Alan R. McCaffery

A behavioural analysis of phase change in the desert locust

A programme of research into phase change in the desert locust, Schistocerca gregaria, is described. The ability to change phase between solitarious and gregarious forms in response to population density is a key feature of locusts and is central to their occasional yet catastrophic impact on humans. Phase polymorphism is an extreme form of phenotypic plasticity. The most labile phase characteristic is behaviour. It is argued that a fully integrated study of behavioural phase change provides a powerful tool for understanding both the mechanisms of phase change and locust population dynamics, both of which offer possibilities for improved management and control of desert locust plagues. An assay for measuring behavioural phase‐state in individual locusts was derived, based on logistic regression analysis. Experiments are described that used the assay to quantify the time‐course of behavioural change, both within the life of individual locusts and across generations. The locust‐related stimuli that provoke behavioural gregarization were investigated. Complex interactions were found between tactile, visual and olfactory stimuli, with the former exerting the strongest effect. Behavioural analysis also directed a study of the mechanisms whereby adult females exert an epigenetic influence over the phase‐state of their developing offspring. Female locusts use their experience of the extent and recency of being crowded to predict the probability that their offspring will emerge into a high‐density population, and alter the development of their embryos accordingly through a gregarizing agent added to the foam that surrounds the eggs at laying. There is also a less pronounced paternal influence on hatchling phase‐state. An understanding of the time‐course of behavioural phase change led to a study of the effect of the fine‐scale distribution of resources in the environment on interactions between individual locusts, and hence on phase change. This, in turn, stimulated an exploration of the implications of individual behavioural phase change for population dynamics. Cellular automata models were derived that explore the relationships between population density, density of food resources and the distribution of resources in the environment. The results of the simulation showed how the extent of gregarization within a population increases with rising population size relative to food abundance and increasing concentration of food resources. Of particular interest was the emergence of critical zones across particular combinations of resource abundance, resource distribution and population size, where a solitarious population would rapidly gregarize. The model provided the basis for further laboratory and field experiments, which are described.

Parental effects on the behaviour and colouration of nymphs of the desert locust Schistocerca gregaria.

Abstract The behavioural phase status of hatchling Schistocerca gregaria Forskal has been quantified using logistic regression, following treatments in which maternal and paternal phase, density during mating and oviposition, and nymphal rearing density were varied. All treatments (having a crowd-reared mother, a crowd-reared father, parents which were crowded only for the period of mating and oviposition, or being reared from birth in a crowd) had a marked gregarizing effect on the behaviour and colour of hatchlings. Such effects have important implications for the generation of locust swarms.

Effects of population density experienced by parents during mating and oviposition on the phase of hatchling desert locusts, Schistocerca gregaria

The behavioural phase state of first-instar desert locusts, Schistocerca gregaria Forskål, was quantified by using logistic regression analysis, following treatments in which the population density experienced by their mother during mating and oviposition was manipulated. Crowding during the period of oviposition caused females that had previously been reared in isolation to produce offspring which behaved in a gregarious manner. However, isolating previously crowd-reared females led to hatchlings which behaved more solitariously. The population density experienced during mating affected the behaviour of larvae from solitary-reared, but not crowd-reared, parents. These results show that the transmission of phase characteristics across generations can be modified even at the final stage of the reproductive cycle, providing considerable flexibility in response to changes in population density.

Pre‐ingestive and post‐ingestive effects of soya bean extracts and rutin on Trichoplusia ni growth

Total ethanolic crude extracts from soya bean (Glycine max L. Merrill) PI 227687 and ‘Davis’, three purified fractions (A, B and C) from PI 227687, and pure rutin were tested for their effect on the growth of Trichoplusia ni (Hübner) (Lepidoptera: Noctuidae) larvae, from egg hatching to pupation. Analysis of covariance (ANCOVA), followed by bicoordinate utilisation plots, was used to remove the effect of feeding time from consumption and weight of pupae and to separate pre‐ ingestive and post‐ ingestive effects of different diets on T. ni growth. Survival, pupal weight, and feeding time of T. ni were all negatively affected by PI 227687 total extracts. Pupal weight and consumption remained affected by diet, after removing the effect of feeding time by ANCOVA. Larvae fed on the ‘Davis’ extract diet grew larger and consumed more rapidly a larger amount of food than those fed on PI 227687 and control diets. T. ni pupal weight was strongly influenced by consumption. In general, those insects that consumed more food were heavier but larvae fed on diet containing PI 227687 extract grew less than the others, irrespective of the amount of ingested and digested food. The feeding time was adversely affected by PI 227687 fraction A, composed of two flavonol glycosides (rutin and quercetin 3‐O‐glucosylgalactoside) and of an isoflavone glycoside (genistin). Survival and pupal weight were not affected, although the consumption and assimilation of ingested food was lower when larvae were fed on fractions A and C. The toxicity of the higher rutin concentration (2%) was similar to that observed when the insects were fed on unfractioned PI 227687, except that effects on food consumption were not observed. Results indicated that the feeding deterrence of PI 227687 must be caused by other compounds from fractions A and C. Moreover, rutin, acting as an antibiotic, appears to account for the adverse effects of PI 227687 on the physiology of T. ni.

Effects of azadirachtin and aqueous neem seed extracts on survival, growth and development of the African armyworm, Spodoptera exempta.

Abstract Treatment of larvae of the African armyworm (Spodoptera exempta) with azadirachtin and aqueous neem seed extracts produced a range of adverse effects that were dose dependent. High doses of up to 10 μg per larva of azadirachtin resulted in 100% larval mortality, but this effect was delayed and prolonged. At lower doses of azadirachtin, however, inhibition and disruption of moulting was observed and larval-pupal intermediates or abnormal pupae were commonly found. Similar results were obtained with the aqueous extracts of neem seeds. The few pupae obtained from larvae treated with lower doses of the extracts (0.01 and 0.1 μg per larva) either failed to develop further or developed into adults that died during eclosion, or had frizzled, curled wings.

Elucidation of detoxication mechanisms involved in resistance to insecticides in the third instar larvae of a field-selected strain of Helicoverpa armigera with the use of synergists

Abstract Synergism studies showed that whereas piperonyl butoxide (PB) synergism was either absent with cis -cypermethrin or slight with trans -cypermethrin and fenvalerate in the third instar larvae of a susceptible strain of Helicoverpa armigera , it was very high in a resistant Thailand strain where it was recorded 11-fold for cis -cypermethrin, 9-fold for trans -cypermethrin, and 139-fold for fenvalerate at the LD 50 level. After synergism with PB, resistance factors for the three pyrethroids declined to the same value of around 3.5 at LD 50 . This indicates a vital role of monooxygenases in the detoxication of pyrethroids in the Thai strain. The synergism of pyrethroids by DEF, either alone or with PB, was much lower in the resistant strain than in the susceptible strain. This implies that esterases may not be important in imparting resistance to the Thailand strain. FDMC did not synergise DDT in the susceptible strain but it produced a very high synergism in the resistant strain, presumably due to the inhibition of DDT-dehydrochlorinase. Carbaryl was considerably synergised by both PB and DEF in the susceptible strain but mildly in the resistant strain, thus suggesting that esteratic and oxidative enzymic mechanisms may not be involved in resistance to carbaryl in the resistant Thai strain.

Evidence for an enhanced metabolism of cypermethrin by a monooxygenase in a pyrethroid-resistant strain of the tobacco budworm (Heliothis virescens F.)

Abstract A pyrethroid-resistant strain (PEG87) and a susceptible strain (BRC) of Heliothis virescens were topically dosed at third instar with trans -(1-[ 14 C]cyclopropyl)cypermethrin in acetone. Penetration studies revealed that trans -cypermethrin was absorbed at a significantly faster rate in the susceptible strain as compared to the resistant strain over a 48-hr period. The times for 50% penetration were 11 and 30 hr for the BRC and PEG87 larvae, respectively. Studies on the metabolism of trans -cypermethrin indicated an increased rate of excretion of radioactivity in PEG87 larvae as compared to BRC larvae. The major metabolic product in both strains was polar-conjugated material; trans -cyclopropane carboxylic acid (Cl 2 CA), a product associated with ester bond cleavage, appeared at later time points. The rate of excretion of conjugate by PEG87 larvae exceeded that of BRC larvae; at 12 hr, PEG87 excreted 20 times more conjugate than larvae of the BRC strain. Radioactivity was detected in the haemolymph of both strains. In the PEG87 strain, radioactivity was associated with two metabolites, hydroxylated trans -cypermethrin and Cl 2 CA, the products of oxidative and esterase attack, respectively. Hydroxylated trans -cypermethrin was not identified in the haemolymph of the BRC strain. Two to four hours after dosing, internal concentrations of 14 C activity were two- to fourfold greater in BRC larvae as compared to PEG87 larvae. In the BRC strain, trans -cypermethrin was the most abundant radiolabelled component, while conjugated material represented the highest percentage of radiolabel within the PEG87 larvae. Pretreatment with piperonyl butoxide resulted in a reduction in the excretion of total radioactivity and conjugate from the PEG87 larvae. The absence of hydroxylated cypermethrin and an increase in the level of parent material in those larvae pretreated with piperonyl butoxide substantiates the importance of the monooxygenase system in the detoxification of cypermethrin in the resistant strain.

The role of female accessory glands in maternal inheritance of phase in the desert locust Schistocerca gregaria

Accessory glands of crowd-reared females of Schistocerca gregaria were ligatured from the lateral oviducts. Hatchlings resulting from egg pods laid after the treatment showed a significant shift towards solitarious behaviour as compared to hatchlings from control-treated females. Morphometric measurement of hatchlings revealed no consistent difference between ligatured and control females, however, one ratio (hind femur length/vertex width) was approaching significance. Hatchlings from eggs of crowd-reared females behaved solitariously when freshly laid eggs were washed with a saline solution. Gregarious behaviour could be restored when washed eggs were treated with a saline extract of the accessory glands. The colouration of hatchlings was not affected by any treatment. Our findings implicate the accessory glands in the production, release or activation of the recently reported gregarizing factor found in the egg pod foam [McCaffery, A.R., Simpson, S.J., Islam, M.S., Roessingh, P., 1998. A gregarizing factor present in the egg pod foam of the desert locust Schistocerca gregaria. Journal of Experimental Biology 201, 347-363].

Novel mutations in the para‐homologous sodium channel gene associated with phenotypic expression of nerve insensitivity resistance to pyrethroids in Heliothine lepidoptera

Coding sequences from the intracellular segment linking repeat domains III and IV of the para‐homologous sodium channel gene were amplified and cloned from two species of Heliothine larvae using RT‐PCR techniques. Sequence comparisons between standard laboratory susceptible and nerve insensitive strains of the tobacco budworm (Heliothis virescens) and the cotton bollworm (Helicoverpa armigera) identified two mutations. These were an aspartic acid (GAC) to valine (GTC ) and a glutamic acid (GAA) to glycine (GGA) mutation. PCR and sequencing was undertaken only on individuals that were demonstrated to be phenotypically nerve insensitive or susceptible to pyrethroids using a neurophysiological technique. The finding of identical mutations across two species suggests that they may be implicated in resistance.

Decreased nerve sensitivity is a mechanism of resistance in a pyrethroid resistant strain of Heliothis armigera from Thailand

Abstract Neurobioassays using cis -cypermethrin were conducted on the third instar larvae of a susceptible laboratory strain and a resistant Thailand strain of Heliothis armigera (Hubner). They consisted of recording the spontaneous multiunit activity (MUA) from the exposed peripheral nerves after superfusion with a saline solution of 4 μ M cis -cypermethrin. After treatment, the MUA first showed a marked excitation and then blockage in both the strains. A consistent pattern of MUA was obtained in all the susceptible insects, whereas the response of the resistant insects was variable and clearly divided into three distinct categories. As in the susceptible insects, half of the resistant insects showed an increased MUA above the control level in the first minute and then it decreased until blockage within 5 to 6 min. Forty percent of the insects maintained their enhanced activity for 15 min above the control level after insecticidal exposure, the activity then decreased steadily but continued for over 36 min before eventual blockage. Ten percent of the resistant insects behaved intermediately. This implies that around 50% of the insects from the Thailand strain appear to be resistant due to the presence of a mechanism of decreased nerve sensitivity.