Screening spruce for white pine weevil resistance

• Project description
• Distribution of P. strobi in North America
• Genetic variation in canadian weevil populations
• Range of Sitka spruce
• Sources tested for resistance in Western North America
• White pine weevil resistance screening trials
• Mass screening for resistance (Jordan River plantation)
• Ghost trial: studies using caged insects clarified the mating system of weevils
• Artificial wounding studies: Studying plant defenses
• Insect - plant relationships
• Are weevil resistant trees also resistant to other insects?
• Conclusions
• Selected references

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Mass screening for resistance (Jordan River plantation)

Animation showing attack progress from 1994-2003

Located on southwestern Vancouver Island, the Jordan River test site is the oldest in this series and was screened by augmentation of the weevil population. This plantation consisted of open-pollinated progeny from 75 Sitka spruce parent trees collected from the Queen Charlotte Islands to Oregon. Each parent was represented by 24 trees. Each dot represents a single tree.

Screening for resistance requires the presence of dense weevil populations in order to create suitable conditions for distinguishing resistant and susceptible trees. The local weevil population at Jordan River was enhanced in 1994 by the release of 2250 insects (3 weevils per tree) in the southeast half of the site. The released weevils originated from the same genetic population as the weevils indigenous to the Jordan River site. The uniform weevil distribution and the elevated weevil attack rate (40% in 1995) caused by the release, provided sufficient conditions to distinguish resistant from susceptible families. Weevil attacks in subsequent years, spread to the rest of the plantation, however, attack rates have since declined throughout the site due to the progressive increase of natural enemies.

Four families, all from the Big Qualicum area, were identified as resistant in this trial. The site has now been recovered by replanting with Douglas-fir and hemlock, which are non-hosts for the weevil. The same families are also replicated at Cowichan Lake, south central Vancouver Island.

Distribution of family attack rates, Jordan River 1998

While many families sustained 50% attack or higher, only four families remained with low attack rates for the duration of the 4-year observation period.

Site to site correlation in % attack by family

Attack levels per Sitka spruce family at Jordan River correlated well with attacks on the same families at the Cowichan Lake site. These trials are located in two different ecosystems, which demonstrates the stability of the resistance in two different environments.

There were four families which sustained low attack levels at both sites and were deemed as "weevil resistant". These families originate from a small area located in the Big Qualicum area of Vancouver Island. Seed from these sources is now commercially available from local seed dealers.

Dynamics of the weevil population at Jordan River: Number of weevils/ha

The initial 2250 weevils released at this site in 1994 produced over 8000 new weevils. Since then, production of new weevils has declined, probably due to a sharp increase in natural enemy populations.

Stability of resistance over time at the Fair Harbour test site

The trial at Fair Harbour was planted by Dr. Cheng Ying of the B.C. Ministry of Forests in 1984.

After eight years of continuous monitoring by the Canadian Forest Service some clonal families remained with low attack rates (Provenance #29 = Haney, #18 = Cedarvale) while others (Provenance #32 = Fair Harbour) were severely attacked. This stability over time indicates useful resistance in these clonal families.

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