Effects of the biological control agent, Tetranychus lintearius, on its host, Ulex europaeus Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/br86b675j

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  • Invasive plants threaten ecosystems and economies worldwide. Classical biological control, introduction of specialist herbivores on invasive plants, is one of the tools employed to reduce the impact of invasive plants. Gorse, Ulex europaeus L. (Fabaceae), an invasive leguminous shrub, is the target of a biological control program in Oregon. The biological control agent, Tetranychus lintearius Dufour (Acari: Tetranychidae), the gorse spider mite, was introduced in 1994. Three studies were conducted to quantify the effects of the gorse spider mite on the growth of gorse: a potted plant study, an experimental field study and an observational field study. The potted plant study quantified the effects of gorse spider mite herbivory at different densities and times of the year under relatively homogenous conditions. Shoot, canopy volume and stem diameter relative growth rates (RGR) and plant dry weights were examined. Average relative mite damage ranged from 1.5 to 4.4% with no significant differences among the inoculation treatments. Shoot RGR was the only plant growth measure affected by mite damage. The late summer high density mite inoculation resulted in slightly higher shoot RGR compared to the control. The amount of mite damage affected shoot RGR but not stem diameter RGR, canopy volume RGR, aboveground thy weight, belowground dry weight or total thy weight. Unexpectedly, increasing mite damage was associated with slightly increased shoot RGR. The experimental field study was conducted to assess the effects of gorse spider mite herbivory on individual gorse plant growth rates under field conditions at two inoculation levels. Stem diameter, canopy volume and shoot RGR were examined Average relative mite damage was 6.5% at the low inoculation level and 7.3% at the high inoculation level and the difference between inoculation levels was not significant. Increased mite damage was associated with decreased shoot RGR. Neither canopy volume RGR nor stem diameter RGR were affected by spider mite damage. The observational study quantified naturally occurring mite damage and gorse shoot length. Gorse shoot length and the amount of mite damage were measured at several sites throughout the summer. At two of the five sites with mites, mite damage was correlated with shoot length. At one site late in the season, higher mite damage was correlated with greater shoot length. At another site increasing mite damage was associated with shorter shoot length. These contradictory results suggest there may be a damage threshold below which the gorse spider mite either does not affect growth or has a mild stimulatory effect on growth. The results of this thesis provide evidence that under certain conditions the gorse spider mite affects the growth of gorse over one season. Further research is needed to determine: under what conditions the gorse spider mite reduces shoot growth, what impact reduced shoot growth has over the lifetime of gorse plants and what implications this may have for gorse dominated plant communities.
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