The Behaviour of Wireworms in Response to Chemical Stimulation

Abstract

1. The behaviour of wireworms Agriotes lineatus, obscurus and sputator in relation to food and to chemical substances of plant origin has been investigated by various types of olfactometer and choice chamber as well as by field experiments. A. lineatus and A. obscurus cannot be distinguished in the larval stages but, when possible, results obtained with these two were checked with A. sputator and in no case was any significant difference found. 2. Although wireworms when seeking food will, under exceptional conditions, walk on the soil surface it has not been possible to detect any response to odours under such conditions. 3. When wireworms encounter certain plant juices or solutions containing either one or more of a number of carbohydrate, fatty or protein substances the biting response is elicited. 4. Of the carbohydrates all the sugars tested elicit biting, certain common plant sugars being among the most active in this respect. It appears that the polyhydric alcohol grouping is responsible for activity as in the case of the human sense of sweetness. The pH activity curve for biting (tested on glucose) shows a marked peak between 6 and 8--most plant juices having a pH of between 5 and 7. 5. Triolein is the only pure fat to which the wireworms have been found to give the biting response, but the sodium salts of certain fatty acids are active in this respect. 6. The proteins so far found to be active in eliciting biting are of animal origin while the plant proteins tested are inactive. Partially broken down proteins may be active though the parent proteins were not, but none of the amino-acids or mixtures of amino acids tested have proved active. 7. As with the feeding of a number of other insects, the threshold for biting is lowered progressively with starvation up to 7 days. 8. In a sand- (or soil-) filled chamber wireworms show the ‘orientating response’, as a result of which they tend to collect in that side of the chamber in which the sand is moistened with aqueous extracts of plant tissues and desert that side which is moistened with an equal quantity of water. This orientation can be induced by a number of the substances which cause the biting response as well as by aqueous solutions in very low concentration of several dibasic acids and amides (e.g. aspartic and asparagine acid) and related substances which are widely distributed in plant tissues. Thus while the activity of sucrose in causing orientation is 2-3 on a logarithmic scale that of aspartic acid is 11, the former sensitivity being comparable to that usually found in organs of taste, while the latter is of the order characteristic of olfactory responses. 9. The sensitivity for orientation tends to vary with the nutritional state (as well as with season and time in relation to moult) as it does for biting. After 7 days' starvation the sensitivity of some individual wireworms gives an activity of 3 for glucose and 13 for asparagines--figures which may be taken as indicating the limits of sensitivity of the receptors concerned. It is shown that orientating wireworms are sensitive to a gradient of active substances. 10. It is shown that orientation is partly the result of orthokinesis and partly of a type of behaviour which partakes both of klinokinesis and klinotaxis. 11. Field experiments are described in which artificial baits were employed. It is shown that burrowing, following the lines of least resistance in the soil but random in respect of chemical stimulation, is the primary basis of food finding. With paper-pulp blocks containing glucose or triolein as artificial baits random burrowing combined with orthokinesis and the biting reaction will cause a very considerable rate of catching. The extreme sensitivity to asparagine and related substances, while conferring no significant advantage when tested with a large rectangular paper-pulp bait, is regarded as a means of increasing the ‘target area’ of a root system and of retaining the insect in the vicinity of fine roots once they have been encountered-- so serving as a fine adjustment in the food-finding process. 12. The prospects of developing the bait method are discussed. It is suggested that combinations of a stable non-water-soluble bait substance with a non-repellent contact poison might provide a method of widespread application for protecting the critical early stages of crops, and that further research should be directed along these lines.