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Resistance of hybrid canola (Brassica napus L.) to flea beetle (Phyllotreta spp.) damage during early growth
Resistance of hybrid canola (Brassica napus L.) to flea beetle (Phyllotreta spp.) damage during early growth R.P. Bodnaryk*, R.J. Lamb, and K.A. Pivnickt
Agriculture Canada, Research Station, Winnipeg, Manitoba, R3T 2M9 Canada and tResearch Station, Saskatoon, Saskatchewan, S7N OX2 Canada
The hybrid canolas, Brassica napus hyb. K4-87 and B. napus hyb. K36-87, were tested for two modes of resistance, antixenosis and tolerance, against flea beetles, Phyllotreta spp. In feeding tests in which choices were offered between the hybrids and a standard inbred canola cultivar, B. napus cv. Westar, the cotyledons of the hybrid K4-87 and the cotyledons and first true leaves of the hybrid K36-87 showed a higher level of antixenosis. As neither parent of K36-87 had a higher level of antixenosis, this trait in the hybrid may be due to heterosis. In no-choice feeding tests, however, cotyledon damage and seedling biomass 1 week after feeding damage did not differ among K4-87, K36-87, and Westar. The hybrids showed no tolerance to feeding damage either in the laboratory or in the field. On the basis of plant biomass and yield data from Manitoba and Saskatchewan, the hybrids do not exhibit sufficient resistance to provide protection in the field. These results do not support the hypothesis that hybrid canola will contribute an agronomically useful level of resistance against flea beetle damage under commercial growing conditions. Keywords: Brassica napus; Phyllotreta; resistance
Apart from the prospect of significantly increased seed yields in hybrid canola, Brassica napus L. (Sernyk and Stefansson, 1983), the vigorous growth displayed by hybrids has aroused expectations that hybrids might be less susceptible to the effects of insect pests. Flea beetles, Phyllotreta spp., are major pests of canola grown in western Canada and must be controlled annually with an insecticide, especially during the first 2-3 weeks of postemergence growth (Lamb, 1984, 1988; Bracken and Bucher, 1986). Alternate control methods, including host-plant resistance, are being sought. This paper describes laboratory and field experiments on the early growth of two hybrid canolas exposed to flea beetle damage to detect possible enhancement of two resistance mechanisms, antixenosis (non-preference) and tolerance, in the hybrids. Materials and methods Plants
Registered seed of canola, Brassica napus cv. Westar and mustard, Sinapis alba cv. Ochre, were obtained from breeding programmes at the Agriculture Canada Research Station, Saskatoon, Saskatchewan. Hybrid canolas B napus hyb. K4-87 and B. napus hyb. K36-87 and the parents of B. napus hyb. K36-87, coded P1 and P2, were provided by Zenon Lisieczko, King Agro, Research and Distribution Centre, Listowel, Ontario, N4W 3G7, Canada. *To whom correspondence should be addressed
For laboratory tests, seeds were planted in Vermiculite in 175 ml styrofoam coffee cups with a 0.5 cm hole punched in the bottom and subirrigated with Hoagland's nutrient medium. The seedlings were held 10--12 cm under wide-spectrum (Gro-Lux) artificial lighting (--200 I~E s-t m-2 of photosynthetically active radiation) at 20 + I°C, 60-70% relative humidity and 18:6 L:D photoperiod. Insects
Field-collected flea beetles, Phyllotreta spp., were used in the laboratory experiments. In eastern Manitoba, the overwhelmingly abundant species is P. cruciferae and in central Saskatchewan it is P. striolata.
Feeding tests
Phyllotreta cruciferae were collected in Winnipeg in late May and early June from volunteer rape, B. napus cv. Westar, in the field and kept for 1 week in the laboratory in screened cages (50 X 50 X 50 cm). The beetles were given water through a cotton dental roll held in a flask and were fed on cabbage leaves, B. oleraceae, obtained from a market. The feeding tests were conducted in the laboratory under conditions given above in screened cages (30 x 30 x 30 cm) with clear polystyrene tops. Pairs of the cultivar Westar, the hybrids and the parents of one hybrid, as given in Table 1, were placed in the cages with known numbers of flea beetles. For feeding tests
0261-2194/94/07/0513-06 (~ 1994 Butterworth-Heinemann Ltd (~ 1994 Department of Agriculture, Governmentof Canada
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Hybrid canola damaged by flea beetles: R.P. Bodnaryk et al. at the cotyledon stage, 12 6-day-old seedlings of each pair were placed randomly in each cage (replicate) with 120 flea beetles and the tests were replicated six times. For feeding tests at the first true leaf stage, pairs of 15day-old plants, the cotyledons of which had been removed with sharp scissors on day 14, were placed in cages with 20 flea beetles and tests were replicated nine times. After 24 h, beetles were removed from all cages and cotyledon or leaf area consumed was measured with an optical grid fitted to a Wild M8 dissecting microscope. Non-choice feeding tests were also conducted with Westar and the hybrids at the cotyledon stage using flea beetles, P. striolata, collected in Saskatoon. Two beetle densities, 10 and 20 beetles per seedling, were used. In these tests, cotyledon damage was estimated by a 0--10 visual rating system in which 0 represents no feeding and 10 represents 100% damage. The dry weights of individual seedlings were obtained 1 week after the termination of the feeding tests and from a control group with no beetles. Tolerance to cotyledon damage Artificial damage was inflicted on 6-day-old seedlings of B. napus cv. Westar, B. napus hyb. K4-87 and B. napus hyb. K36-87 with scissors by removing 20, 50 or 75% of the total cotyledon area. The fresh weight of the above-ground portion of each seedling was obtained on day 15. Forty-eight seedlings were used for each damage level. The weight of each plant was regressed against the amount of tissue removed and the resulting linear estimates of the slopes of the lines were used to assess the relative tolerance of Westar and the hybrids to damage. Field study In one series of experiments in Olenlea, Manitoba, Westar and the two hybrids, together with S. alba cv. Ochre, were exposed to natural infestations of flea beetles for various periods during their development, and then weighed to determine their ability to gain weight when damaged by flea beetles. Sinapis alba was included because it shows a high level of tolerance (Bodnaryk and Lamb, 1991). Three treatments were used - a control in which the plants were exposed to flea beetles throughout the experiment and two treatments in which the plants were protected from beetle damage by covering the rows with perforated polyethylene crop cover for 1 or 2 weeks from the onset of emergence. The experiments were terminated 28-34 days after seeding at the time when beetles were no longer causing appreciable damage. The experiment was repeated four times with seeding dates on 14, 22, 27 and 30 May 1991. Each experiment was conducted with the three treatments distributed as a randomized complete block design with four replicates, and applied to a four-row main plot. The four crops were arranged as single-row sub-plots randomized within each main plot. Each 3 m row was sown with 150 seeds at a depth of 2-3 cm. Rows were separated by 30 cm and plots were separated by 1 m. The assigned four-row main plots were loosely covered with polyethylene sheets from the day plants began to
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emerge, by burying the edges of each sheet 30 cm beyond the edge of the plot. Experiments were terminated by counting the number of plants in each row, cutting them at the soil surface, placing the foliage in plastic bags, and weighing the foliage. The average weight of a plant in each row was calculated by dividing the weight of the foliage in the row by the number of plants. Each experiment was analysed separately, as a split-plot experiment, assigning the variance to a treatment effect (i.e. duration of protection) and a sub-plot effect (i.e. crop type), and interaction. Differences among means were separated using Tukey's multiple range test (SAS Institute Inc., 1990). The analyses were repeated excluding S. alba, which had a response to the treatments differing from that of B. napus. Westar and B. napus hyb. K4-87 were also assessed at Saskatoon, Saskatchewan, as part of a large field test of the performance of oilseed rape cultivars and lines. A randomized complete block design was used with eight blocks - four with 1 g carbofuran-impregnated granules per plot as an in-furrow application and four without insecticide. Each plot consisted of one 6 m row with 60 cm spacings and 300 seeds per row. The experiment was repeated twice with seeding dates on 22 and 30 May 1991. Seedling damage was assessed visually on a scale of 0-10 for one cotyledon on each of ten seedlings in a row (e.g. a rating of 1 = 10% surface damage). Damage was assessed on seedlings at 30 cm intervals in a row when all seedlings had two expanded true leaves. Samples of ten seedlings per row were cut at ground level 4 weeks after seeding and the dry weight of this foliage was determined. At the same time, the number of seedlings in each row was counted. The time to maturity, designated as the date when 50% of seeds had changed colour, was determined for each row by examining rows three times weekly. Rows were harvested individually, cleaned by sieving, and the weight of 100 seeds as well as the yield were determined. Westar and the hybrid were compared separately for the two seeding dates and two insecticide treatments in a two-way analysis of variance followed by an 1.s.d. test to discriminate means.
Results Feeding tests The feeding rate of P. cruciferae on the cotyledons of B. napus hyb. K4-87 and B. napus hyb. K36-87 was significantly lower than that on the cotyledons of B. napus cv. Westar in paired feeding tests in the laboratory (Table 1). The feeding rate on the first true leaves of K36-87 was also significantly lower than that on the first true leaves of Westar. Flea beetles showed no feeding preference for cotyledons or leaves of either hybrid when the hybrids themselves were pair-tested. Beetles also showed no preference for the cotyledons or leaves of the parents P1 or P2 of K36-87 when these were pair-tested with Westar (Table 1). The parents of K4-87 were not tested because we had insufficient seed. In no-choice feeding tests, there were no differences in the percentage of cotyledon damage or seedling dry weight 1 week after the termination of feeding by P.
Hybrid canola damaged by flea beetles: R.P. Bodnaryk et aL Table 1. Feeding rates of flea beetles, P. cruciferae, on the cotyledons or first true leaves of candle, Brassica napus cv. Westar, two hybrid canolas and the parents of one hybrid (Pl and P2) in paired feeding tests a
Paired feeding test
Feeding rate (mm 2 per beetle day-t) + s.d. Cotyledons
(p, paired t test)
First true leaves
(p)
B. napus cv. Westar B. napus hyb. K4-87
0.55 _+ 0.03 0.41 _+ 0.02
<0.01
0.44 + 0.07 0.42 + 0.13
n.s.
B. napus cv. Westar B. napus hyb. K36-87
0.71 +_ 0.11 0.48 +_ 0.08
<0.05
0.40 + 0.07 0.19 +_ 0.12
<0.01
B. napus hyb. K4-87 B. napus hyb. K36-87
0.38 ___0.22 0.50 _+ 0.12
n.s.
0.66 + 0.17 0.51 + 0.10
n.s.
B. napus cv. Westar B. napus hyb. K36-87 P,
0.44 _+ 0.20 0.30 _+ 0.05
n.s.
0.42 + 0.09 0.44 +__0.19
n.s.
B. napus cv. Westar B. napus hyb. K36-87 P2
0.43 +__0.04 0.44 _+ 0.18
n.s.
0.43 _+ 0.1)4 0.44 _+ 0.18
n.s.
"Tests were replicated six times and each test used 12 pairs of plants
Table 2. Cotyledon damage and seedling dry weight of canola, B. napus cv. Westar, and two hybrid candles in no-choice feeding tests at two flea beetle densitiesa
Flea beetles (n)
Experiment
Cotyledons damage, % (n)
Seedling dry wt, g (n)
0
B. napus cv. Westar B. napus hyb. K4-87 B. napus hyb. K36-87
-
18.7 _+ 2.3 (11) 17.8 _+ 2.5 (10) 18.2 _+ 1.3 (10)
10
B. napus cv. Westar B. napus hyb. K4-87 B. napus hyb. K36-87
36.9 + 7.2 (12) 39.5 +_ 8.1 (10) 43.4 + 6.2 (11)
12.2 _+ 2.2 (12) 9.1 _+ 1.9 (10) 10.4 _+ 1.6 (11)
20
B. napus cv. Westar B. napus hyb. K4-87 B. napus hyb. K36-87
67.9 + 6.9 (13) 73.3 + 7.4 (10) 67.0 _+ 7.1 (10)
6.7 + 2.1 (13) 4.5 +__1.7 (10) 7.7 _+ 2.6 (10)
~Beetles, P. striolata, were allowed to feed on l-week-old seedlings for 24 h and then removed for damage estimates. The seedlings were then kept for an additional week at 21°C, cut at their bases and dried to a constant weight. Values are means -+ s.e. No significant differences, A N O V A and Ls.d.
striolata a m o n g W e s t a r , K4-87 and K36-87 at two flea beetle densities ( T a b l e 2). Cotyledon damage
M e c h a n i c a l d a m a g e to the c o t y l e d o n s of W e s t a r and the hybrids c a u s e d reductions in weight o f the plants p r o p o r t i o n a l to the d a m a g e ( F i g u r e 1). T h e relationship for W e s t a r was linear with a slope of --0.011 _+ 0.0008 (i.e. for each 1% d a m a g e , s u b s e q u e n t weight is r e d u c e d by 0 . 9 % ) , and t h e r e f o r e W e s t a r has very little capacity to c o m p e n s a t e in the 8 days following d a m a g e . F o r e x a m p l e , w h e n 75% of the foliage was r e m o v e d , 8 days later seedling weight was 29% of that of und a m a g e d plants. T h e slope and intercept of K4-87 did not differ significantly f r o m that of W e s t a r (data not s h o w n ) . T h e slope o f K36-87 was significantly less than that o f W e s t a r ( F i g u r e 1), indicating that it was less able t h a n W e s t a r to c o m p e n s a t e for the d a m a g e .
o b s e r v e d for S. alba. F e e d i n g d a m a g e was evident on all seedlings in the first seeding. In the later seeding dates, no seedling mortality was attributed to the flea beetles, but feeding d a m a g e was d e t e c t e d in the s e c o n d a n d third seeding dates. V e r y little flea beetle d a m a g e was o b s e r v e d on seedlings in the final seeding. T h e a v e r a g e weights o f seedlings at the end of the e x p e r i m e n t s differed significantly (p < 0.01) a m o n g t r e a t m e n t s ( T a b l e 3). F o r B. n a p u s , plant weight increased with the d u r a t i o n of protection, in the first
1.5
g ~
1.0
7
~
0.5
m
Field t e s t s
In the first seeding at G i e n l e a , M a n i t o b a , the beetles c a u s e d significant (p < 0.05) mortality to seedlings of W e s t a r and the two hybrids, with the result that a b o u t half as m a n y seedlings in the control plots survived as in the c o v e r e d plots (data not shown). N o mortality was
0.0
~ 0
i 2.5
I 50
i 75
FOLIAGE EXCISED, % Figure 1. Effect of mechanical damage to cotyledons of Brassica napus cv. Westar (0) and B. napus hyb. K36-87 (0) on mean seedling fresh weight (+ s.c.) 8 days later
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Hybrid canola damaged by flea beetles: R.P. Bodnaryk et al. o r p r o t e c t i o n f o r 1 w e e k o n l y ) . S. alba a n d B. napus p l a n t s h a d s i m i l a r w e i g h t s in t h e f o u r t h e x p e r i m e n t o r where the plants were protected for 2 weeks, when flea beetle damage was minimal. No significant differences (p > 0 . 0 5 ) w e r e d e t e c t e d b e t w e e n W e s t a r a n d t h e t w o h y b r i d s f o r a n y o f t h e t r e a t m e n t s in a n y o f t h e experiments. At Saskatoon, Saskatchewan, the insecticide treatment reduced flea beetle damage, increased the dry weight of seedlings, speeded maturity and increased y i e l d at b o t h s e e d i n g d a t e s (Table 4). F l e a b e e t l e d a m a g e w a s m o r e s e v e r e in t h e e a r l i e r - s e e d e d e x p e r i m e n t a n d , in this c a s e , t h e i n s e c t i c i d e - t r e a t e d p l a n t s also had higher survival rates. When protected with insecticide, the hybrid had a consistently higher plant stand, higher yield and larger seeds than Westar,
three experiments. For the fourth seeding date, where little flea beetle damage was observed, no consistent treatment e f f e c t w a s d e t e c t e d . I n t h e first t h r e e e x p e r i m e n t s , t h e w e i g h t o f B. napus s e e d l i n g s i n c r e a s e d with later seeding dates, when seedlings were not protected or were protected for 1 week. With 2 weeks' protection, no consistent pattern of weight was detected in s u c c e s s i v e e x p e r i m e n t s . Sinapis alba p l a n t s s h o w e d n o c o n s i s t e n t p a t t e r n o f v a r i a t i o n in w e i g h t a m o n g t r e a t m e n t s o r a m o n g t h e four times of seeding, except that weights were lower t h a n a v e r a g e in t h e s e c o n d s e e d i n g a n d h i g h e r t h a n a v e r a g e f o r t h e f o u r t h s e e d i n g (Table 3). S. elba p l a n t s w e i g h e d s i g n i f i c a n t l y (p < 0 . 0 5 ) m o r e t h a n B. napus p l a n t s w h e n f l e a b e e t l e d a m a g e w a s h e a v y (i.e. in t h e first t h r e e e x p e r i m e n t s w h e r e t h e r e w a s n o p r o t e c t i o n ,
Table 3. Mean fresh weight (g ± s.e.) of 28- to 34-day-old plants of four crops seeded at Glenlea, Manitoba on four dates and protected from damage from flea beetles, P. cruciferae, for 0, 1 or 2 weeks after emergence a
Duration of protection (weeks) Crop
Seeding date (May, 1991)
Sinapis elba cv. Ochre
Brass±canapus cv. Westar
Brass±canapus hyb. K4-87
Brass±canapus hyb. K36-87
0
1
2
14 22 27 30
4.7 2.3 4.9 8.1
± ± ± ±
2.0 0.3 0.3 0.7
3.) 2.4 4.5 9.5
± ± ± ±
2.3 0.3 0.4 1.2
6.5 2.0 6.3 7.0
± ± ± ±
2.6 0.5 1.1 0.6
14 22 27 30
0.2 0.5 1.2 6.6
± ± ± ±
0.1 0.1 2.0 0.5
0.5 1.4 1.9 8.9
± ± ± ±
0.3 0.1 0.4 0.6
4.6 2.0 4.1 6.5
± ± ± ±
2.5 0.4 0.8 1.0
14 22 27 30
0.3 0.5 1.0 4.8
± ± ± ±
0.1 0.3 0.2 0.5
0.3 1.1 1.9 7.0
± ± ± ±
0.1 0.2 0.5 0.7
6.0 2.7 2.9 6.4
± ± ± ±
1.9 0.1 0.8 0.8
14 22 27 30
0.2 0.7 1.2 6.6
± ± ± ±
0.1 0.2 0.2 0.2
0.3 2.1 2.8 7.7
+ ± ± ±
0.1 0.4 0.9 0.4
5.7 2.5 3.4 6.9
± ± ± ±
3.3 0.3 0.8 0.9
"For differences among means, see text Table 4. Response (~ ± s.e.) of Brassica napus cv. Westar and 13. napus hyb. K4-87 to attack by flea beetles, P. striolata, when plants in single-row plots (n = 4) at Saskatoon, Saskatchewan, were treated at seeding with carbofuran granules or no insecticide Seeded 22May Parameter
Crop
Cotyledon damage (%)
K4-87 Westar
Dry wt per row (g)
K4-87 Westar
Plant stand (%)
K4-87 Westar
Maturity (days)
Untreated 75 + 8 86 ± 1
Carbofuran
Seeded 30May Untreated
Carbofuran
24 ± 6 26 ± 4
42 ± 4 44 ± 7
12 ± 5 9 + 4
58 ± 15 56 ± 23
121 ± 28a 42 ± lib
241 + 117 137 ± 19
53 _+ 6 32 ± 5
69 ± 13 54 ± 8
53 ± 4a 31 ± 4b
45 ± 9 33 ± 2
K4-87 Westar
101 ± la 96 ± Ib
97 + la 94 ± 0b
99 ± la 92 ± lb
93 ± I 89 + 3
Yield (g)
K4-87 Westar
486 ± 66 323 ± 9
878 ± 103 799 + 46
467 +__74a 336 ± 72b
655 + 59a 471 ± 69b
100-seed wt (mg)
K4-87 Westar
325 ± 13 302 ± 20
330 ± 10 318 ± 9
337 ± 8 337 ___4
232 ± 9 294 ± 14
7 ± la" 2 ± lb
"Means in the same column and for the same variable, and followed by different letters, differ significantlybased on ANOVA and I.s.d., p < 0.(}5
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Hybrid canola damaged by flea beetles: R.P. Bodnaryk et aL
although in the later-seeded plants the only significant difference was for yield. Westar always matured more quickly than the hybrid. When Westar and K4-87 were not protected by insecticide, the two lines showed no difference in damage or seed weight, although there was a consistent trend for the hybrid to receive less damage and to have larger seeds (Table 4). The hybrid plants also had higher dry weights than Westar and this difference was significant in both experiments. The hybrid had a higher plant stand and yield than Westar, although the differences were significant only for the later seeding date. The proportional difference between K4-87 and Westar (i.e. K4-87/Westar) was higher for plant stand and dry weight when plants were untreated than when they were protected by insecticide. However, no significant difference was detected in yield between K4-87 and Westar in the first experiment when damage was highest, and the difference between the hybrid and Westar (39%) in the second experiment was the same for the untreated and treated plots. Discussion
Of the three modes of insect resistance described for plants (Painter, 1951), it is currently possible to test antixenosis (non-preference) and tolerance but not antibiosis against flea beetles in the laboratory (Bodnaryk and Lamb, 1991). Two hybrid canolas, when tested in this study against the commercial cultivar Westar, showed increased antixenosis to P. cruciferae at the cotyledon stage and one of the hybrids, K36-87, also showed increased antixenosis at the first true leaf stage. It is noteworthy that the parents of K36-87 had the same level of antixenosis as found in Westar, indicating that the increased antixenosis found in the hybrid did not arise through parents with an enhanced level of this trait. Conceivably, increased antixenosis in K36-87 is a product of heterosis, but mechanisms are unknown. Although P. cruciferae preferred to feed on the cotyledons of Westar in paired choice tests with hybrids, differences in the amount of feeding were not evident among the cultivars in the no-choice tests with P. striolata. Choice and no-choice feeding tests commonly give different estimates of antixenosis (Overman and MacCarter, 1972; Goonewardene et al., 1975, 1979; Cantelo and Sanford, 1984), but in some crucifers the same level of antixenosis is seen in both types of feeding test and the mechanism contributes to durable resistance in the field (Bodnaryk and Lamb, 1991). In the case of the canola hybrids, however, there is no evidence of a useful amount of antixenosis under field conditions. Neither hybrid tested in this study showed tolerance to damage that was inflicted at the seedling stage under exacting laboratory conditions designed to detect this type of plant resistance (Bodnaryk and Lamb, 1991), or in the field. In similar tests, a selection of B. rapa cv. Tobin did show significantly higher tolerance than Tobin in the laboratory and in the field, resulting in a measurably improved resistance to flea beetles (Lamb et al., 1993). Sinapis alba revealed the expected resistance to flea beetle damage (Bodnaryk and Lamb,
1991) compared with B. napus, and S. alba seedlings weighed more than B. napus seedlings whenever flea beetle damage was evident. The variation in weight among B. napus plants was also consistent with the observed level of flea beetle damage, decreasing with the duration of protection and in later seedings. At least 2 weeks' protection is required to prevent flea beetle damage to B. napus (Lamb, 1984; Bracken and Bucher, 1986). Flea beetle damage is known to decline as the season progresses and so later-seeded crops often suffer less damage than crops seeded in mid-May (Lamb, 1984, 1988). Under the field conditions that occurred in southern Manitoba in 1991, there was no evidence that either hybrid canola exhibited any tolerance to flea beetle damage. The vigorous growth and high yields displayed by hybrid canolas have led to the expectation that hybrids might be less susceptible to the effects of early flea beetle damage than are inbred cultivars. The two hybrids did show antixenosis in one of the laboratory tests, but not tolerance, and neither mode of resistance was sufficient to provide protection in the field. The results do not support the hypothesis that these hybrids will contribute an agronomically useful level of resistance against flea beetle damage under commercial growing conditions. The possibility remains, however, that hybrids may be less affected by other canola pests, especially lepidopteran defoliators, that damage the crop at a later stage of maturity when hybrids might be more tolerant.
Notes
Winnipeg Research Station, Contribution No. 1520.
References
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(1993) A selectionof oilseed rape, Brassica rapa, with resistanceto flea beetles, Phyllotreta cruciferae (Coleoptera: Chrysomelidae). Can. Entomol. 125, 703-713 Overman, J.L. and MacCarter, L.E. (1972) Evaluatingseedlingsof
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Hybrid canola damaged by flea beetles: R.P. Bodnaryk et al. cantaloupe for varietal non-preference type resistance to Diabrotica spp. J. Econ. Entomol. 65, 1140-1144 P~Later, R.H. (1951) Insect Resistance in Crop Plants, University of Kansas, Lawrence, Kansas SAS Institute Inc. (1990) SAS/STA T User's Guide, Version 6 Edition, SAS Institute Inc., Cary, NC, 1686 pp
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Sernyk, J.L. and Stefansson, B.R. (1983) Heterosis in summer rape (Brassica napus L.). Can. J. Plant Sci. 63, 407-413 Received 5 November 1992 Revised 1 September 1993 Accepted 26 November 1993
Agriculture Canada, Research Station, Winnipeg, Manitoba, R3T 2M9 Canada and tResearch Station, Saskatoon, Saskatchewan, S7N OX2 Canada
The hybrid canolas, Brassica napus hyb. K4-87 and B. napus hyb. K36-87, were tested for two modes of resistance, antixenosis and tolerance, against flea beetles, Phyllotreta spp. In feeding tests in which choices were offered between the hybrids and a standard inbred canola cultivar, B. napus cv. Westar, the cotyledons of the hybrid K4-87 and the cotyledons and first true leaves of the hybrid K36-87 showed a higher level of antixenosis. As neither parent of K36-87 had a higher level of antixenosis, this trait in the hybrid may be due to heterosis. In no-choice feeding tests, however, cotyledon damage and seedling biomass 1 week after feeding damage did not differ among K4-87, K36-87, and Westar. The hybrids showed no tolerance to feeding damage either in the laboratory or in the field. On the basis of plant biomass and yield data from Manitoba and Saskatchewan, the hybrids do not exhibit sufficient resistance to provide protection in the field. These results do not support the hypothesis that hybrid canola will contribute an agronomically useful level of resistance against flea beetle damage under commercial growing conditions. Keywords: Brassica napus; Phyllotreta; resistance
Apart from the prospect of significantly increased seed yields in hybrid canola, Brassica napus L. (Sernyk and Stefansson, 1983), the vigorous growth displayed by hybrids has aroused expectations that hybrids might be less susceptible to the effects of insect pests. Flea beetles, Phyllotreta spp., are major pests of canola grown in western Canada and must be controlled annually with an insecticide, especially during the first 2-3 weeks of postemergence growth (Lamb, 1984, 1988; Bracken and Bucher, 1986). Alternate control methods, including host-plant resistance, are being sought. This paper describes laboratory and field experiments on the early growth of two hybrid canolas exposed to flea beetle damage to detect possible enhancement of two resistance mechanisms, antixenosis (non-preference) and tolerance, in the hybrids. Materials and methods Plants
Registered seed of canola, Brassica napus cv. Westar and mustard, Sinapis alba cv. Ochre, were obtained from breeding programmes at the Agriculture Canada Research Station, Saskatoon, Saskatchewan. Hybrid canolas B napus hyb. K4-87 and B. napus hyb. K36-87 and the parents of B. napus hyb. K36-87, coded P1 and P2, were provided by Zenon Lisieczko, King Agro, Research and Distribution Centre, Listowel, Ontario, N4W 3G7, Canada. *To whom correspondence should be addressed
For laboratory tests, seeds were planted in Vermiculite in 175 ml styrofoam coffee cups with a 0.5 cm hole punched in the bottom and subirrigated with Hoagland's nutrient medium. The seedlings were held 10--12 cm under wide-spectrum (Gro-Lux) artificial lighting (--200 I~E s-t m-2 of photosynthetically active radiation) at 20 + I°C, 60-70% relative humidity and 18:6 L:D photoperiod. Insects
Field-collected flea beetles, Phyllotreta spp., were used in the laboratory experiments. In eastern Manitoba, the overwhelmingly abundant species is P. cruciferae and in central Saskatchewan it is P. striolata.
Feeding tests
Phyllotreta cruciferae were collected in Winnipeg in late May and early June from volunteer rape, B. napus cv. Westar, in the field and kept for 1 week in the laboratory in screened cages (50 X 50 X 50 cm). The beetles were given water through a cotton dental roll held in a flask and were fed on cabbage leaves, B. oleraceae, obtained from a market. The feeding tests were conducted in the laboratory under conditions given above in screened cages (30 x 30 x 30 cm) with clear polystyrene tops. Pairs of the cultivar Westar, the hybrids and the parents of one hybrid, as given in Table 1, were placed in the cages with known numbers of flea beetles. For feeding tests
0261-2194/94/07/0513-06 (~ 1994 Butterworth-Heinemann Ltd (~ 1994 Department of Agriculture, Governmentof Canada
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Hybrid canola damaged by flea beetles: R.P. Bodnaryk et al. at the cotyledon stage, 12 6-day-old seedlings of each pair were placed randomly in each cage (replicate) with 120 flea beetles and the tests were replicated six times. For feeding tests at the first true leaf stage, pairs of 15day-old plants, the cotyledons of which had been removed with sharp scissors on day 14, were placed in cages with 20 flea beetles and tests were replicated nine times. After 24 h, beetles were removed from all cages and cotyledon or leaf area consumed was measured with an optical grid fitted to a Wild M8 dissecting microscope. Non-choice feeding tests were also conducted with Westar and the hybrids at the cotyledon stage using flea beetles, P. striolata, collected in Saskatoon. Two beetle densities, 10 and 20 beetles per seedling, were used. In these tests, cotyledon damage was estimated by a 0--10 visual rating system in which 0 represents no feeding and 10 represents 100% damage. The dry weights of individual seedlings were obtained 1 week after the termination of the feeding tests and from a control group with no beetles. Tolerance to cotyledon damage Artificial damage was inflicted on 6-day-old seedlings of B. napus cv. Westar, B. napus hyb. K4-87 and B. napus hyb. K36-87 with scissors by removing 20, 50 or 75% of the total cotyledon area. The fresh weight of the above-ground portion of each seedling was obtained on day 15. Forty-eight seedlings were used for each damage level. The weight of each plant was regressed against the amount of tissue removed and the resulting linear estimates of the slopes of the lines were used to assess the relative tolerance of Westar and the hybrids to damage. Field study In one series of experiments in Olenlea, Manitoba, Westar and the two hybrids, together with S. alba cv. Ochre, were exposed to natural infestations of flea beetles for various periods during their development, and then weighed to determine their ability to gain weight when damaged by flea beetles. Sinapis alba was included because it shows a high level of tolerance (Bodnaryk and Lamb, 1991). Three treatments were used - a control in which the plants were exposed to flea beetles throughout the experiment and two treatments in which the plants were protected from beetle damage by covering the rows with perforated polyethylene crop cover for 1 or 2 weeks from the onset of emergence. The experiments were terminated 28-34 days after seeding at the time when beetles were no longer causing appreciable damage. The experiment was repeated four times with seeding dates on 14, 22, 27 and 30 May 1991. Each experiment was conducted with the three treatments distributed as a randomized complete block design with four replicates, and applied to a four-row main plot. The four crops were arranged as single-row sub-plots randomized within each main plot. Each 3 m row was sown with 150 seeds at a depth of 2-3 cm. Rows were separated by 30 cm and plots were separated by 1 m. The assigned four-row main plots were loosely covered with polyethylene sheets from the day plants began to
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emerge, by burying the edges of each sheet 30 cm beyond the edge of the plot. Experiments were terminated by counting the number of plants in each row, cutting them at the soil surface, placing the foliage in plastic bags, and weighing the foliage. The average weight of a plant in each row was calculated by dividing the weight of the foliage in the row by the number of plants. Each experiment was analysed separately, as a split-plot experiment, assigning the variance to a treatment effect (i.e. duration of protection) and a sub-plot effect (i.e. crop type), and interaction. Differences among means were separated using Tukey's multiple range test (SAS Institute Inc., 1990). The analyses were repeated excluding S. alba, which had a response to the treatments differing from that of B. napus. Westar and B. napus hyb. K4-87 were also assessed at Saskatoon, Saskatchewan, as part of a large field test of the performance of oilseed rape cultivars and lines. A randomized complete block design was used with eight blocks - four with 1 g carbofuran-impregnated granules per plot as an in-furrow application and four without insecticide. Each plot consisted of one 6 m row with 60 cm spacings and 300 seeds per row. The experiment was repeated twice with seeding dates on 22 and 30 May 1991. Seedling damage was assessed visually on a scale of 0-10 for one cotyledon on each of ten seedlings in a row (e.g. a rating of 1 = 10% surface damage). Damage was assessed on seedlings at 30 cm intervals in a row when all seedlings had two expanded true leaves. Samples of ten seedlings per row were cut at ground level 4 weeks after seeding and the dry weight of this foliage was determined. At the same time, the number of seedlings in each row was counted. The time to maturity, designated as the date when 50% of seeds had changed colour, was determined for each row by examining rows three times weekly. Rows were harvested individually, cleaned by sieving, and the weight of 100 seeds as well as the yield were determined. Westar and the hybrid were compared separately for the two seeding dates and two insecticide treatments in a two-way analysis of variance followed by an 1.s.d. test to discriminate means.
Results Feeding tests The feeding rate of P. cruciferae on the cotyledons of B. napus hyb. K4-87 and B. napus hyb. K36-87 was significantly lower than that on the cotyledons of B. napus cv. Westar in paired feeding tests in the laboratory (Table 1). The feeding rate on the first true leaves of K36-87 was also significantly lower than that on the first true leaves of Westar. Flea beetles showed no feeding preference for cotyledons or leaves of either hybrid when the hybrids themselves were pair-tested. Beetles also showed no preference for the cotyledons or leaves of the parents P1 or P2 of K36-87 when these were pair-tested with Westar (Table 1). The parents of K4-87 were not tested because we had insufficient seed. In no-choice feeding tests, there were no differences in the percentage of cotyledon damage or seedling dry weight 1 week after the termination of feeding by P.
Hybrid canola damaged by flea beetles: R.P. Bodnaryk et aL Table 1. Feeding rates of flea beetles, P. cruciferae, on the cotyledons or first true leaves of candle, Brassica napus cv. Westar, two hybrid canolas and the parents of one hybrid (Pl and P2) in paired feeding tests a
Paired feeding test
Feeding rate (mm 2 per beetle day-t) + s.d. Cotyledons
(p, paired t test)
First true leaves
(p)
B. napus cv. Westar B. napus hyb. K4-87
0.55 _+ 0.03 0.41 _+ 0.02
<0.01
0.44 + 0.07 0.42 + 0.13
n.s.
B. napus cv. Westar B. napus hyb. K36-87
0.71 +_ 0.11 0.48 +_ 0.08
<0.05
0.40 + 0.07 0.19 +_ 0.12
<0.01
B. napus hyb. K4-87 B. napus hyb. K36-87
0.38 ___0.22 0.50 _+ 0.12
n.s.
0.66 + 0.17 0.51 + 0.10
n.s.
B. napus cv. Westar B. napus hyb. K36-87 P,
0.44 _+ 0.20 0.30 _+ 0.05
n.s.
0.42 + 0.09 0.44 +__0.19
n.s.
B. napus cv. Westar B. napus hyb. K36-87 P2
0.43 +__0.04 0.44 _+ 0.18
n.s.
0.43 _+ 0.1)4 0.44 _+ 0.18
n.s.
"Tests were replicated six times and each test used 12 pairs of plants
Table 2. Cotyledon damage and seedling dry weight of canola, B. napus cv. Westar, and two hybrid candles in no-choice feeding tests at two flea beetle densitiesa
Flea beetles (n)
Experiment
Cotyledons damage, % (n)
Seedling dry wt, g (n)
0
B. napus cv. Westar B. napus hyb. K4-87 B. napus hyb. K36-87
-
18.7 _+ 2.3 (11) 17.8 _+ 2.5 (10) 18.2 _+ 1.3 (10)
10
B. napus cv. Westar B. napus hyb. K4-87 B. napus hyb. K36-87
36.9 + 7.2 (12) 39.5 +_ 8.1 (10) 43.4 + 6.2 (11)
12.2 _+ 2.2 (12) 9.1 _+ 1.9 (10) 10.4 _+ 1.6 (11)
20
B. napus cv. Westar B. napus hyb. K4-87 B. napus hyb. K36-87
67.9 + 6.9 (13) 73.3 + 7.4 (10) 67.0 _+ 7.1 (10)
6.7 + 2.1 (13) 4.5 +__1.7 (10) 7.7 _+ 2.6 (10)
~Beetles, P. striolata, were allowed to feed on l-week-old seedlings for 24 h and then removed for damage estimates. The seedlings were then kept for an additional week at 21°C, cut at their bases and dried to a constant weight. Values are means -+ s.e. No significant differences, A N O V A and Ls.d.
striolata a m o n g W e s t a r , K4-87 and K36-87 at two flea beetle densities ( T a b l e 2). Cotyledon damage
M e c h a n i c a l d a m a g e to the c o t y l e d o n s of W e s t a r and the hybrids c a u s e d reductions in weight o f the plants p r o p o r t i o n a l to the d a m a g e ( F i g u r e 1). T h e relationship for W e s t a r was linear with a slope of --0.011 _+ 0.0008 (i.e. for each 1% d a m a g e , s u b s e q u e n t weight is r e d u c e d by 0 . 9 % ) , and t h e r e f o r e W e s t a r has very little capacity to c o m p e n s a t e in the 8 days following d a m a g e . F o r e x a m p l e , w h e n 75% of the foliage was r e m o v e d , 8 days later seedling weight was 29% of that of und a m a g e d plants. T h e slope and intercept of K4-87 did not differ significantly f r o m that of W e s t a r (data not s h o w n ) . T h e slope o f K36-87 was significantly less than that o f W e s t a r ( F i g u r e 1), indicating that it was less able t h a n W e s t a r to c o m p e n s a t e for the d a m a g e .
o b s e r v e d for S. alba. F e e d i n g d a m a g e was evident on all seedlings in the first seeding. In the later seeding dates, no seedling mortality was attributed to the flea beetles, but feeding d a m a g e was d e t e c t e d in the s e c o n d a n d third seeding dates. V e r y little flea beetle d a m a g e was o b s e r v e d on seedlings in the final seeding. T h e a v e r a g e weights o f seedlings at the end of the e x p e r i m e n t s differed significantly (p < 0.01) a m o n g t r e a t m e n t s ( T a b l e 3). F o r B. n a p u s , plant weight increased with the d u r a t i o n of protection, in the first
1.5
g ~
1.0
7
~
0.5
m
Field t e s t s
In the first seeding at G i e n l e a , M a n i t o b a , the beetles c a u s e d significant (p < 0.05) mortality to seedlings of W e s t a r and the two hybrids, with the result that a b o u t half as m a n y seedlings in the control plots survived as in the c o v e r e d plots (data not shown). N o mortality was
0.0
~ 0
i 2.5
I 50
i 75
FOLIAGE EXCISED, % Figure 1. Effect of mechanical damage to cotyledons of Brassica napus cv. Westar (0) and B. napus hyb. K36-87 (0) on mean seedling fresh weight (+ s.c.) 8 days later
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Hybrid canola damaged by flea beetles: R.P. Bodnaryk et al. o r p r o t e c t i o n f o r 1 w e e k o n l y ) . S. alba a n d B. napus p l a n t s h a d s i m i l a r w e i g h t s in t h e f o u r t h e x p e r i m e n t o r where the plants were protected for 2 weeks, when flea beetle damage was minimal. No significant differences (p > 0 . 0 5 ) w e r e d e t e c t e d b e t w e e n W e s t a r a n d t h e t w o h y b r i d s f o r a n y o f t h e t r e a t m e n t s in a n y o f t h e experiments. At Saskatoon, Saskatchewan, the insecticide treatment reduced flea beetle damage, increased the dry weight of seedlings, speeded maturity and increased y i e l d at b o t h s e e d i n g d a t e s (Table 4). F l e a b e e t l e d a m a g e w a s m o r e s e v e r e in t h e e a r l i e r - s e e d e d e x p e r i m e n t a n d , in this c a s e , t h e i n s e c t i c i d e - t r e a t e d p l a n t s also had higher survival rates. When protected with insecticide, the hybrid had a consistently higher plant stand, higher yield and larger seeds than Westar,
three experiments. For the fourth seeding date, where little flea beetle damage was observed, no consistent treatment e f f e c t w a s d e t e c t e d . I n t h e first t h r e e e x p e r i m e n t s , t h e w e i g h t o f B. napus s e e d l i n g s i n c r e a s e d with later seeding dates, when seedlings were not protected or were protected for 1 week. With 2 weeks' protection, no consistent pattern of weight was detected in s u c c e s s i v e e x p e r i m e n t s . Sinapis alba p l a n t s s h o w e d n o c o n s i s t e n t p a t t e r n o f v a r i a t i o n in w e i g h t a m o n g t r e a t m e n t s o r a m o n g t h e four times of seeding, except that weights were lower t h a n a v e r a g e in t h e s e c o n d s e e d i n g a n d h i g h e r t h a n a v e r a g e f o r t h e f o u r t h s e e d i n g (Table 3). S. elba p l a n t s w e i g h e d s i g n i f i c a n t l y (p < 0 . 0 5 ) m o r e t h a n B. napus p l a n t s w h e n f l e a b e e t l e d a m a g e w a s h e a v y (i.e. in t h e first t h r e e e x p e r i m e n t s w h e r e t h e r e w a s n o p r o t e c t i o n ,
Table 3. Mean fresh weight (g ± s.e.) of 28- to 34-day-old plants of four crops seeded at Glenlea, Manitoba on four dates and protected from damage from flea beetles, P. cruciferae, for 0, 1 or 2 weeks after emergence a
Duration of protection (weeks) Crop
Seeding date (May, 1991)
Sinapis elba cv. Ochre
Brass±canapus cv. Westar
Brass±canapus hyb. K4-87
Brass±canapus hyb. K36-87
0
1
2
14 22 27 30
4.7 2.3 4.9 8.1
± ± ± ±
2.0 0.3 0.3 0.7
3.) 2.4 4.5 9.5
± ± ± ±
2.3 0.3 0.4 1.2
6.5 2.0 6.3 7.0
± ± ± ±
2.6 0.5 1.1 0.6
14 22 27 30
0.2 0.5 1.2 6.6
± ± ± ±
0.1 0.1 2.0 0.5
0.5 1.4 1.9 8.9
± ± ± ±
0.3 0.1 0.4 0.6
4.6 2.0 4.1 6.5
± ± ± ±
2.5 0.4 0.8 1.0
14 22 27 30
0.3 0.5 1.0 4.8
± ± ± ±
0.1 0.3 0.2 0.5
0.3 1.1 1.9 7.0
± ± ± ±
0.1 0.2 0.5 0.7
6.0 2.7 2.9 6.4
± ± ± ±
1.9 0.1 0.8 0.8
14 22 27 30
0.2 0.7 1.2 6.6
± ± ± ±
0.1 0.2 0.2 0.2
0.3 2.1 2.8 7.7
+ ± ± ±
0.1 0.4 0.9 0.4
5.7 2.5 3.4 6.9
± ± ± ±
3.3 0.3 0.8 0.9
"For differences among means, see text Table 4. Response (~ ± s.e.) of Brassica napus cv. Westar and 13. napus hyb. K4-87 to attack by flea beetles, P. striolata, when plants in single-row plots (n = 4) at Saskatoon, Saskatchewan, were treated at seeding with carbofuran granules or no insecticide Seeded 22May Parameter
Crop
Cotyledon damage (%)
K4-87 Westar
Dry wt per row (g)
K4-87 Westar
Plant stand (%)
K4-87 Westar
Maturity (days)
Untreated 75 + 8 86 ± 1
Carbofuran
Seeded 30May Untreated
Carbofuran
24 ± 6 26 ± 4
42 ± 4 44 ± 7
12 ± 5 9 + 4
58 ± 15 56 ± 23
121 ± 28a 42 ± lib
241 + 117 137 ± 19
53 _+ 6 32 ± 5
69 ± 13 54 ± 8
53 ± 4a 31 ± 4b
45 ± 9 33 ± 2
K4-87 Westar
101 ± la 96 ± Ib
97 + la 94 ± 0b
99 ± la 92 ± lb
93 ± I 89 + 3
Yield (g)
K4-87 Westar
486 ± 66 323 ± 9
878 ± 103 799 + 46
467 +__74a 336 ± 72b
655 + 59a 471 ± 69b
100-seed wt (mg)
K4-87 Westar
325 ± 13 302 ± 20
330 ± 10 318 ± 9
337 ± 8 337 ___4
232 ± 9 294 ± 14
7 ± la" 2 ± lb
"Means in the same column and for the same variable, and followed by different letters, differ significantlybased on ANOVA and I.s.d., p < 0.(}5
516
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Hybrid canola damaged by flea beetles: R.P. Bodnaryk et aL
although in the later-seeded plants the only significant difference was for yield. Westar always matured more quickly than the hybrid. When Westar and K4-87 were not protected by insecticide, the two lines showed no difference in damage or seed weight, although there was a consistent trend for the hybrid to receive less damage and to have larger seeds (Table 4). The hybrid plants also had higher dry weights than Westar and this difference was significant in both experiments. The hybrid had a higher plant stand and yield than Westar, although the differences were significant only for the later seeding date. The proportional difference between K4-87 and Westar (i.e. K4-87/Westar) was higher for plant stand and dry weight when plants were untreated than when they were protected by insecticide. However, no significant difference was detected in yield between K4-87 and Westar in the first experiment when damage was highest, and the difference between the hybrid and Westar (39%) in the second experiment was the same for the untreated and treated plots. Discussion
Of the three modes of insect resistance described for plants (Painter, 1951), it is currently possible to test antixenosis (non-preference) and tolerance but not antibiosis against flea beetles in the laboratory (Bodnaryk and Lamb, 1991). Two hybrid canolas, when tested in this study against the commercial cultivar Westar, showed increased antixenosis to P. cruciferae at the cotyledon stage and one of the hybrids, K36-87, also showed increased antixenosis at the first true leaf stage. It is noteworthy that the parents of K36-87 had the same level of antixenosis as found in Westar, indicating that the increased antixenosis found in the hybrid did not arise through parents with an enhanced level of this trait. Conceivably, increased antixenosis in K36-87 is a product of heterosis, but mechanisms are unknown. Although P. cruciferae preferred to feed on the cotyledons of Westar in paired choice tests with hybrids, differences in the amount of feeding were not evident among the cultivars in the no-choice tests with P. striolata. Choice and no-choice feeding tests commonly give different estimates of antixenosis (Overman and MacCarter, 1972; Goonewardene et al., 1975, 1979; Cantelo and Sanford, 1984), but in some crucifers the same level of antixenosis is seen in both types of feeding test and the mechanism contributes to durable resistance in the field (Bodnaryk and Lamb, 1991). In the case of the canola hybrids, however, there is no evidence of a useful amount of antixenosis under field conditions. Neither hybrid tested in this study showed tolerance to damage that was inflicted at the seedling stage under exacting laboratory conditions designed to detect this type of plant resistance (Bodnaryk and Lamb, 1991), or in the field. In similar tests, a selection of B. rapa cv. Tobin did show significantly higher tolerance than Tobin in the laboratory and in the field, resulting in a measurably improved resistance to flea beetles (Lamb et al., 1993). Sinapis alba revealed the expected resistance to flea beetle damage (Bodnaryk and Lamb,
1991) compared with B. napus, and S. alba seedlings weighed more than B. napus seedlings whenever flea beetle damage was evident. The variation in weight among B. napus plants was also consistent with the observed level of flea beetle damage, decreasing with the duration of protection and in later seedings. At least 2 weeks' protection is required to prevent flea beetle damage to B. napus (Lamb, 1984; Bracken and Bucher, 1986). Flea beetle damage is known to decline as the season progresses and so later-seeded crops often suffer less damage than crops seeded in mid-May (Lamb, 1984, 1988). Under the field conditions that occurred in southern Manitoba in 1991, there was no evidence that either hybrid canola exhibited any tolerance to flea beetle damage. The vigorous growth and high yields displayed by hybrid canolas have led to the expectation that hybrids might be less susceptible to the effects of early flea beetle damage than are inbred cultivars. The two hybrids did show antixenosis in one of the laboratory tests, but not tolerance, and neither mode of resistance was sufficient to provide protection in the field. The results do not support the hypothesis that these hybrids will contribute an agronomically useful level of resistance against flea beetle damage under commercial growing conditions. The possibility remains, however, that hybrids may be less affected by other canola pests, especially lepidopteran defoliators, that damage the crop at a later stage of maturity when hybrids might be more tolerant.
Notes
Winnipeg Research Station, Contribution No. 1520.
References
Bodnaryk,R.P. and Lamb, R.J. (1991) Mechanismsof resistance to the flea beetle, Phyllotreta cruciferae (Goeze), in mustardseedlings, Sinapis alba L. Can. J. Plant Sci. 71, 13-20 Bracken, G.K. and Bueber, G.E, (1986) Yield losses in canola caused by adult and larval flea beetles, Phyllotreta cruciferae (Coleoptera: Chrysomelidae).Can. Entomol. il8, 319-324 Cantelo, W.W. and Sanford,L.L. (1984) Insect populationresponse to mixed and uniform plantings of resistant and susceptible plant material. Environ. Entomol. 13, 1443-1445 Goonewardene,
H.F.,
Kwolek,
W.F.,
Dolphin,
D.E.
and
Williams, E.B. (1975) Evaluatingresistance of apple fruits to four insect pests. Hort. Sci. 10, 393-394 Goonewardene,
H.F.,
Kwolek,
W.F.,
Mouzin,
T.E.
and
Williams, E.B. (1979) A 'no-choice' study for evaluatingresistance of apple fruits to four insect pests. Hort. Sci. 14, 165-166 Lamb, R.H. (1984) Effects of flea beetle PhyUotreta spp. (Chrysomelidae:Coleoptera),on the survival,growth,seedyieldand quality of canola, rape and yellow mustard. Can. Entomol. 116, 269-280 Lamb, R.J. (1988) Assessingthe susceptibilityof cruciferseedlings to flea beetle (Phyilotreta slap.) damage. Can. J. Plant Sci. 611,85-93 Lamb, R.J., Palaniswamy, P., Pivnick, K.A. and Smith, M.A.H.
(1993) A selectionof oilseed rape, Brassica rapa, with resistanceto flea beetles, Phyllotreta cruciferae (Coleoptera: Chrysomelidae). Can. Entomol. 125, 703-713 Overman, J.L. and MacCarter, L.E. (1972) Evaluatingseedlingsof
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Hybrid canola damaged by flea beetles: R.P. Bodnaryk et al. cantaloupe for varietal non-preference type resistance to Diabrotica spp. J. Econ. Entomol. 65, 1140-1144 P~Later, R.H. (1951) Insect Resistance in Crop Plants, University of Kansas, Lawrence, Kansas SAS Institute Inc. (1990) SAS/STA T User's Guide, Version 6 Edition, SAS Institute Inc., Cary, NC, 1686 pp
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Sernyk, J.L. and Stefansson, B.R. (1983) Heterosis in summer rape (Brassica napus L.). Can. J. Plant Sci. 63, 407-413 Received 5 November 1992 Revised 1 September 1993 Accepted 26 November 1993