W. M. Blaney

Antifeedant effects of azadirachtin and structurally related compounds on lepidopterous larvae.

The antifeedant activity of azadirachtin, azadirachtin‐derivatives and related limonoids was assessed in choice and no‐choice bioassays against four species of Lepidoptera: Spodoptera littoralis, Spodoptera frugiperda, Heliothis virescens and Heliothis armigera. The choice bioassay showed that the feeding behaviour of S. littoralis was affected by more of the compounds than that of either S. frugiperda or H. virescens. H. armigera was the least affected. Azadirachtin and dihydroazadirachtin were the most potent of the 40 compounds tested. The results showed that hydrogenation of the C‐22,23 double bond did not decrease antifeedant activity and the nature of the substitutes at C‐1, C‐3 and C‐11 were important. Molecules with bulky substitutes at either C‐22 or C‐23 were usually ineffective antifeedants as were compounds lacking an epoxide. Compounds recorded as active antifeedants in the choice bioassay were not always as active in the no‐choice test. The value of the bioassays in assessing the mode of action of the compounds is discussed.

Actions of azadirachtin, a plant allelochemical, against insects

Investigations of the antifeedant mode of action of azadirachtin and four synthetic analogues, 22,23-dihydroazadirachtin, 3-tigloylazadirachtol, 11-methoxydihydroazadirachtin and 22,23-bromoethoxydihydroazadirachtin have revealed that both polyphagous and oligophagous insects are behaviourally responsive to azadirachtin, with the most responsive species being able to differentiate extremely small changes in the parent molecule. In Lepidoptera the antifeedant response is correlated also with increased neural activity of the chemoreceptors. When locusts are treated on crop plants, the antifeedant and physiological actions of azadirachtin and analogues work in concert and result in feeding deterrence, growth and moulting aberrations and mortality with the same order of potency as for antifeedancy. Specific binding studies using [3H]dihydroazadirachtin carried out on locust testes and Spodoptera Sf9 cells have shown that the competitive binding of the different analogues of azadirachtin to these binding sites occurs in a similar order of potency to that found with antifeedant and IGR bioassays. This suggests a causal link between specific binding to membrane proteins and the ability of the molecule to exert biological effects.

A comparison of dietary selection behaviour in larval Locusta migratoria and Spodoptera littoralis

ABSTRACT. . Final instar nymphs of the oligophagous acridid Locusta migratoria (L.) and larvae of the polyphagous noctuid Spodoptera littoralis (Boisduval) were fed for 4, 8 or 12 h, the conditioning period, on one of four artificial diets. Of these, diet PC contained 20% protein and 10% digestible carbohydrate; another, P, contained 20% protein but with the digestible carbohydrate component replaced by cellulose; a third, C, had the protein component substituted by cellulose, and the fourth, O, had both protein and digestible carbohydrate replaced. After this conditioning period, insects were given a choice of two diets, P and C, and hence an opportunity to select for the nutrients, if any, which were lacking in their previous food. Amounts eaten and selection behaviour were then recorded in detail for a total of 9 h. This paper deals with total amounts of diet eaten during the conditioning and choice periods. Spodoptera larvae were more sensitive than the locusts to being fed a nutritionally inadequate conditioning diet, and ate only small quantities of the P, C and O diets as compared with the PC diet, irrespective of the duration of conditioning. Locusts, on the other hand, when restricted to the P diet continued to eat relatively large amounts of it throughout a 12 h conditioning period. Those nymphs fed the C diet ingested large quantities (more than of the PC diet) up until 8 h, after which intake fell. When offered a choice, both species selected for the nutrients missing from the conditioning diet, even if the conditioning period had been as short as 4 h. During the first hour of choice locusts selected the P diet if they had been previously fed C and the C diet if previously fed P. Those deprived of both nutrients increased consumption of both P and C diets. Spodoptera larvae were more sensitive to prior deprivation of digestible carbohydrate than of protein. During the first hour of choice they selected the C diet if previously fed P or O but did not choose the P diet if previously fed C. In the subsequent 8 h of choice, however, a strong selection for the P diet after previous deprivation became apparent. In the locust, the selection for nutrients missing from the conditioning diet continued for the following 8 h of choice but became masked by a tendency, shown by all nymphs, to select C over P. The functional significance and possible physiological basis of all these responses is discussed.

Variation in chemosensitivity and the control of dietary selection behaviour in the locust

Investigations into the behavioural and underlying physiological mechanisms of dietary selection are presented for the locust, Locusta migratoria. Locusts were fed for 4, 8 or 12 h on one of four chemically defined artificial diets: diet PC, which was nutritionally complete; diet P, containing no digestible carbohydrate; diet C, containing no protein; and diet O, which lacked both protein and digestible carbohydrate. Following this pretreatment, the locusts were provided with both the P and the C diet in a choice test. Detailed analyses of selection behaviour indicated that diets lacking a nutrient for which the insect was deficient were either rejected before a meal was initiated, or, if feeding commenced, eaten in meals of only short duration, while those containing the appropriate nutrients were accepted more readily and eaten in longer meals. Electrophysiological studies showed that this behaviour was paralleled by nutrient-specific changes in gustatory responsiveness. Locusts pretreated for 4h on C diet had increased gustatory responsiveness to stimulation with an amino acid mix, but not to sucrose, while insects fed on P diet showed increased responsiveness to stimulation with sucrose, but not to the amino acid mix. This result is consistent with earlier experiments in which levels of blood nutrients were shown to modulate taste responsiveness in the locust.

THE FUNCTIONS OF THE MAXILLARY PALPS OF ACRIDIDAE (ORTHOPTERA)

The functions of the maxillary palps of acridids have been investigated in experiments with Locusta migratoria migratorioides (R. & F.), Schistocerca gregaria (Forsk.) und Chorthip‐pus purallelus (Zett.). The palp makes small irregular flicking movements and more extensive rapid vibrations described as palpation. Both activities involve the same basic movements of the palp segments. Palpation is essentially related to feeding and in normally feeding insects the palps play an important role in food selection, but as starvation is prolonged they become unimportant. Selection depends on appropriate chemical stimulation from the cuticle on the surface of the leaf. Palpation on this leads to head lowering and to biting if appropriate tactile stimuli are then received.

The antifeedant activity of clerodane diterpenoids from Teucrium

Abstract Clerodane diterpenoid compounds from Teucrium (Labiatae) were assayed for antifeedant activity against larvae of Spodoptera littoralis and Heliothis armigera . The functional groups responsible for antifeedant activity are discussed.

Phomopsichalasin, a novel antimicrobial agent from an endophytic Phomopsis sp.

Abstract The antimicrobial agent phomopsichalasin (1) was isolated from an endophytic Phomopsis sp. fermented on shredded wheat. The structure was determined by UV, IR, NMR and mass spectroscopy. Phomopsichalasin represents the first cytochalasin-type compound with a three ring system replacing the cytochalasin macrolide ring.

Food selection in adults and larvae of three species of Lepidoptera: a behavioural and electro physiological study

Adults and larvae of Spodoptera littoralis, Heliothis virescens and Heliothis armigera were tested with a range of sugars, amino acids, sugar alcohols and allelochemics. Feeding behaviour was correlated with the electrophysiological responses of maxillary styloconic sensilla in the larvae and proboscis styloconic sensilla in the adult. The neural response was more vigorous in larvae than in adults but otherwise the spectra of responses were similar in the two life stages. Phagostimulants and antifeedants stimulated maximally different sensilla in larvae but not in adults. The responses of adult sensilla to sugars and amino acids were significantly correlated to behaviour in all three species, but only in H. armigera was there a similar correlation with the sugar alcohols and allelochemics.

An electrophysiological and behavioural study of insect antifeedant properties of natural and synthetic drimane-related compounds

ABSTRACT. A range of drimane‐related natural and synthetic compounds have been tested in behavioural and electrophysiological screens against larvae of Spodoptera exempta (Walker), S. littoralis (Boisduval), S. frugiperda (J. E. Smith), Heliothis armigera (Hubner) and H. virescens (Fabricius). The most effective compounds are identified and their potential as antifeedants and metabolic poisons is assessed. A consideration of structure in relation to function allows the identification of important functional groups within molecules.

Synthesis of a hydroxy dihydrofuran acetal related to azadirachtin: A potent insect antifeedant

Abstract The synthesis of novel hydroxy dihydro and tetrahydrofuran acetals modelled on the insect antifeedant azadirachtin is described.