Fieldtesting For Resistance

This method involves the planting of candidate genotypes in replicated trials. Genotypes are discriminated as resistant or susceptible by exposure to natural (Alfaro and Ying 1990; Kiss and Yanchuk 1991) or artificial (King and Alfaro 2001) infestations. Releasing weevils in plantations to create artificial infestations is recommended, because screening is faster and more reliable than depending on natural infestations. Artificial infestations are initiated once spruce trees reach attackable height (1-2 meters in height in coastal BC). Results are obtained within one or two years after infestation. This method is cheap and effective; however, it provides only a general ranking of the resistance level, with only limited information on the mechanisms involved.

One type of information that we found useful in understanding resistance mechanisms to shoot insects was measuring the rate of Attack Failure among various genotypes. Failed attacks are those cases in which the insect has laid eggs that failed to develop into viable larval populations due to drowning of the eggs and young larvae in toxic resin. Although the shoot survives a failed attack, egg niches and scars due to mining by the larvae are visible.

A wide range of resistance to the white pine weevil was revealed by studying rates of oviposition by P. strobi among Sitka spruce clones and ramets from nine provenances tested at Fair Harbour, BC (Ying 1991) (Figure 3). In the susceptible provenances (Aberdeen, Fair Harbour, Moresby, Tasu, Muir) virtually every leader with eggs was killed (Figure 3). Clones from provenances with intermediate resistance (Cedarvale, Kitwanga, Green Timbers) sustained high rates of oviposition, suggesting strong attraction, and low feeding and oviposition repellency. However, the leaders were killed at moderate levels, suggesting only mild antibiotic effects, i.e. mild toxicity was exerted on the weevil larvae. Clones from the most resistant provenance (Haney families 0 and 1) had significantly lower oviposition and leader kill rates than the rest, suggesting high levels of repellency (physical or chemical) and toxicity. The most resistant genotype (Haney family 0, clone 2, also known for its registration number as #898), sustained consistently low oviposition rates, with about 12% of the ramets having oviposition at the Fair Harbour trial, of which only about 6% resulted in leader kill.

Cumulative percent weevil attack, 1986-95, Fair Harbour by S ilka spruce Provenance. Family and Clone

Cumulative percent weevil attack, 1986-95, Fair Harbour by S ilka spruce Provenance. Family and Clone

2 5 6 7 lilt 2345 2589 23 123? 1238 1231 123-1 1 SS)

Clone number within Provenance

Figure 3. Weevil attack rates among ram els of Sitka spruce clones tested at Fair Harbour, BC, Canada, between 19X6 and 1995. Length of bar indicates the percentage oframets with oviposit ion in one or more years; the solid portion of the bar indicates the percentage of ramets with leader kill in one or more years. Provenances are indicated on top of the bars (two families were tested in the Haney provenance) and are arranged from left to right in descending order of resistance.

2 5 6 7 lilt 2345 2589 23 123? 1238 1231 123-1 1 SS)

Clone number within Provenance

Figure 3. Weevil attack rates among ram els of Sitka spruce clones tested at Fair Harbour, BC, Canada, between 19X6 and 1995. Length of bar indicates the percentage oframets with oviposit ion in one or more years; the solid portion of the bar indicates the percentage of ramets with leader kill in one or more years. Provenances are indicated on top of the bars (two families were tested in the Haney provenance) and are arranged from left to right in descending order of resistance.

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