R. G. McKinlay

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.

Studies of the growth, development and reproductive performance of the aphid Myzus persicae on artificial diets containing potato glycoalkaloids

Peach potato aphids Myzus persicae (Sulzer) (Homoptera: Aphididae) were reared on artificial diets containing the steroidal glycoalkaloids (GAs) α‐solanine and α‐chaconine in concentrations lower or similar to those observed in potato leaves. The adults proved to be susceptible to high concentrations (80–160 mg GA /100 ml of diet) showing reduced fecundity, diet uptake and increased mortality in comparison to controls. Ingestion of these artificial diets by nymphs delayed maturity and decreased the intrinsic rate of natural increase. GAs in low concentrations marginally stimulated the reproductive performance and diet acceptability of this aphid. The possibility of GAs exerting a defensive role in potato plants against aphids is discussed.

Interactions of aphid herbivory and nitrogen availability on the total foliar glycoalkaloid content of potato plants.

In plant growth room (PGR) and open-air pot (OAP) experiments, potato cvs King Edward and Maris Piper were grown under two nitrogen levels or two different nitrogen release patterns. Plants were subjected to infestation by peach potato aphids Myzus persicae (Homoptera: Aphididae). Total glycoalkaloid (GA) levels were measured in the foliage of both infested and non-infested plants, before, during and after aphid infestation. In the PGR experiment, aphid infestation reduced the amounts of total GAs in both cultivars. This reduction is attributed to the sugar deficiency induced in the plants owing to the dense aphid colonization. Results from the OAP experiment showed a temporal increase of GAs produced by potato cv. King Edward plants subjected to aphid infestation. Elevated amounts of nitrogen in the nutrient solutions (PGR experiment) reduced total GAs, while no differences were observed between manure and fertilizer treated plants (OAP experiment). It is concluded that the source of available nitrogen does not affect foliar GA synthesis in potatoes, and as a consequence, does not affect its endogenous chemical defense against insect herbivory. The case for insect-induced chemical defense mechanisms as triggered by low rates of aphid infestation is discussed.

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).

The role of brassica leaf surface chemicals in antixenotic resistance to Delia floralis

Turnip root fly, Delia floralis (Fall.), is a major pest of brassicas in Northern Europe. Understanding the mechanisms of host plant resistance is important to the development of integrated pest management. Antixenosis is a key component of resistance (Birch, 1988) because damage is reduced by deterring oviposition. The oviposition behaviourial sequence of D. floralis was characterised by Havukkala & Virtanen (1985). Later work by Stadler & Schoni (1990) demonstrated the importance of leaf surface chemicals in host plant selection by cabbage root fly, Delia radicum (L.). The objective of this work is to isolate the chemical stimuli which influence the host selection of D. floralis.