Thesis

Ph.D. Thesis Title:

Interactions between the peach aphid Myzus persicae and the entomopathogenic fungus Verticillium lecanii.

Supervisors:

Dr A. K. Charnley and Prof. N. R. Franks,

School of Biology and Biochemistry, University of Bath (Bath, BA2 7AY, UK)

Period: 1996 - 1999

Funding

Work funded by FAIR- BM – 960878

Project title: ‘Interactions of the parasitoids Aphidius colemani and the entomopathogenic fungi Verticillium lecanii against the peach aphid Myzus periscae’

• Investigation of fitness cost of insecticide resistant development in green peach aphid Myzus persicae.

• Effects of insecticide resistance on susceptibility to entomopathogenic fungus V. lecanii

• Effects of insecticide resistance on aphid behaviour and fecundity (growth rate)

• Host selection and survival of parasitoid Aphidius colemani on insecticide susceptible and resistant aphids with and without the presence of fungus V. lecanii.

• Development of different spore pickup bioassays and involvement in pathogenesis

• Effect of fungal infection on aphid behaviour; investigation of different host attack strategies by fungal isolates and effects on host behaviour

• Behaviour manipulation of aphid behaviour and improved efficacy of fungal spore application

Work funded by FAIR3 – CT94 – 1352

Project title: ’Insect pathogenic fungi for environmentally friendly pest control in the greenhouse’

• Investigation of possible case of synergism between V. lecanii fungal isolates with different host attack strategy on insecticide resistance aphid clones

Abstract

The aphid Myzus persicae is a major crop pest, that has developed resistance to major groups of insecticides over the past few years. The present work was aimed at improving the control achieved by an alternative pest control method, the use of entomogenous fungus Verticillium lecanii.

Four M. persicae clones, with different resistance levels, were studied; S (susceptible clone), R1 (low resistant), R2 (medium resistant) and R3 (highly resistant). No differences were found between the dose responses of the four clones to V. lecanii (Vertalec) when the fungal inoculum was applied in situ. The dose response of the two extreme clones, R1 an R3, was also studied using two more spore application methods. The 'direct impact' method, was designed to allow spore pick up only from the applied spore suspension, and consequently known levels of inoculum. Absence of significant differences using this application method suggested absence of susceptibility differences to the fungus. The 'secondary pick up' method allowed spore pick up only from previously inoculated leaf surfaces, and as a result of the above findings, the absence of dose response differences was associated with possible absence of differences in activity between insects of the two clones.

Visual analysis of time-lapse video recordings of aphid behaviour, suggested that both susceptible and resistant aphid clones (S and R3) demonstrated similar and very low activity levels during the experiments. The two clones also exhibited similar honeydew excretion rates, closely associated with feeding rates. Similar low activity levels were recorded for clone R1 using computer analysis (tracking program) of aphid mobility. Differences were found in the reproductive rate between the aphid clones but there was no correlation with the level of resistance.

Visual analysis of behaviour of V. lecanii infected aphids indicated possible increase of aphid activity in the first days of the infection development. Indeed, computer analysis indicated that an aphid infected with isolate Vertalec covered significantly more distance that control aphids 1 and 2 days after application. Using the isolate KV42 the aphids demonstrated increased mobility only one day after application. This was associated with differences in host attack strategies between the two isolates.

Aphid alarm pheromones (EBF) increased the activity of the aphid clones, resulting in significantly higher mortality due to enhanced secondary spore pick up. However, EBF occasionally produced inconsistent results due to its unstable chemical structure. Use of sublethal systemic doses of the insecticide imidacloprid also significantly increased aphid mobility and mortality in secondary pick up experiments. There was no synergism between the fungus and the insecticide other than increased levels of spore pick up. Use of sublethal doses of imidacloprid, as an irritant and antifeedant, could possibly provide a practical method to improve fungal efficiency in field applications.