Walking to survive : searching feeding and egg production of carabid beetle Pterostichus coerulescens L. (= Poecilus versicolor Sturm)

Abstract

This study concerns the prey-searching and feeding behaviour of the polyphagous groundbeetle Pterostichus coerulescens L. ( = Poecilus versicolor Sturm), a common species on sandy soils. This ground beetle rarely flies, thus preysearching behaviour involves walking. The beetle is diurnal. As object of research, predators of this kind are very suitable because they can be handled easily, their behaviour can be observed directly or filmed with a video camera. Furthermore they are abundantly available. Much research has been done on its biology (van Dijk, 1979a, 1979 b, 1982, 1986), population dynamics ( Baars & van Dijk, 1984,a,b; den Boer, 1977) and dispersal in the field (Baars, 1979; den Boer, 1971). This study aims to contribute to the role of groundbeetles as important natural enemies of some agricultural pests. The ultimate aim of this study is to investigate the role of the density and distribution of the prey on the searching behaviour of the groundbeetle P. coerulescens and the resulting predation and egg production.This study is tackled with the help of system analysis and simulation. Firstly, behaviour was divided in its major components of searching, acceptation and feeding. Subsequently the factors governing these components, the motivation, were for looked for. The investigation of the motivation, food ingestion and egg production will be discussed in part I of this thesis. The relationship between motivation and searching behaviour and prey acceptation are the subject of part 11. The relationships between motivation and behavioural components were quantified experimentally as far as possible and subsequently integrated in a simulation model. How the different behavioural components take part in the functional and numerical response was sorted out with help of simulations concerning prey discovery, prey capture and egg production for a range of prey densities and different prey distributions.Behaviour is influenced by external and internal factors. The internal factors are determined by the physiological condition or state of the beetle and they form the motivation for behaviour. This motivation results from the state of different organs and is strongly connected with food consumption and digestion, which are quantitatively determined by body size, maximum gut capacity, emptiness of the gut, whether reproductive or not etc. These internal states are influenced by external factors of which temperature and daylength are the most important. Assuming that hunger is the most important motivating factor in the predation process, the emptyness of the gut is taken as a measure of the hunger level of the beetle. It was found that the emptyness of the gut is determined by a complex of closely connected factors including the rates of food intake, digestion and defaecation. Digestion rate depends on the reproductive state of the beetle. The size of a meal depends on prey size and on gut capacity. In turn gut capacity depends on maximum gut capacity and on the room in the abdomen of the beetle. How much room is available depends on the size of the other organs in the abdomen. The size of these other organs, for example the ovaries, number of maturing eggs in the oviduct, the fat body, for the storage of reserves, is determined by the physiological phase of the beetle, whether in diapause or in the reproductive phase, and of course by the quantity of food ingested in the course of time. To be able to understand and estimate the influence of this complex system at hunger level, a simulation programme was developed in which these external and internal factors are integrated. Experiments were carried out to estimate gut emptying rates at different temperatures, the assimilation efficiency, the respiration rate at different temperatures and how long the maturing eggs stay in the oviduct. Gut capacity was assessed in relationship to the filling of the abdomen by fatbody, ovaries and maturing eggs. With this model it is possible to estimate the motivation of the beetle under a range of state variables, while the egg production can be calculated in relation to the quantity of food ingested at fluctuating field temperatures. Hunger level expressed as relative satiation level (RSATL) could be related to the different components of behaviour that play a role in the foraging process of the beetle. Hunger level has a strong influence on the locomotory activity of the beetle. Hungry beetles (gut filled 80%) are active for not more one hour per day. Also walking behaviour is strongly influenced by the hunger level. At 20°C, the walking speed of hungry beetles is twice that of beetles with more than 5% of the gut filled. Just as many other predators this beetle shows an intensive, tortuous walking behaviour after consumption of a prey. The duration of this intensive searching behaviour depends on fullness of the gut, hungry beetles search for about twelve minutes intensively while this intensive search behaviour is absent when the gut is more than 80% full. How winding the walking pattern is depends on the walking speed, the faster the beetle walks the straighter its pattern. The success ratio (number of successful encounters divided by the total number of encounters with prey) depends on the prey species and on the relative satiation level of the beetle.Firstly, a simulation model was constructed of the individual searching behaviour. With this model the effect of walking speed (fast in a hungry state, intermediate when the relative satiation level is more than 5% and tortuous after prey consumption) on the discovery of prey could be investigated. The advantages of the different walking types are best seen in aggregated prey distributions. In random prey distributions, intensive search after prey consumption is disadvantageous. By coupling of the motivational model to the searching and predation model, in which both walking behaviour, locomotory activity and success ratio are integrated, the functional and numerical response of the beetle to changes in prey density and prey distribution, both for short and for long periods, can be estimated. In this combined model the beetle seems to react very flexibly to changes in prey density or in prey aggregation. This model shows that intensive search is an advantage at generally low prey densities (less than 1 prey/m 2) only when prey is aggregated. Higher than this low prey density, compensation for one behaviour by another occurs in such a way that prey distribution has hardly any effect on prey capture and egg production. Longer walking behaviour compensates partly for the negative effect of intensive search in random prey distributions.During simulation, the practical use of these models is apparent. Because of the stochastic character of the processes, it is very difficult if not impossible, to carry out those experiments in the laboratory not mention those in the field, because they require many repetitions before we can have confidence in the means obtained. Besides, experiments with a range of prey densities and prey distributions, carried out over a season, are completely impracticable. Over longer periods, feedback mechanisms between state variables of the beetle produce effects that are difficult to estimate. With the help of this modelling approach more insight into them is obtained. The value of laboratory experiments for estimating functional and numerical responses can be better assessed.With the help of these models the area of the dispersal of beetles at specific food densities and distributions can be estimated, which may be important for assessing the exchange of individuals between local populations. Such data can be especially important in nature conservation because when not enough exchange takes place, measures can be taken to prevent local populations from becoming extinct