Abstract. Douglas-fir seedlots were treated with the Incubation Drying Separation (IDS) method to test whether seeds infested with the seed chalcid, ...
NewForests 5: 327--334, 1991. © 1991 KluwerAcademic Publishers. Printed in the Netherlands.
Applying the IDS method to remove seeds infested with the seed chalcid, Megastigmus spermotrophus Wachtl, in Douglas-fir, Pseudotsuga menziesii (Mirb.) Franco J. D. SWEENEY, 1 Y. A. EL-KASSABY, 2 D. W. TAYLOR, 3 D. G. W. EDWARDS 3 and G. E. MILLER 3 i Forestry Canada -- Maritimes Region, P.O. Box 4000, Fredericton, NB, E3B 5P7, Canada; 2 Canadian Pacific Forest Products, 8067 East Saanich, Saanichton, BC, Canada; 3 Forestry Canada -- Pacific and Yukon Region, 506 West Burnside Road, Victoria, B C VSZ 1M5, Canada Received 3 April 1991; accepted 29 August 1991 Key words: germination capacity, insect removal, seed treatment
Application. The Incubation Drying Separation (IDS) method was used to remove most of the chalcid-infested seeds from Douglas-fir seedlots with no significant adverse effects on germination parameters. The IDS method should be useful for treating Douglas-fir seedlots that would otherwise require multiple sowing due to chalcid infestation.
Abstract. Douglas-fir seedlots were treated with the Incubation Drying Separation (IDS) method to test whether seeds infested with the seed chalcid, Megastigmus spermotrophus Wachtl, could be separated from non-infested seeds. Seeds were soaked in distilled water for 24 h, drained, placed in plastic bags and incubated at 15 °C for 3 days. The seeds were dried for either 0.5, 1, or 2 h at 25 °C, and then separated into floaters and sinkers in a water column. An average of 97% of the infested seed floated. The drying period did not affect the separation of infested seeds but significantly fewer sound seeds floated in the seedlots dried for 0.5 h than those dried for 1 h. Germination capacity of IDS-treated seeds did not differ from that of untreated seeds but the germination rate was significantly faster for IDS-treated seeds when all lots were stratified.
Introduction
The Douglas-fir seed chalcid, Megastigmus spermotrophus Wachtl, is a common pest of Douglas-fir, Pseudotsuga menziesii (Mirb.) Franco, seed in Europe and North America. Losses often exceed 20% in North America (Schowalter and others 1985) and have approached 100% in western Europe where the insect has been introduced and established for many years (Hussey 1954; Roques 1981). The seed chalcid overwinters inside the seed and may remain in the seed in a state of prolonged
328 diapause for up to 5 years (Annila 1982). Seed chalcid infestation reduces a seedlot's value in three ways: 1) percent germination is reduced so that multiple sowing may be required; 2) average seed moisture content, used to determine the optimal conditions for seed storage, may not represent that of healthy seeds and thus, storage conditions may not be optimal for maximum viability; 1 and 3) most countries place import restrictions on insect-infested seeds, thereby reducing international sales. The seed chalcid produces no external symptoms of infestation and seed must be either dissected or X-rayed to determine whether or not it is infested. The mature seed chalcid larva weighs about the same as a filled Douglas-fir seed (Hedlin and others 1980) so infested seeds are not removed during operational, gravity-based seed cleaning and separation procedures. Seed fumigation, with carbon disulfide or hydrocyanic acid, has been successful in killing several species of Megastigmus in conifer seeds, but it can reduce germination capacity (Richardson and Roth 1968; Roth 1968; Roth and Strasser 1971). Ruth and Hedlin (1974) showed that heat treatment of Douglas-fir seed (45 °C for 33 h) killed all seed chalcid larvae with little effect on germination. However, separate trials with heat treatment of Abies spp. seeds have resulted in reductions in the number of viable seed. 1 The Incubation Drying Separation (IDS) technique was developed in Sweden as a method for separating live seeds from dead-filled seeds of conifers such as Pinus sylvestris and P. contorta Dougl. (Simak 1981, 1984; Simak and others 1985). The IDS method works on the principle that healthy seeds lose absorbed water at a much slower rate than dead seeds. When a mixture of seeds are soaked in water, dried a short time and then placed back in water, the healthy seeds should sink while dead seeds float (Simak 1981). The floating seeds can then be discarded to produce a smaller, but higher quality seedlot. We hypothesized that seeds infested with the seed chalcid would absorb very little water and, like dead seeds, would be found in the floating portion of if)S-treated seed.
329
Materials and methods Seed collection
Douglas-fir cones were collected from squirrel caches in eight locations near Duncan and Shawnigan Lake, British Columbia, on 12--15 December 1988. The cones were air-dried in an open shade house and the seeds were removed from the open cones using a cone tumbler. Seeds were de-winged by gently rubbing them inside a canvas bag and the chaff and debris were removed using an aspirator cleaner (Edwards 1979). Samples of the discard portion of chaff and seeds were X-rayed and the vacuum adjusted to ensure that no infested or apparently filled seeds were removed. IDS treatment
Seeds from all locations were bulked, thoroughly mixed, and then split into 20 samples of about 2000 seeds each. Five samples were set aside as untreated controls. The remaining 15 samples were soaked in distilled water for 24 h at room temperature (20--22 °C), then drained. Seeds were placed in plastic bags and incubated for three days at 15 °C. Following incubation, any excess water was gently removed with paper towels and seeds were spread one-seed thick on blotters, to ensure uniform drying, and placed in an incubator set at 25 °C for 0.5, 1, or 2 h. Five samples were randomly assigned to each of the three drying periods. Immediately after the prescribed drying period, seeds were poured into a large beaker of distilled water and stirred to break the surface tension. Seeds that floated were separated from those that sank and both portions were airdried overnight at room temperature. All samples were then X-rayed and the numbers of chalcid-infested, empty, and filled seeds determined. Germination tests
Two random samples of 50 seeds each were taken from both the floating and sinking portions of the 15 IDS-treated samples and from each of the five untreated samples. These were X-rayed to determine the numbers of tilled seed and then subjected to a standard germination test (Edwards 1987). Prior to being placed in the germinator, half of the seed samples were stratified by soaking them in water for 24 h, draining, then refrigerating them at -t-2 °C for 21 days; the remaining samples were unstratified. All seed samples were spread on moistened cellulose wadding (Kimpak) and filter paper inside clear plastic germination boxes which were then
330 placed in a germinator set at an alternating temperature of 30 °C for 8 h followed by 20 °C for 16 h. Light, at about 1000 lux, was provided during the high-temperature period. Germinants were counted on alternate days for 28 days and the results expressed as: germination capacity (GC), the total percentage of normal germinants at the end of the test; and germination rate (Rs0), the number of days required for 50% germination. Both variables were calculated using filled seeds only.
Statistical analysis The numbers of infested and filled seeds that floated were compared with those that sank using paired t-tests. The percentage of total filled seeds from each sample that floated was compared among drying periods by one-way ANOVA and the Student-Newman-Keuls' range test (P -- 0.05) (Steel and Torrie 1960) on data transformed by arcsin square root. The effect of IDS treatment on GC and R50 was tested by separate one-way ANOVA's and the Student-Newman-Keuls' range test for stratified and unstratified seed samples. For each ANOVA, there were seven treatments consisting of the floating and sinking portions at the three drying periods and the untreated controls. The data for GC were transformed by arcsin square root prior to analysis.
Results and discussion
The IDS treatment was successful in removing most of the chalcid-infested seeds from each seedlot regardless of drying period. Although an average of only 1.3% of the seeds were infested with the chalcid, the floating portion of treated seedlots contained an average of 95--99% of the total number of chalcid-infested seeds (Fig. 1). A significantly larger proportion of filled seeds floated in the samples dried for 1 h compared to those dried for only 0.5 h (Fig. 1). Only in seedlots dried for 0.5 h did significantly fewer filled seeds float than sink (paired t-test, P = 0.008, n -- 5). The IDS-treatment did not adversely-affect the germination of filled seed. The GC of IDS-treated seeds did not differ significantly from those of untreated seeds, whether they were stratified or not (Fig. 2). The Rs0 of IDS-treated seed was significantly lower (faster germination) than that for untreated control seed at all drying periods when seeds were stratified, as well as in seeds that sank when dried for 0.5 h and were unstratified (Fig. 2). Contrary to our expectations and the results of others (Simak 1981, 19841 ) the filled seeds that floated were at least as viable as those that sank. For mS-treated seeds that were dried for 1 h and stratified, the GC
331
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Fig. 1. Mean percentage and standard error of total filled seeds and total seed infested with seed chalcid found in the floating (floaters) and sinking (sinkers) portions of IDS-treated Douglas-fir seed samples dried for 0.5, 1, or 2 hours. Means with different letters are significantly different ( A N O V A and Student-Newman-Keuls (P = 0.05) on data transformed by arcsin square root). STRATIFIED
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Fig. 2. Mean total percentage germination and germination rate of filled seeds in IDStreated Douglas-fir seeds (floaters and sinkers) dried for 0.5, 1, or 2 hours and untreated control seeds. Stratified (a, c) and unstratified (b, d) seeds were analysed separately. Within each graph, means with different letters are significantly different ( A N O V A and StudentNewman-Keuls, P = 0.05).
332 was significantly higher for seeds that floated than for those that sank (Fig. 2). We have no explanation for this. The utility of this method with respect to individual seedlots will depend on the proportion of infested seeds, the GC of non-infested seeds, the commercial and genetic values of the seeds, and the objectives of the seed handler. For entomological work or for the export of chalcid-free Douglas-fir seeds, IDS treatment will reduce the amount of X-ray analysis needed to ensure complete removal of infested seeds, regardless of percent infestation. For domestic seedling production, IDS should be practical whenever the level of chalcid infestation is so high that multiple sowing is required in untreated seedlots. Improving a seedlot's quality can significantly affect production costs in container nurseries. Sowing rates are determined by the source and GC of each seedlot. In turn, the need for crop thinning or, conversely, the number of empty cavities carried through a crop rotation are determined by sowing rates. For example, the sowing rate for class-A Douglas-fir seeds, i.e., those produced in a seed orchard, increases from 1 seed per cavity to 2, 3 or 4 seeds per cavity when the GC drops below 90, 86 and 66%, respectively.2 The sowing rate for a class-A Douglas-fir seedlot with 80% germination in its non-infested seeds would increase from 3 seeds per cavity to 4, if more than 18% of the total seeds were infested with the seed chalcid. If the GC of noninfested seeds was 95%, however, the sowing rate would double with only 6% chalcid infestation. The gain (or loss) in potential seedlings due to IDS-treatment is given by: SFt:/SF~Ds X p where SF U and SFDs are the sowing factors for untreated and IDS-treated "sinkers" respectively, and p = the proportion of the original seedlot's viable filled seed retained in the IDS-treated "sinkers". For example, by reducing the sowing factor from 2 seeds to 1 seed, 1.5 times as many seedlings would be produced with IDS-treated seeds compared with untreated seeds, despite the loss of 25% of viable seeds in the "floaters" (2/1 X 0.75). With further testing and refinements of drying times and liquid densities relative to the seed density, it may be possible to increase the proportion of viable seeds in the sinkers while retaining the infested seeds in the floaters. Seeds of many other conifer species, including Picea spp., Pinus spp. and Abies spp., are also commonly infested with Megastigmus spp. and, as with Douglas-fir, there are no external signs of damage on infested seeds and little difference in weight between infested and normal seeds (Hedlin and others 1980). We hypothesize that the IDS treatment could be used to
333 isolate chalcid-infested seeds in other conifers such as spruce, pine and fir but this remains to be tested. The IDS method has been found to be effective in improving the seedlot quality in white spruce in British Columbia and in Eastern Canada (Edwards 1986; Edwards and Banerjee 1989; Downie and Bergsten 1990), as well as in Douglas-fir (Edwards 1986). If the above-mentioned hypothesis is true, then the IDS method will provide an added benefit by removing chalcid-infested seeds.
Conclusions The results indicate that the IDS method can be used to remove most of the chalcid-infested seeds from Douglas-fir seedlots with no significant adverse effects on germination parameters. Of the drying periods tested, 0.5 h conserved the highest proportion (75%) of filled seeds in the sinking portion.
Acknowledgments We thank Doug Ruth and Remy Claire for their help in collecting and handling the cones, and Dale Simpson, Dan Quiring, and the anonymous journal reviewers for comments and criticisms of the manuscript. This work was supported in part by funding from the Federal-Provincial Development Agreement on Forestry in British Columbia.
Notes 1. 2.
Edwards, D. G. W. Forestry Canada, Pacific and Yukon Region, unpub, data. British Columbia Ministry of Forests Sowing Rules for 1990, Memorandum, Silviculture Branch, Victoria, B.C.
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