Differences in the relative resistance of two carrot cultivars to carrot fly attack over five seasons

A comparison was made over five seasons of carrot fly (Psila rosae) damage on two carrot cultivars, Sytan and Danvers, which represented the extremes of resistance discovered in screening trials at Wellesbourne. Plants were harvested regularly during the seasons and at each harvest the number and weight of roots was recorded and carrot fly damage was assessed using various techniques; in 1979-80 the numbers of carrot fly larvae and pupae in and around root samples were counted. The experiments confirmed repeatedly the relative resistance of cv. Sytan compared with Danvers. The estimated reduction in carrot fly larvae on Sytan relative to Danvers in early November in four seasons ranged from 40% to 67%. When tested against first generation carrot fly attack the reduction in larvae on Sytan was 54%. There were 45% fewer mines per root on Sytan resulting in less damage at each harvest in all seasons. Larvae took longer to develop on Sytan than Danvers and 18% more plants of Sytan survived carrot fly attack. Differences in seedstock, season, sowing time, generation of carrot fly, plant size and density did not account for observed differences in damage between the two cultivars. The studies indicated satisfactory practical techniques for assessing carrot fly damage in cultivar screening trials.     

The complementary effects of plant resistance and the choice of sowing and harvest times in reducing carrot fly (Psila rosae) damage to carrots

The benefits of combining a partially-resistant carrot cultivar with different sowing and lifting dates to reduce carrot fly, Psila rosae, damage were investigated at Wellesbourne in 1983 and 1984-85. The partially-resistant cv. Sytan was less damaged and supported fewer insects than the susceptible cv. Danvers on all lifting dates. The estimated reduction of carrot fly larvae on Sytan compared with Danvers ranged from 33 to 95%. Nine combinations of sowing and lifting dates provided more than 75% marketable roots of Sytan compared with only three combinations of dates for Danvers. An early June sowing of both cultivars provided roots of a marketable size with the least attack. More than 90% of Sytan roots were still marketable in December and fewer insects were produced by the end of the season on these roots than on those sown earlier. In addition, sowing in June decreased the number of pupae produced on cv. Danvers by 10 times compared with earlier sowings. Combining partial resistance with specific sowing and lifting times enabled satisfactory yields of marketable carrots to be obtained in a field infested by high populations of carrot fly.     

The consistency of the resistance of eight carrot cultivars to carrot fly attack at several centres in Europe

Eight carrot cultivars representing the range of resistance to carrot fly damage as assessed in England were grown at 12 sites in five European countries in 1977 and in 1978. The trials provided evidence of agreement with the findings in England for both years in the ranking of cultivars with respect to their resistance to carrot fly damage. The Nantes cultivar Clause's Sytan Original was the most resistant cultivar tested. Consistent results were obtained from sites where carrots were assessed at harvest and carrot fly attack was severe. A significant interaction between sites and cultivars was largely due to sites where carrots were stored prior to assessment.     

Exploitation of the resistance to carrot fly in the wild carrot species Daucus capillifolius

Under field conditions a wild Daucus species from Libya, D. capillifolius, supported less than one tenth as many carrot flies (Psila rosae) as the susceptible carrot cultivar Danvers Half Long 126. Breeding lines developed from crosses between D. capillifolius and three different carrot types were grown in a series of field experiments at Wellesbourne between 1980 and 1989. Each year selections were made for agronomic quality and/or for increased resistance to carrot fly. The programme produced lines which for size, shape and colour represented most of the commercially-important carrot types. Some of these lines were also significantly more resistant to carrot fly than selections from the partially-resistant cv. Sytan. However, the best lines were not as resistant as the wild parent. The highest quality resistant lines were sold to seed companies for variety production.     

Development of carrot inbreds with resistance to carrot fly using a single seed descent programme

Two carrot cultivars, ‘Sytan’ and ‘Long Chantenay’, representing commercially important carrot types and selected for their partial resistance to the carrot fly (Psila rosae) were crossed as the basis for a single seed descent programme. The resulting F₁ progeny were mass pollinated to produce an F₂ generation and approximately 2000 plants were raised from this segregating family in the glasshouse in 1981. By careful choice of sowing date and glasshouse temperatures it was possible to stimulate the plants to flower within 10 months. Individual king umbels were enclosed within bags and pollinated with blowflies. Resulting seed was sown in pots in the following August and the process of seed production repeated in a 12 month cycle. Each inbred line was selfed in this way over three generations until the F₅ stage. Stocks of seed were then multiplied. A total of 753 inbreds were produced by 1987. The vigour of each inbred was evaluated in glasshouse tests and the level of chlorogenic acid was determined by a fluorescence technique. The most vigorous lines with the lowest levels of chlorogenic acid were tested in field experiments against carrot fly at Wellesbourne. Nine inbreds with promising agronomic quality and moderate resistance to carrot fly were selected and seeded. These nine lines were submitted to seed companies with the aim of developing new cultivars of carrot.     

Host-plant susceptibility to the carrot fly, Psila rosae. 1. Acceptability of various host species to ovipositing females

A preference hierarchy of the oligophagous carrot fly for 30 umbelliferous host-plant species or varieties and six non-hosts was assessed. Foliage of the test plants was presented to a laboratory population of the fly in oviposition choice experiments together with leaves of a standard plant, the susceptible carrot cultivar “Danvers”. Only two species (Anthriscus cerefolium, Carum carvi) were more acceptable than the standard plant, while about half the species received significantly fewer eggs. Some umbellifers (Anethum graveolens, Pimpinella major) had a low acceptability similar to that of non-hosts (non-umbelliferous plants). The results obtained with dual and multiple choice assays were in agreement. A multiple choice assay with potted plants yielded a similar ranking of the species as assays using detached leaves, indicating that foliage is representative for whole plants. Variability in the exploratory runs performed by the females on the leaves prior to egg-laying is described and quantified for hosts and non-hosts. Differences in post-alighting pre-ovipositional behaviour of the female flies on the leaves accounted for a major part of the variation recorded in egg-laying.     

Induced preference for host plant chemicals in the tobacco hornworm: contribution of olfaction and taste

Many herbivorous insects induce preferences for host plants. Recent work in Manduca sexta indicates that induced preferences are mediated by a “tuning” of the peripheral taste system to chemicals within host plant foliage. We tested this hypothesis by rearing caterpillars on artificial diet or potato foliage, and then examining olfactory- and taste-mediated responses to potato foliage extract. First, we confirmed earlier reports that consumption of potato foliage tunes the peripheral taste system by reducing responsiveness to glucose and increasing responsiveness to foliage extract. Second, we offered caterpillars a choice between disks treated with foliage extract (experimental) or solvent alone (control). The foliage-reared caterpillars approached and consumed the experimental disks disproportionately, whereas the diet-reared caterpillars approached and consumed both disks indiscriminately. This indicated that induced preferences involve olfaction and taste. Third, we ran choice tests with foliage-reared caterpillars deprived of either olfactory or gustatory input. Caterpillars lacking olfactory input approached both disks indiscriminately, but fed selectively on experimental disks. In contrast, caterpillars lacking gustatory input approached experimental disks selectively, but fed indiscriminately on both types of disk. We conclude that even though olfaction helps caterpillars locate potato foliage, it is the “tuned” gustatory response that ultimately mediates the induced preference.     

Preliminary studies of carrot susceptibility to carrot fly attack

Eleven carrot cultivars were tested to compare their susceptibility to carrot fly attack at Wellesbourne and at Mepal, Cambridgeshire in 1969. Significant differences in the percentages of unattacked roots were observed but not all the cultivars behaved consistently in this respect at the two sites. Foliage height and the percentage unattacked roots were not significantly correlated for these cultivars. Four of the cultivars representing the different levels of susceptibility were retested in fen and in mineral soil in microplots at Wellesbourne in 1970. Soil type significantly affected plant growth but not the relative susceptibilities of the cultivars to carrot fly attack. Plant size was positively correlated with insect attack and accounted for most, but not all, of the differences in susceptibility between the cultivars. In 1971 also, cv. Royal Chantenay was significantly less attacked than Speed's Norfolk Giant from August to December even after allowing for the large differences between the two cultivars in plant size. The experiments demonstrated a consistent difference in the relative susceptibilities of cvs Royal Chantenay and Speed's Norfolk Giant. The difference was correlated with plant size but allowances for foliage and root size failed to account for all of the difference. The results suggested a more fundamental basis for part of the difference in the susceptibility of the cultivars to carrot fly attack.     

Stem galls affect oak foliage with potential consequences for herbivory

Abstract.   1. On two dates, foliar characteristics of pin oak, Quercus palustris , infested with stem galls caused by the horned oak gall, Callirhytis cornigera , were investigated, and the consequences for subsequent herbivory assessed.
2. Second-instar caterpillars of the gypsy moth, Lymantria dispar , preferred foliage from ungalled trees.
3. Ungalled trees broke bud earlier than their galled counterparts.
4. Galled trees produced denser leaves with higher nitrogen and tannin concentrations, but foliar carbohydrates did not differ among galled and ungalled trees.
5. Concentrations of foliar carbohydrates in both galled and ungalled trees increased uniformly between the two assay dates. Nitrogen concentrations were greater in leaves from galled trees, and decreased uniformly in galled and ungalled trees over time. Foliar tannins were also greater in foliage from galled trees early in the season; however, foliar tannins declined seasonally in galled tissue so that by the second assay date there was no difference in tannin concentrations between galled and ungalled foliage.
6. In spite of differences in foliar characteristics, performance of older, fourth instar gypsy moth caterpillars did not differ between galled and ungalled trees.     

The responses of carrot fly larvae, Psila rosae, to components of their physical enrivonment

Abstract. 1. The responses of third instar Psila rosae (F.) larvae to light, temperature, humidity and soil moisture were investigated in the laboratory.
2. Larvae were photonegative and preferred a temperature of about 15°C. Temperatures between 30 and 40°C adversely affected movement and over 40° C were lethal.
3. In choice chambers, larvae preferred humidities of 70–100% r.h. and larvae in sand avoided dry conditions (2.5% field capacity). The latter response became more marked as larvae approached the pre-pupal stage when moistures of 40% field capacity and lower were avoided.
4. Most larvae were found at a depth of 8 cm in sand of uniform moisture content and temperature, but variation in moisture content could alter this preference.
5. In August, most larval damage in the field occurred near the tip of the carrot tap root but was more evenly distributed over the roots in November. It is uncertain whether this was due to soil near the surface being drier in August or whether it was caused by behavioural differences between the two generations of carrot fly larvae.
6. During the summer of 1975, low soil moisture levels resulted in the total absence of larval mines on the carrot roots even though pupae were found at depths of 20–30 cm in the soil. Temperature had no effect on the distribution of mines on carrot roots except at the top 2 cm of the soil profile.     

Physical features of grass leaves influence the placement of eggs within the plant by the Hessian fly

Hessian fly eggs are more likely to be found on adaxial rather than abaxial surfaces of wheat leaves. These leaf surfaces differ in their physical features: the adaxial side of the leaf has parallel grooves and ridges while the abaxial side is relatively smooth. We used leaf models to investigate the relationship between Hessian fly egglaying and these physical features. When both sides of a green paper leaf model were treated with a wheat leaf extract, but only one side of the model was scored with parallel grooves, the grooved side received more eggs than the smooth side. As the number of grooves per surface increased from 0 to 10, eggs per model increased. When grooves and the wheat leaf extract were tested together and separately, the grooves significantly increased egg numbers in the presence, but not the absence, of wheat extract. In contrast, wheat extract increased egg numbers both in the absence and presence of grooves. Molding techniques were used to recreate the physical features of the adaxial and abaxial leaf surfaces of five grasses. For four of the grass genotypes (a triticale, two common wheats, and a durum wheat), patterns of egglaying on real leaves and molded models were similar, i.e., adaxial leaf surfaces and adaxial molded models were preferred over abaxial leaf surfaces and abaxial models. On the fifth grass, oat, preferences for the adaxial side of real leaves and for adaxial models were not as obvious. We conclude that the adult female Hessian fly obtains information about the leaf surface through her tactile and/or kinesthetic senses and uses this information when making egglaying decisions.     

Colour preference and potato cultivar oviposition choice by onion thrips,Thrips tabaci (Thysanoptera: Thripidae)

Thrips tabaci (Lindeman) is an important polyphagous pest, and vector of Tomato spotted wilt virus responsible for sporadic, but devastating epidemics in potato. T. tabaci shows significant preferential differences between potato cultivars that may be important for field resistance. To investigate the role of visual cues in host choice we tested colour preference using two-choice assays. Thrips tabaci showed a significant preference for mid-green over red, blue, and white coloured cards, a preference for both light-green and mid-green over dark-green, and light-green over yellow, but no preferential difference between mid-green and yellow, and between light-green and mid-green. Analysis of the spectral reflectance of potato cultivars differing in thrips preference, revealed significant differences within the 400–700 nm wavelengths. In most cases cultivars preferred by T. tabaci had lighter green foliage and higher reflectance at 552 nm, and thus colour preference may be important for host selection. Oviposition choice of T. tabaci for potato cultivars was determined from counts of larvae and unhatched eggs from leaf disks under choice and no-choice conditions. In contrast to the colour choice experiments, onion thrips showed oviposition preference for cultivars with darker green foliage and lower reflectance at 552 nm.     

Foliar form, colour and surface characteristics influence oviposition behaviour of the carrot fly

Various leaf models made of paper were presented to carrot flies, Psila rosae (F.) (Diptera: Psilidae) in choice assays to investigate the effect of non-chemical plant traits on oviposition behaviour. The surrogate leaves differed in colour, shape, surface coating, size and stem length. In the presence of host-plant extracts, physical factors strongly influenced oviposition. Green, yellow and orange three-dimensional models similar in shape to host-plant leaves (pinnately or ternately compound or dissected) and with a thin cover of paraffin wax were most acceptable to the females. Egg-laying was not affected by leaf size, but was negatively correlated with stem length. The results obtained by testing models with simple leaf silhouettes were confirmed in an experiment using more lifelike imitations of real host and non-host leaves. The findings are discussed by an extensive review of similar studies in three other phytophagous fly species (cabbage root fly, onion fly, Hessian fly).     

Resistance in apple to shoot infection by Venturia inaequalis

Eight apple genotypes, including cultivars and breeding selections resistant and susceptible to Venturia inaequalis on foliage, were screened for shoot infection and the development of wood pustules following inoculation of shoot tips of 1–year maiden trees in the greenhouse. Where genotypes were highly resistant in terms of foliar symptoms (cvs Prima and Gavin), no shoot infection was observed. Where genotypes were highly susceptible in terms of foliar symptoms, then shoot tissue was either resistant (cv. Shinko) or susceptible (cv. Starking). In one experiment, shoot tips were inoculated sequentially as shoots extended. No pustules developed where inoculations were made later than May. In a further experiment, two leaf internode positions were inoculated on one occasion. Pustules were only observed where inoculations were made above the youngest unrolled leaf. The results suggested that shoot tips were more susceptible during early extension growth of the shoot. Pustules were noted in abundance on petioles of susceptible cultivars, and these probably contributed to early leaf abscission.     

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.     

Inferring changes in foliar mass and area from foliage cover: A mechanistic model

Measuring foliar area or mass directly is destructive, and precludes long‐term, repeated observations of individual trees as they suffer or recover from foliar damage. Instead, foliage cover indices are often used as a proxy for foliar mass. Patterns of fluctuations in foliage cover indices can be used to infer qualitative changes in canopy health. However, foliage cover is not necessarily linearly related to foliar area or mass, and this may confound the detection of significant foliar damage, and comparisons of herbivore browse impacts between individual trees, tree species or sites. I derived a mechanistic model to quantify the relationship between foliar area or mass and foliage cover measured as the proportion of sky occluded by leaves. This one parameter model is close to linear for single‐tiered trees, but increasingly non‐linear for multi‐tiered trees. I compared the non‐linear model to a linear model using foliage cover data from an artificial defoliation experiment on two single‐tiered, sub‐canopy species and from simulated photographic images of single‐ and multi‐tiered canopies. The non‐linear model had lower errors than the linear model, and errors did not increase with foliage density (leaf area per unit area), variation (of leaf sizes within and between canopies) or leaf geometry. The non‐linear model can be easily parameterized from relatively low‐cost observations of foliage cover, independently of empirical measurements of foliar area or mass, and is applicable to a wide range of tree species. It should therefore help managers quantify how changes in foliage cover due to natural fluctuations or foliar damage affect foliar area and mass, and can be used to quantify parameters for models of browse impacts in mixed forest.     

Insect responses to plant water deficits. I. Effect of water deficits in soybean plants on the feeding preference of Mexican bean beetle larvae

Abstract. 1. We examined the preference of larvae of the Mexican bean beetle, Epilachna varivestis Mulsant (Coleoptera: Coccinellidae), for foliage of soybean ( Glycine max [L.] Merr.) grown under several levels of water deficit. Third instar larvae were exposed simultaneously to excised foliage from plants that were either well-watered (control) or had experienced water deficits (treatment).
2. Water-deficient plants were re-watered 12 h prior to initiating feeding trials to eliminate physical differences between control and treatment foliage such as leaf water potential, diffusive resistance, relative water content, and foliage toughness.
3. Examination of the free amino acid contents of re-watered and excised foliage indicated that amino acid concentrations increased markedly in foliage grown under water deficits, and that this increase persisted during the preference tests.
4. Larvae preferred control foliage, but shifted preference to treatment foliage under mild water deficits. When the leaf water potential of water-deficient treatment foliage was lower than – 1.13 MPa or when it was more than 0.5 MPa lower than that of control foliage, larvae preferred to feed on foliage from well-watered control plants.
5. The expression of preference for well-watered control foliage was coincident with increases in the concentrations of total free amino acids and individual free amino acids in the water-deficient treatment foliage.
6. These results are inconsistent with White's (1974) hypothesis because Mexican bean beetle larvae avoid plants grown under water deficits that have increased concentrations of free amino acids.     

Vapour–phase activities of essential oils against antibiotic sensitive and resistant bacteria including MRSA

Aims:  To determine whether essential oil (EO) vapours could reduce surface and airborne levels of bacteria including methicillin-resistant Staphylococcus aureus (MRSA).
Methods and Results:  The antibacterial activity of geranium and lemongrass EO individually and blended were evaluated over a range of concentrations by direct contact and vapour diffusion. The EO were tested in vitro against a selection of antibiotic-sensitive and -resistant bacteria, including MRSA, vancomycin-resistant Enterococci (VRE), Acinetobacter baumanii and Clostridium difficile . An EO blend containing lemongrass and geranium was used to formulate BioScent^TM that was dispersed into the environment using the ST Pro^TM machine. The effects were variable depending on the methods used. In a sealed box environment, MRSA growth on seeded plates was reduced by 38% after 20 h exposure to BioScent^TM vapour. In an office environment, the ST Pro^TM machine dispersing BioScent^TM effected an 89% reduction of airborne bacteria in 15 h, when operated at a constant output of 100%.
Conclusions:  EO vapours inhibited growth of antibiotic-sensitive and -resistant bacteria in vitro and reduced surface and airborne levels of bacteria.
Significance and Impact of the Study:  Results suggest that EO vapours, particularly Bioscent^TM, could be used as a method of air disinfection.     

Susceptibility of strawberry cultivars to the vine weevil Otiorhynchus sulcatus (Coleoptera: Curculionidae)

Abstract 1 The feeding preference of vine weevil, Otiorhynchus sulcatus (Fabricius), adults for foliage from 21 commercial cultivars of strawberries is investigated using binary choice tests with leaf disks, using ‘Honeoye’ as a standard against which all other cultivars are compared. ‘Delmarvel’, ‘Idea’, ‘Lester’, ‘Primetime’ and ‘Seneca’ are not preferred. 2 Variation in leaf nitrogen content is correlated with feeding preference, but does not explain all the variation, because outliers exist for both preferred and nonpreferred cultivars. 3 Removal of leaf hairs with adhesive tape permits their role in deterrence to be evaluated. Eight of the 21 cultivars have deterrent leaf hairs. Paradoxically, some highly preferred cultivars (e.g. ‘Latestar’ and ‘Tristar’) have deterrent leaf hairs, and four of the five nonpreferred cultivars lack significantly deterrent leaf hairs. 4 The 21 tested cultivars do not differ in their suitability for vine weevil larval development. 5 Differences in cultivar susceptibility to vine weevil in the field may involve interactions between the palatability and nutritional value of foliage, which influence fecundity and egg placement, and the tolerance of strawberry plants to larval feeding, determined by root biomass. The nonpreference traits found in the foliage of commercial cultivars of strawberry are present by chance. Accordingly, further selective breeding to enhance strawberry varietal tolerance to vine weevils may be possible.     

Oviposition preference of a Eucalyptus herbivore and the importance of leaf age on interspecific host choice

1. Paropsine chrysomelids are eucalypt folivores for which it has been hypothesized that the availability of suitable age class foliage (i.e. young foliage) is a more important determinant of host use than the physical and/or chemical characteristics of foliage from different hosts. This prediction was tested by assessing the oviposition preference of Chrysophtharta bimaculata (Olivier) on excised branches of Eucalyptus regnans F. Mueller and E. nitens (Deane & Maiden) Maiden.
2. In cage trials, and as observed in the wild, beetles oviposited preferentially on E. regnans when the branches of both eucalypt species displayed the full range of foliar developmental classes, from immature to fully expanded. However, when the branches were pruned so that both species displayed only immature and expanding leaves, no difference in oviposition between the species occurred. Further, when the branches were pruned so that E. regnans displayed only fully expanded leaves and E. nitens displayed only immature and expanding leaves, females oviposited entirely on the latter species.
3. These results support and extend the hypothesis that C. bimaculata 's ovipositional preference is dictated more by leaf age class than by the tree species from which the leaves derive. In the discussion it is proposed that a eucalypt that initiates and expands leaves rapidly will be less preferred for oviposition than one that develops and expands leaves slowly, because of the smaller area of its canopy with less sclerophyllous foliage.