We have interests in the physiology and ecology of aphids and their natural enemies from both fundamental and applied aspects. Aphids are notorious plant pests capable of direct damage by feeding but are also the most effective vectoring agents of plant viral diseases. They are, therefore, of interest to plant health issues but they also have fascinating biologies that allow switching between asexual and sexual modes of reproduction and the development of five or more different adult forms. A number of insects use aphids as hosts and as prey items. Our studies also involve such insects as parasitoids, which lay their eggs inside aphids, and in lacewings that eat aphids. We are particularly intrigued by how aphids find the specific host plants on which they can develop and how their natural enemies can, in turn, locate the aphids. Random processes would not allow for survival and evolutionary mechanisms have acted to improve the efficiencies of host/prey location. The chemical communication between the insects and plants are of crucial importance here and form the basis for the study of chemical ecology.
Host-plant location by aphids
To varying degrees, plant-eating insects are host specific and can only develop and reproduce on certain plants. Aphids are, therefore, extremely good botanists and use all of their senses to select appropriately. Thus, the visual system is particularly sensitive to light wavelengths reflected from plant leaves, the olfactory system is sensitive to both general and specific odours emanating from plants and the taste organs detect chemicals with plant tissues.
Aphids feed from the phloem vessels of plants with hypodermic-like mouthparts that penetrate into the leaves. The process of feeding can be monitored electrically and we have recently shown that chemical constituents in peripheral plant cells are used by the insect to determine plant suitability for reproduction before they reach the nutritional phloem tissue. We are trying to identify crucial chemicals.
Environmental and physiological control of phenoptype
Wingless asexual forms reproduce fast and give birth to some 100 offspring over a week or two. Populations can thus build up rapidly and when they do winged adults develop which fly to fresh host plants with the trade-off that their reproductive capacity is impaired, only producing 70 or so offspring. In many cases it is the mechanical stimulation of sensory hairs that induces the formation of winged adults.
The mode of reproduction, asexual or sexual, and the associated adult forms are controlled by day length. This photoperiodic response was the first example discovered in animals back in 1925. It requires that the insect possesses a receptor that can distinguish day from night, a biological clock that can measure the length of day, or more usually night, a photoperiodic counter or memory which stores information of day length from successive light dark cycles and an endocrine effector mechanism which controls the relevant developmental pathway. Unravelling these mechanisms and their interplay is a major task.
Parasitoid and lacewing behaviour
Parasitoids lay their eggs in the aphid and their larvae develop inside the host, keeping vital systems intact but eventually kill the host before emerging as adults from a 'mummified' aphid corpse. Adult female parasitoids have a well-developed sense of smell and prefer odours emanating from the plant on which they developed. They can distinguish between not only different plant species but also different varieties of the same species. However, if host aphids on that plant are not available they will deposit eggs in hosts on other plants and rapidly learn the new odour. Thus the probability of finding a suitable host is increased.
Whilst working with aphid sex pheromones and how male aphids can locate the sexual females, we found male lacewings in field-placed traps. The most frequently caught species was new to Britain. How and why this attraction has evolved is being investigated.
Research Student Supervision
Dewhirst,S, Chemical ecology of aphids
Kati,A, The influence of parasitoids on the normal development of their aphid hosts
Martinez,A, Chemical ecology of aphid predators and parasitoids
Salisbury,A, Chemical ecology of the red lily beetle
Vamvatsikos,P, Olfactory behaviours associated withhost-aphid location in a generalist parasitoid wasp
Webster,B, Host location mechanisms in herbivorous insects