The foundation for a future decision support system using these models for the sustainable management of D. coarcata risk is described. It should be noted that these models represent a stepping-stone towards a decision support system and that further model validation over a wider geographic range is required.     

Changes in wheat bulb fly (Delia coarctata) populations in East Anglia in relation to crop rotations, climatic data and damage forecasting

Records of wheat bulb fly egg (Delia coarctata) population densities in fields sampled throughout East Anglia in the autumns of 1953–1990 are presented. In descending order of risk, fallow, potato, pea (mainly vining), sugar beet and oilseed rape are the main crops preceding wheat or barley which attract oviposition. A declining trend of egg populations observed over the study period may be associated with climatic changes as well as with the elimination of fallow in the rotation and the dramatic increase in the use of insecticides against the pest. Negative correlations (P < 0.05) in mean annual egg numbers were found with departures from average of July temperature and January air or soil temperature; positive correlations (P<0.05) with departure from average of August raindays. Stepwise regression analysis was used to identify the most important relationships of meteorological variables with mean annual egg numbers, or the proportion of fields sampled with egg numbers in excess of the action threshold of 2.5 million eggs/ha. Up to 59% of the variation in the annual proportion of fields above threshold was accounted for in a regression equation using departures from average of July temperature, August raindays and the percentage of average of October (preceding year) rainfall. Estimated mean annual egg populations and the proportion of fields above threshold showed a good fit with the observed values. The findings are discussed and compared with previous work. The forecasting model may be readily incorporated as a regional risk-prediction component of a knowledge-based system for the management of wheat bulb fly control. Regional forecasts of wheat bulb fly oviposition from this work should be tested and modified as necessary according to experience or changing climatic or agricultural factors.     

A comparison of a corer and a shovel for sampling populations of wheat bulb fly Delia coarctata eggs

The most commonly used tool for sampling soil for eggs of the wheat bulb fly Delia coarctata Fall.) is a corer, but in Scotland samples are taken with a modified shovel. The sampling properties of a corer and shovel were compared in two exercises in which soil samples were taken from the ridge, sides and furrow of drills of maincrop potatoes at several sites in east Scotland. Although each tool sampled approximately the same surface area of soil, the corer sampled three times as much soil as the shovel. The numbers of wheat bulb fly eggs estimated by the corer and shovel were similar in both sampling exercises and there were no indications that the comparison of the two tools was affected by the sampling position on the potato ridge. The results of the two exercises gave no reason to believe that in similar conditions the shovel is an inferior sampling tool or that population estimates obtained with a shovel in Scotland are not comparable with those obtained with a corer. Evidence from some sites suggested that wheat bulb flies lay more eggs on the potato ridge than in the furrow.