The role of naturally occurring insect pathogenic fungi in regulating aphid populations on vegetable brassica crops

Aphids are serious pests of a wide range of crops and are a threat to food security. They are normally controlled using chemical insecticides, however this is becoming more difficult due to product withdrawals following new EU legislation, the development of heritable resistance in target pest aphid populations, and pressure on farmers from retailers to produce crops with zero detectable pesticide residues. Therefore new methods of aphid control are needed that makes crop protection more effective and sustainable.

This project concerns biological pest control with natural enemies. It focuses on Brevicoryne brassicae, which is an important pest of vegetable brassica crops. Aphid populations on horticultural crops increase in the spring and the autumn, interspersed with a period in midsummer when they decline rapidly, or ‘crash’. Data collected at the Warwick Crop Centre, through HDC-funded projects, suggest that the crash is caused by naturally occurring fungal pathogens. Unfortunately, the timing of this crash cannot yet be predicted with accuracy. If the crash could be predicted, control programmes could be modified and our reliance on insecticide sprays would be reduced.

The PhD will test the hypothesis that fungal epizootics are one of the principle factors causing the mid-season crash in populations of aphids on horticultural brassicas. The research will be done by following four Objectives:
• Firstly, insect pathogenic fungi will be isolated from aphid populations and identified using a combination of morphological characters and nucleotide sequence data (rDNA ITS).
• Laboratory bioassays will be used to measure the susceptibility of cabbage aphid to fungi collected from the field and compared with commercial microbial control agents. Work will also be done to measure the susceptibility of naturally occurring aphid pathogenic fungi to fungicides routinely applied to brassica crops, since it is possible that fungicide applications could be inhibiting natural epizootics.
• Aphid populations will be monitored on sequentially planted brassicas to study the link between the mid-summer population crash and epizootics of insect pathogenic fungi. The numbers of other natural enemies of aphids within the plots will also be recorded. At the same time, data on environmental parameters (temperature, leaf wetness, and canopy humidity) will be recorded.
• The effect of temperature and moisture on the pathogenicity of fungi will be determined in laboratory experiments. The interactions between the aphid, fungus, temperature and moisture availability will then be modelled to provide a simple forecast of the outbreak of fungal epizootics.

The results will be communicated to the industry at grower meetings and through written summaries, such as articles in HDC News.

Key experimental skills involved:
Training in the assimilation and summary of information from a variety of sources, design and execution of field and laboratory experiments, fungal molecular biology, data analysis, insect rearing techniques, writing and presentation skills