Ecological Nutrition of Phaulacridium Marginale (Walker) (Acridoidea: Catantopidae)

Abstract

This project presents the results of an integrated study of the ecological nutrition of the New Zealand lowland grasshopper Phaulacridium marginale. The work was divided into field nutrition, artificial diets and plant diets. Field nutrition of P. marginale was assessed using faecal analysis of live insects. Quantification of these data in relation to availability of the food-plants gave a measure of preferences. This research was conducted over 2 years with a total 15 quadrats at 3 localities: coastal vegetation; naturalised grassland; and indigenous tussock shrubland. The principal coastal food-plants consumed were: Coprosma propinqua var latiuscula, Microlaena stipoides and Muehlenbeckia complexa. The principal naturalised grassland food-plants were: Anthoxanthum odoratum, Lolium perenne and Taraxacum officinale. The principal food-plants associated with the indigenous tussock grassland were: Cyathodes fraseri, Muehlenbeckia axillaries and Racomitrium lanuginosum, a moss. For most of these principal food-plants the proportion in the diet was correlated with the availability, except T. officinale which was consumed at almost 3 times its availability. Other highly preferred species included Hypochoeris radicata and Leontodon taraxacoides was well as reproductive tissue. In general, naturalised food-plants were preferred, as were members of the Compositae. The total number of plant species consumed was 69, representing 58% of all the plant species present. P. marginale was reared on a total of 27 artificial diets from first instar onwards. There were 3 diet types: preliminary meridic, meridic wheat germ diets, and chemically-defined diets. The lowest % mean survival occurred on the preliminary meridic diets and the highest on a wheatgerm diet (WG3). Increasing the ascorbic acid concentration to 6/ Mg/g diet increased survival to 20% by the 10th week and cholesterol at 8 mg/g diet gave 22% survival at week 10. The chemically-defined alginate diet resulted in 16% survival at week 10. Addition of microbial inhibitors to the diets prolonged nymphal development. Single deletion of amino acids from the chemically-defined diet resulted in the 10 amino acids considered essential for mammals and insects also being essential for P. marginale. Among the lipids, inositol, choline, linoleic acid and cholesterol were all essential for this insect. The seven vitamins of the β-complex were essential for P. marginale. Sucrose and glucose were essential for this insect, but the hexose sugars, the disaccharides and trisaccharides substituted for sucrose and glucose. Ascorbic acid concentrations of 0.5 M to 0.05 M were phagoinhibitory, while 1 M and 0.5 M sucrose concentrations and 0.5 M fructose and arabinose were phagostimulatory. 0.1 M atropine, 0.5 M coumarin, 0.01 M digitonin and 0.1 M gramine were all significant feeding inhibitors. P. marginale was reared on 21 food-plants from nine plant families. There was a wide range of response from 100% mortality within 4 days on Lotus pedunculatus and Chionochloa rubra to survival for 14 weeks on Taraxacum officinale. The plant species were divided into 3 groups on the basis of P. marginale development: plants which supported nymphal development only; and plants which enabled adult development of P. marginale. For the Gramineae that supported P. marginale development, % water content was correlated with approximate digestibility. For the Dicotyledons which supported adult development, % water content was correlated with mean relative growth rate. P. marginale was successfully able to detoxify allelochemics present in the plant diets such as: the glycoside ranunculin in Ranunculus repens; the quinine chrysophanol in Rumex obtusifolius; malic acid in Lolium perenne and Dactylis glomerata; ‘tannic acid’ in Taraxacum officinale. It is concluded that under field conditions, P. marginale is generally not food-limited except for highly preferred species. The adult insects can survive on food-plants unfavourable for reproduction and this is an adaptive advantage. Food-plant selection in P. marginale is mediated by the absence of feeding inhibitors in leaf tissue and the presence of adequate phagostimulants to ensure continued feeding.