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.     

Problems in field selection for resistance to carrot fly (Psila rosae) in carrot cv. Long Chantenay

In field trials between 1973 and 1982 families of carrots of cv. Long Chantenay were assessed for their resistance to carrot fly (Psila rosae). In some but not all years, families selected for their observed resistance in earlier trials seemed more resistant than the parent line. In more than half the trials there was an association between carrot fly damage and root density, there being less damage on those families from which more roots were harvested. Thus, family selection within cv. Long Chantenay did not lead to a consistent improvement in resistance to carrot fly. Alternative strategies for achieving plant resistance to this pest are discussed.     

Electroantennogram responses of the carrot fly, Psila rosae, to volatile plant components

ABSTRACT. Electroantennogram (EAG) responses of male and female carrot flies, Psila rosae F. (Diptera: Psilidae), were recorded to thirty-six volatile plant constituents. The most distinct EAG responses were obtained to: (1) the general green leaf volatiles 1-hexanol, trans-2-hexen-1-ol and cis-3-hexen-1-ol, their isomers cis-2-hexen-1-ol and trans-3-hexen-1-ol, the alcohol 1-heptanol, the ester cis-3-hexenyl acetate and the leaf aldehydes hexanal and trans-2-hexenal, and (2) from four compounds associated with the umbelliferous host plants of this insect, namely trans-methyl-iso-eugenol, β-caryophyllene, linalool and trans-2-nonenal. Higher responses were elicited by the leaf aldehydes than by the corresponding alcohols. Although the absolute amplitude of the female response was over twice that of the male, there were no differences between the relative responses to the compounds tested in both sexes, with the exception of a much higher response to the leaf aldehydes in the male. The shape of the EAG evoked by the various compounds was consistently different, with the slowest recovery being recorded for trans-methyl-iso-eugenol. While the antennal olfactory receptors of the carrot fly are sensitive to the closely related general green leaf volatiles, they are most specifically tuned to the aldehyde component of this green odour complex. In addition, the ability of this insect to discriminate between different plants may be augmented by the perception of a group of more host specific volatiles. The conformity of the responses of males and females to the compounds tested may indicate that host plant volatiles plays an additional role as an aggregation cue for both sexes.     

Relationship between root volatiles of some carrot cultivars and their resistance to the carrot fly, Psila rosae

Field experiments investigated the resistance of some carrot cultivars to Psila rosae. In addition, headspace vapour and steam distillate from the roots of resistant and susceptible varieties were compared by gas-liquid chromatography. The field data confirmed that resistance may operate by decreasing the numbers of eggs laid indicating a nonpreference by the female Psila. Root resistance to the larva was also confirmed but the mechanism was unclear. A new finding was that root resistance is independent of the effect of egg laying, some cultivars evincing one or the other effect and some such as Regulus Imperial displaying both. It was clear that root resistance to the larva is the crucial prerequisite in breeding resistant varieties.One consistent difference was detected by the chemical comparisons: intact roots of resistant varieties released substantially less volatiles. Specifically, Regulus released almost five times less of the volatiles already shown to positively influence host-finding behaviour by the larva.

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Résumé La résistance à P. rosae de quelques cultivars de carotte a été étudiée en plein champ. Parallèlement, les substances volatiles diffusées et celles extraites par la vapeur des racines de variétés résistantes et sensibles, ont été comparées en chromatographie gaz-liquide (GLC). Les résultats en champ ont confirmé que la résistance peut être due à une diminution du nombre d'oeufs pondus, révélant une absence d'attractivité pour les femelles de P. rosae. La résistance des racines aux larves a été aussi confirmée, mais les raisons n'en étaient pas claires. Un aspect nouveau est que la résistance des racines est indépendante de l'effet de la ponte, quelques cultivars présentant l'un ou l'autreeffet et certains, comme Regulus Imperial, manifestant les deux. Il est net que la résistance racinaire aux larves est la condition essentielle pour la sélection de variétés résistantes. Une différence importante a été mise en évidence par les comparaisons chimiques: les racines intactes de variétés résistantes libèrent nettement moins de substances volatiles. Précisément, Regulus a libéré 5 fois moins de substances volatiles déjà connues comme influençant positivement le comportement de découverte de l'hôte par la larve.

     

Behavioural responses of the carrot fly larva, Psila rosae, to carrot root volatiles

ABSTRACT. The behaviour of the carrot fly larva, Psila rosae F. (Diptera, Psilidae), was investigated in the presence of six concentrations of thirty-four volatiles associated with carrot root and of trans -methyl-iso-eugenol, a constituent of carrot herb oil. Bomyl acetate, 2,4-dimethyl styrene, α-ionone, SbT-ionone and biphenyl were the most consistently preferred compounds. Klinotaxis and klinokinesis were the orientation responses near the stimulus source. Trans -2-nonenal was the most consistently avoided compound. Extracts derived by steam distillation of carrot roots and by concentration of vapour from roots were resolved, by gas liquid chromatography, into forty-five and twenty-two major constituents, respectively. The five preferred compounds are micro-components of these extracts, collectively representing 1% and 0.6% of root extract oil and vapour, respectively. It is suggested that these secondary plant constituents are specific host finding cues for the larva, and that frara-2-nonenal is a carrot defence compound.     

Oriented responses of carrot fly larvae, Psila rosae, to plant odours, carbon dioxide and carrot root volatiles

ABSTRACT. The responses of carrot fly larvae, Psila rosae , to host plant volatiles were studied. Two criteria for an oriented response were used: one based on whether an individual reached within 1.5 cm of the odour source and one involving analysis of the track patterns of larvae within 3 cm of the odour source. All larval instars were attracted to both host and non-host plant material, though to a lesser degree by the latter; they were also attracted to CO₂, but not to N₂. A steam volatile fraction of carrot roots, and methyl eugenol, a volatile component of this fraction, also attracted the larvae, differentially at different concentrations. The possible roles of both CO₂ and the host-specific secondary plant volatiles in the host plant finding behaviour of soil inhabiting arthropods are discussed.     

Bionomics of the carrot fly Psila rosae F.; the infestation and sampling of carrot crops

The headlands of a carrot field are generally more heavily attacked by carrot fly larvae than the remainder of the field.
The methods of sampling a carrot crop are outlined: it has been found that, for fields up to 10 acres, samples of carrots taken in alternate twos and threes in the midfield, and in twos on the headlands, give a reliable measure of the infestation. For larger fields the number of carrots is doubled. Transects of headlands and whole fields have been made, and are useful in showing the actual amount of damage in a crop. Thus it is possible to represent each field as a three-dimensional body showing the amount and distribution of the attack.
The deterioration of attacked carrot crops during autumn and winter has been followed. Deterioration is measured by (1) percentage carrots attacked, (2) number of mines per 100 carrots and (3) the percentage of carrots unsaleable or unfit for market. The relations between (1) mines per 100 carrots and time, (2) percentage attack and time, (3) mines per 100 carrots and percentage attack, and (4) mines per 100'carrots and percentage unsaleability are stated. Based on the above relationships, methods of prediction of deterioration have been worked out.
The importance of shelter in determining the degree and position of attack is discussed and the relative effects of different types of shelter on infestation are compared.     

Using physical barriers to prevent carrot fly (Psila rosae (Fabricius)) damage in domestic production

A field experiment was used to assess the effectiveness of different barriers in protecting garden‐scale carrot production from carrot fly (Psila rosae (Fabricius)) damage. Some of the vertical barriers tested were found to provide a useful method of protecting early season carrots from carrot fly in terms of the percentage of carrots free from damage but, under cumulative pest pressure of several generations of carrot fly, such barriers were found to provide insufficient protection. Gardeners should therefore completely cover their carrot crop to attain an acceptable level of control, and this was found to be especially important for carrots harvested later in the season. There were positive effects of some barrier types on yield which may be due, at least in part, to the protection given by the barriers to carrot seedlings.     

The occurrence of Entomophthorales on the carrot fly [Psila rosae F.] in the field during two successive seasons

Three species of Entomophthorales were found on adult carrot flies (Psila rosae F.) during two successive seasons:Entomophthora muscae (C.) Fres.,Condiobolus apiculatus (Thax.) Remaud. & Keller andErynia sp.E. muscae was by far the most common species and caused epizootics in one carrot fly population up to 3 times per year. Flies cuaght in the hedge showed a higher infection level than flies caught in the field close to the hedge. Flies caught 200 m away from the hedge showed the lowest infection level. Apparently the hedge was the site of infection, since carrot flies, killed byE. muscae, were found there attached to the underside of the leaves. Flies caught on yellow sticky traps developed only to a limited extend symptoms and gave little information about the fungus infection levels.     

Further investigations of the host range of the carrot fly, Psila rosae (F.)

Techniques were developed for testing umbelliferous species against naturally-occurring populations of carrot fly in the field. The most efficient technique involved exposure of plants to either first or second generations of the insect in the field, followed by caging of infested plots and trapping of emerged flies in water in yellow water dishes. In a series of field experiments between 1981 and 1989 inclusive, a total of 132 umbelliferous species and sub-species were tested against carrot fly. Seventy-eight of these proved to be new hosts, 27 were confirmed as hosts and 27 failed to support any carrot flies. Six non-umbelliferous plant species failed to be colonised by carrot fly. Sources of variability in investigations of the host range of insects are discussed.     

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.     

Monitoring populations of the carrot fly, Psila rosae, with sticky and water traps

The relative, effectiveness of Rebell®, small cylinder, large cylinder, windmill and water traps, the five types of trap used currently for monitoring populations of the carrot fly, Psila rosae, was assessed in nine field experiments, three in south west Lancashire, four in the Fens (Suffolk, Norfolk, Cambridgeshire), one in East Suffolk and one at Wellesbourne, Warwickshire. Regression analysis of the numbers of flies caught on each type of trap against the numbers caught on the Rebell® trap indicated that each trap samples a constant proportion of the fly population relative to the other traps. Therefore, provided the fly population was sufficiently large for insects to be caught on the least effective traps, any of the five traps would monitor adequately fluctuations in carrot fly populations. However, the Rebell® trap caught 4–17 times as many flies/trap and 5–7 times as many flieshnit area of trap as any of the other traps tested.
Operators considered the Rebell® trap to be the easiest to use. However, it was more expensive than any of the other traps tested.     

Methods of analysing data to compare effects on populations of carrot fly (Psila rosae) larvae on carrots of treatments with exponentially-increasing doses of granular insecticide formulations

Effects of insecticide treatments were assessed by comparing the percentages of undamaged carrot roots on insecticide-treated and untreated check plots. A dose-response relationship based on a log-log transformation was derived and illustrated with data for six insecticides from two field experiments. Formulae are provided for estimating doses corresponding to given levels of decrease in the numbers of carrot fly larvae. Guidance is given on sample sizes needed and constraints on the accuracy of the estimates are stated. Examples are given of the use of the dose-response relationship to compare insecticide treatments within and between experiments.     

Bionomics of the carrot fly (Psila rosae Fab.): II. Soil populations of carrot fly during autumn, winter and spring

The behaviour of the carrot-fly population in the soil during autumn and winter has been investigated in East Anglia over the three seasons 1943-5. The results show that the pest overwinters in both the larval and pupal conditions, and that the proportion of the individuals in each stage varies from crop to crop and from season to season. Samples taken from a May (1943) sown plot showed that pupation, which was 452% complete in October, rose to 742% in March and was not complete until late April. From a June (1944) sown plot of carrots samples showed a rise in percentage pupation from 8-6% in October to 935% in March, the process probably being complete by early April. The changes in proportion of the soil larvae, carrot larvae and puparia in both the May and June plots were followed and are shown graphically. The changes in the size groups of the soil larvae and in the carrot larvae were also traced. The immature larvae persist longer in the carrots than in the soil. The size composition of the larvae from the soil of the June (1944) plot was recorded over the period September to April and the results are given graphically. Samples taken in October 1945 contained highly organized puparia from which flies emerged in a few days. These flies (third generation) gave rise to progeny which overwintered in the larval condition. The population in December 1945, composed chiefly of second generation puparia and third generation larvae, was very similar to that occuring in the previous two years. Mortality in the overwintering carrot-fly larvae and puparia varied considerably but was frequently heavy. The principal agencies concerned were an hymenopterous insect, Dacnusa gracilis Nees., and fungus and bacterial diseases. Other insect parasites occasionally important were Loxotropa tritoma Thorns, and Aleochara sparsa Heer. Nematode parasites were also recorded.     

A method of using the proportions of undamaged carrots or parsnips to estimate the relative population densities of carrot fly (Psila rosae) larvae, and its practical applications

The plant-to-plant distribution of carrot fly damage was studied on carrots and parsnips in field and microplot experiments. The observed data were well-fitted by the relationship In (–In q) = In L+ constant, where q is the proportion of undamaged roots and L is the number of larval mines in a sample and is proportional to the number of associated larvae, N. Hence, if N₁ and N₂ are the numbers of larvae at two levels of infestation on one occasion, the relative numbers present, N₂:N₁ are estimated by In q₂: In q₁ In this way the relative efficiencies of insecticide treatments can be determined within an experiment from only q₁ and q₂ and compared between occasions. A nomogram relating q₁, q₂ and treatment efficiency is provided. The method can also probably be used to compare the relative susceptibilities of host-plant cultivars to carrot fly attack. Guidance is given on the sample sizes needed to estimate q within 95% confidence limits and constraints on the application of the method are stated. An appropriate statistical procedure for analysing data sets on carrot fly infestations is by analysis of deviance and an example is given in an Appendix. The principles described can probably be adapted to other crop/pest problems.     

Host-plant susceptibility to the carrot fly, Psila rosae. 3. The role of oviposition preferences and larval performance

The acceptability of various plant species to ovipositing carrot flies was weakly, but significantly correlated with the host's suitability for larval development. Both adult host-plant preferences and larval performance as determined in laboratory experiments explained a part of the variation in susceptibility among the various test plants observed in the field. Across the whole set of plant species examined, antixenosis contributed more substantially to resistance than antibiosis, while the reverse seemed to be true for carrot cultivars.     

The protection of carrots against carrot fly (Psila rosae) with granular and emulsifiable concentrate formulations of chlorfenvinphos incorporated in gels used for drilling pre germinated seed in a sandy loam

Field experiments were done in a sandy loam in 1979 and 1980 to investigate the performance against carrot fly on carrots of e. c. and granular formulations of chlorfenvinphos incorporated in gels used for drilling pre-germinated seed. With 9·4–66·7 mg a. i./m row, the two formulations performed similarly in a natural hydrocolloid and a mineral colloid gel. Thus the emergence of carrot seedlings 6–7 wk after drilling was similar on plots with untreated gel and those with insecticide-treated gel and, about 6 months after drilling when > 90% of carrots grown without insecticide were damaged by carrot fly larvae, an approximately six-fold increase in dose of both formulations increased the estimated larval mortality from about 70 to 95%. Mean concentrations of residues of total (Z + E) isomers of chlorfenvinphos in harvested carrots treated with 9·4–56·5 mg a. i./m row were50% of the insecticide had been released into both gels.     

Responses of carrot fly larvae, Psila rosae, to the odorous and contact-chemostimulatory metabolites of host and non-host plants

ABSTRACT. The responses of third instar carrot fly larvae, Psila rosae , to plant metabolites were studied in a moving air olfactometer. Directed responses over distances greater than 7.5 cm were obtained to both host and non-host plant odours, to CO₂ and methyl eugenol, a volatile secondary metabolite of carrot roots. Larvae also oriented to gradients of contact-chemostimuli. The role of both olfactory and contact-chemostimuli in the host plant finding behaviour of P. rosae larvae is discussed, and observational evidence for the behavioural mechanisms involved presented.