A. N. E. Birch

Oviposition and chemosensory stimulation of the root flies Delia radicum and D. floralis in response to plants and leaf surface extracts from resistant and susceptible Brassica genotypes

In Brassica crops differences in susceptibility to root fly attack can be largely attributed to antixenotic resistance. Plants of four genotypes (two swedes and two kales) with widely differing resistance in field trials, were compared in laboratory choice assays for their susceptibility to oviposition by the root flies Delia radicum (L.) and D. floralis (Fallen) (Diptera, Anthomyiidae). For both species the preference among the genotypes corresponded to the susceptibility of the genotypes in the field. The preference ranking in response to surrogate leaves treated with methanolic surface extracts of the four genotypes was identical to the preference among potted plants, demonstrating that chemical factors on the leaf surface mediate host preference for oviposition in these species.

Influence of Increasing Herbivore Pressure on Modification of Glucosinolate Content of Swedes (Brassica napus spp. rapifera)

The effect of increasing herbivore pressure, in the form of larval feeding damage by the turnip root fly, Delia floralis, on the glucosinolate content of swede roots (Brassica napus ssp. rapifera) was investigated. Only one of the 14 root glucosinolates detected, 3-indolyl methyl glucosinolate, rose significantly with increasing levels of insect attack. Although other root glucosinolate concentrations altered following damage, the induced changes were no greater from inoculation with 20 eggs/root than with 5 eggs/root. Swedes roots that had been damaged by D. floralis contained approximately three times the concentration of total indolyl glucosinolates of control roots. This change was strongly influenced by a fourfold increase in the concentration of 1-methoxy-3-indolyl methyl glucosinolate. The total glucosinolate concentration found in swede roots remained unchanged overall as a result of a fall in the concentration of five of the aliphatic glucosinolates, which balanced the rise in aromatic glucosinolates. The relevance of these results to studies of crucifer–insect interactions are discussed.

Molecular analysis of ribosomal DNA from the aphid Amphorophora idaei and an associated fungal organism

The ribosomal DNA cistron from the large raspberry aphid, Amphorophora idaei (Hemiptera: Aphididae), has been mapped by restriction analysis. The results showed that the map of A. idaeiwas similar to those of the previously characterized aphids Schizaphis grami‐num and Acyrthosiphon pisum. An extra Bglti site was found in some of the ribosomal DNA intergenic spacer repeats in A. idaei. Using in‐situ hybridization to aphid mitotic chromosomes it was demonstrated that probes derived from this region mapped to the pair of X chromosomes and it was therefore aphid in origin. Polymerase chain reaction using conserved rDNA primers also detected significant amounts of a fungal genome in the DNA samples. Microscopic investigation showed that the external surface of A. idaei harboured fungal propagules, hyphae and yeast‐like organisms.

Behavioural and chemosensory responses of the turnip root fly (Delia floralis) to glucosinolates

This study investigates the behavioural and neural mechanisms involved in the oviposition behaviour of the turnip root fly, Delia floralis (Fallen). Behavioural studies showed that glucosinolates modulated the oviposition behaviour of the flies on artificial leaves as well as the number of eggs laid in the soil at the base of these leaves. Electrophysiological responses to glucosinolates were obtained from type A and type D sensilla on the prothoracic and mesothoracic tarsi, as well as from the long contact sensilla on the labellum. The neural responses from these sensilla were positively correlated with the oviposition behaviour of the flies and with the number of eggs laid. Of the eleven glucosinolates tested in the behavioural and electrophysiological tests, the flies were most responsive to glucobrassicanapin, gluconapin and glucobrassicin. The type D tarsal sensilla were more responsive to the glucosinolates than either the type A tarsal sensilla or the labellar sensilla. The structure‐activity investigations showed that slight modifications to the chemical composition of the glucosinolates resulted in changes in neural activity.

LEAF SURFACE COMPOUNDS AND OVIPOSITION PREFERENCE OF TURNIP ROOT FLY Delia floralis: THE ROLE OF GLUCOSINOLATE AND NONGLUCOSINOLATE COMPOUNDS

The role of leaf surface compounds influencing the oviposition of the turnip root fly, Delia floralis, was investigated using bioassays and fractionation of leaf surface extracts from four Brassica genotypes. Polar leaf surface extracts contained between 65 and 175 nM/g leaf equivalent of glucosinolates. However, following fractionation it was found that nonglucosinolates were the major stimuli for D. floralis oviposition. Electrophysiological studies of leaf surface extracts and their fractions were performed by using D. radicum, the cabbage root fly, as an analytical tool. The most behaviorally active fractions contained stimulatory compound(s) that had an activity profile identical to that previously described for recently discovered nonglucosinolate compounds. The role of leaf surface chemicals in influencing antixenotic resistance to D. floralis is discussed.

A time-course study of chemical and physiological responses in Brassicas induced by turnip root fly (Delia floralis) larval feeding

Many pests and pathogens of Brassica crops (family Cruciferae) are influenced by a group of sulphur-containing secondary metabolites called glucosinolates. The effects of these chemicals on plant-pest interactions can differ greatly, depending on many complex factors: e.g. whether an insect is a ’generalise or Crucifer ‘specialist’ and on the concentrations in specific plant tissues at the time of insect attack. Recent studies at SCRI (Birch et al., 1990, 1992; Hopkins et al., 1992b, 1995) have shown that root fly attack induces multiple chemical changes (e.g. of glucosinolates, sugars, lignin, volatiles) in different Brassica host plant tissues following root damage. Root fly-induced effects on host plant metabolism can be localized and/or systemic (i.e. from root to leaf). These chemical changes in the host plant genotype, but cannit be mimick be artificial root damage (Griffiths et al. , 1994).

The relationship between cabbage root fly (Delia radicum) larval feeding and the freeze-dried matter and sugar content of Brassica roots

Five genotypes of swede (Brassica napus var. napobrassica), two genotypes of kale (B. oleracea var. acephala), and two genotypes of rape (B. napus var. napus) were each inoculated at the 8–10 true leaf stage with five cabbage root fly (Delia radicum) eggs. The percentage pupation after larval feeding on individual plant genotypes ranged from 45 to 78%, and the mean pupal weight from 6.5 to 13.0 mg. After 5 weeks, larval feeding damage had reduced root weight by up to 47%, compared with uninoculated plants. The dry matter content of undamaged roots was higher in the kales and rapes than in the swedes. Whilst the dry matter content of the rapes and swedes were not changed by D. radicum damage, that of the kales was elevated. The ethanol‐soluble sugar content of the root was reduced in all cases by D. radicum larval damage. However, the effect of D. radicum damage on the concentrations of individual sugars (glucose, fructose and sucrose) was crop‐ and genotype‐dependent. In the roots of kales and rapes, the glucose and fructose concentrations were either very low or unaffected by D. radicum damage, whilst both glucose and fructose were generally reduced in swede roots by D. radicum damage. The root sucrose concentration was either reduced or not significantly affected by D. radicum damage in all of the crop types tested. The percentage pupation and the mean pupal weight of D. radicum were inversely correlated to root freeze‐dried matter content. D. radicum pupal weight was positively correlated with root fructose, glucose and ethanol‐soluble sugar contents.

The decision to reject an oviposition site: sequential analysis of the post-alighting behaviour of Delia floralis

Abstract.The post‐alighting behaviour of gravid female turnip root fly, Delia floralis (Fallén) (Diptera: Anthomyiidae), was observed in the laboratory, using four plant genotypes differing in antixenotic resistance, to stimulate a range of oviposition‐related behavioural events. Analysis of the behavioural sequences of individual female flies suggested that during the post‐alighting behaviour of D. floralis, the decision to reject a highly resistant plant was predominantly based on plant cues received during a stationary period immediately after landing on the leaf (the leaf contact phase). Female flies that rejected a plant immediately after the leaf contact phase did so after spending approximately twice as long in the leaf contact phase as those flies that continued to explore the plant. On a plant genotype of intermediate susceptibility, D. floralis females on the stem of the plants increased proboscis extension. The mechanisms involved in early host plant rejection decisions and the possible ecological advantages of such behaviour patterns to female flies seeking suitable oviposition sites are explored. The potential advantages of using detailed studies of individual behaviour sequences to focus studies seeking to elucidate chemical oviposition stimuli on the plant’s surface are also discussed.

Analysis of sequential behaviour patterns: the oviposition decision of the turnip root fly, Delia floralis

The decision to oviposit on or reject a plant is central to the evolution and maintenance of insect host range. However, detailed analyses of oviposition sequences are uncommon. Important information can be retrieved from behavioural sequences of individual insects, and standard statistical techniques can often be applied to such sequences to provide answers to ethological or ecological questions. Studies have described or analysed the probabilistic nature of oviposition behaviour. St/idler & Schrni (1990) produced a descriptive ethogram of the behavioural transitions of D. radicum on surrogate and real leaves. In addition, Harris & Miller (1991) contrasted the Conditional Transition Frequencies between behavioural events of ovipositing and non-ovipositing D. antiqua on onions, the latter showing consistently low transitions to running behaviours. The methods outlined here were developed to examine the sequence of behavioural units of individual insects in relation to the acceptance or rejection of an oviposition site. Havukkala & Virtanen (1985) divided the host selection behaviour of Delia floralis (Diptera: Anthomyiidae) (Fall.) into six events. Preliminary examination of the pattern of rejection on plants of differing susceptibility to D. floralis indicated that many individuals rejected potential oviposition sites early in the host selection sequence (Hopkins et al., 1992). More detailed studies separated 30 exclusive behavioural events (Hopkins, 1994). The behavioural sequence of individual females which do and do not oviposit must be different. By discovering the point in the sequence at which behavioural patterns differ, information can be gained about the decision to reject a plant. In this study, groups of insects were observed in different environments (i.e. different plants). The main factors investigated were the choice of next behaviour by individuals, and the relationship between the duration of a behaviour and a subsequent decision to accept or reject the plant. The behavioural patterns described are those of individuals which are assessing a plant as a potential oviposition site. For the purposes of these, and previous, studies it is referred to as oviposition behaviour, although the actual performance of oviposition is one of two potential outcomes of the assessment of the plant (acceptance or rejection).

Oviposition and larval survival of Dasineura tetensi on four blackcurrant Ribes cultivars

Monitoring of an unsprayed infested fieldsite using watertraps in S.E. Kent revealed four generations of Dasineura tetensi (Rubs) (Diptera: Cecidomyiidae) occurring between April and September 1996. Ribes nigrum L. cultivars ‘Baldwin’ (susceptible), ‘Ben Alder’ (susceptible) and ‘Ben Connan’ (resistant) were sampled for eggs in the field and assessed for midge damage throughout the season. Oviposition was indiscriminate, but plant damage varied significantly between cultivars. In laboratory choice experiments, mated female midges showed no preference between susceptible shoots of ‘Ben Alder’ and resistant shoots of ‘Ben Connan’ for oviposition. Olfactory responses of D. tetensi to leaf volatiles of ‘Ben Alder’ and ‘Ben Connan’ were also tested in a 4‐way olfactometer. Mated females did not discriminate between volatiles of susceptible and resistant host plants. Larvae reared on cv. ‘Ben Connan’ shoots were significantly smaller than those reared on shoots of cv. ‘Ben Alder’. Larval antibiosis and not female antixenosis appears to be the main mechanism for resistance to D. tetensi in ‘Ben Connan’.