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STUDIES ON PREDATOR -PREY INTREACTIONS IN THE SOIL
Nur Tjahjadi
UPSI Tanjung Malim, www.upsi.edu.my
Abstract
Three techniques, pitfall traps, rhizotron observation and time lapse photography were used to study the effect of predators on insect resting stages above and below ground. Locust and stick insect eggs, mustard beetle and mealworm pupae were compared as possible baits for increasing activity. Mealworm pupae (Tenebrio molitor) were selected. A time lapse cinematographic technique was developed and using an analysing projector, it was found that an eight minute interval between frames maximised the number of significant results obtained per unit cost. The parameters measured were pupal disappearance, contacts between predators and prey and the identity of predators. The results showed that predation was highest at the surface and lowest at 6 cm. Predator activity differed with depth according to taxa. Staphylinid activity was greatest at the soil surface, carabid activity predominated at the surface and 3 cm, whereas geophylid centipedes and campodeans proved to be subterranean with greatest activity at 3 cm and 6 cm. They were rarely or never encountered in pitfall traps. Observations on seasonal changes in predator activity above and below ground showed that predators were most active at contacting pupae in the spring and summer. However, variabilityin activity within a season such as the summer could be as great as between season. It was concluded that predation is a significant risk to below ground resting stages throughout the the year. All predatory groups were more active during darkness. A crop insecticide and a soil insecticide affected predatory activity mostly at the surface during the first three days only.
Keywords: Predator, prey, pitfall traps rhizotron, time lapse cinematography
INTRODUCTION
Plant material in the soil is characterised by dense root systems, the biomas of which may exceed that above ground several-fold (Sims & Singh, 1978). These dense root systems develop in part in response to intense competition between plants and soil fauna (Newman et al., 1989). Many animals spend sometime underground, and many pests are also underground root attackers or spend part of their life cycle below ground. The British species of crop pests listed in Gratwick (1992) has been used to compile Table 1. This shows the proportion of species with underground resting stages (eggs and pupae). Also shown is the proportion of animal which over winter in the soil in some stages of their life cycle. The result show that 41 per cent of pest species lay their eggs in the soil, 65 per cent pupate there and 54 per cent overwinter in the soil (Table 1.1). The impact of below-ground predation on plant productivity has been far less studied than that of phytophagous feeding on aerial parts.
A knowledge of the activity, population density, dispersal and foraging patterns of soil predators would be of value in determining the feasibility of using them as biocontrol agents of crop pests (Vickerman & Sunderland, 1980; Best et al., 1981; Chiverton, 1986; Luff, 1987; Booij & Noorlander, 1992 and Bailey, 1994). If suitable, they may reduce the need for pesticide inputs, an aim of much present day farm practice. Our knowledge of predation below ground is still very limited. Control of underground pests is notoriously difficult, especially when they are in a resting stage (eggs or pupae). These stages are often very difficult to control with pesticides, few of which are able withstand the hostile environment for synthetic chemicals the soil provides.
One question which arises is why resting stages are placed below ground. Possible answers are that the soil offers more physically stable conditions, protection from predators and parasitoids and a relatively moister environment than above-ground.
