ABSTRACT

A three-year field study was conducted on four cultivars of Brassica napus to investigate the influence of seed size on agronomic performance and tolerance of seedlings to feeding damage from crucifer flea beetles, Phyllotreta cruciferae. Cultivars included a hybrid, doubled haploid and two open-pollinated types. Seedlots were sieved through graded screens to obtain samples of small seed (1.4-1.6mm diameter), medium seed (1.6-1.8mm) and large seed (1.8-2.0mm). Sized seeds were planted in six-row plots without chemical protectants at 200 seeds per 6.0m row. Flea beetle damage to cotyledon surfaces 21 days after planting (DAP) varied depending on the year and size of seed planted. Damage declined as seed size increased. Agronomic performance varied depending on the year, cultivar and seed size. Seedling counts, seedling fresh weight, biomass (21 DAP) and seed yield increased in relation to size of seed planted. Compared to small seed, large seed improved seedling establishment, fresh weight, biomass and seed yield 2.0, 1.5, 3.0 and 1.5 times, respectively. In most instances, year by size interactions were significant whereas cultivar by year, size by cultivar and year by size by cultivar interactions were not significant. Findings indicate that in hybrid, doubled haploid and open-pollinated breeding types of B. napus, plants grown from large seed are agronomically superior and more tolerant to flea beetle damage than plants grown from small seed or medium seed.

KEYWORDS: flea beetles, feeding damage, seedling establishment, biomass, yield

INTRODUCTION

The crucifer flea beetle, Phyllotreta cruciferae (Goeze), is a serious insect pest of oilseed Brassica crops in most regions of western Canada (Lamb 1989). Adult beetles feed on the cotyledons and young true leaves causing seedling mortality, reduced seedling growth, delayed maturity and lower seed yield. Cultural practices and biological control agents provide limited regulation of flea beetle populations so producers are reliant on several methods of chemical control including seed treatments, granular insecticides and insecticidal sprays. These treatments are not only costly economically but also pose potential hazards to the applicator and beneficial non-target species. Clearly, alternate management strategies that reduce reliance on chemical control would be beneficial economically and environmentally.

Vigourous seedling growth is a desirable attribute in regions where abiotic and biotic factors limit seedling emergence and seedling growth. In some Brassica species, seedling establishment and growth are influenced by the size of seed planted (Major 1977; Mendham et al. 1981; Heather and Sieczka 1991; Shanmuganathan and Benjamin 1992). Studies have shown that large seeds provide better establishment, larger plants, more vigourous plants and higher biomass than small seed. Laboratory studies also suggest that seedlings grown from large seed suffer less flea beetle damage and are more tolerant to flea beetle damage than seedlings grown from small seed (Bodnaryk and Lamb 1991). The objectives of this study were to investigate the influence of seed size on flea beetle damage and agronomic performance of open-pollinated, doubled haploid and hybrid cultivars of B. napus.

MATERIALS AND METHODS

Replicated field tests were conducted on spring-seeded canola at Saskatoon in 1995, 1996 and 1997. Test entries included two open-pollinated cultivars (Profit, AC Elect), a doubled haploid cultivar (Cyclone) and hybrid (AC H102). Seedlots were sieved through graded screens to obtain samples of small seed (1.4-1.6mm diameter), medium seed (1.6-1.8mm) and large seed (1.8-2.0mm). Sized seeds were planted without chemical protectants in six-row plots at 200 seeds per 6.0m row and 0.30m row-spacing. Tests were replicated four times and employed a randomized split-plot design with cultivars as main plots and seed size as subplots. Depending on the year and moisture conditions, seeds were planted at 1.5-2.5cm depths with a six-cone, double-disc drill during the third week of May.

Flea beetle damage to the cotyledons of 20 seedlings in each subplot was assessed 21 days after planting (DAP) using a 10-point rating system. Visual ratings were based on the percentage of cotyledon surface eaten by flea beetles. Seedlings along the centre rows of each subplot were counted 21 DAP. Ten seedlings were collected from the outer two rows of each subplot and transported on crushed ice to the laboratory. After the roots were removed, samples were cleaned and weighed. A computer-based imaging system (Decagon Devices, Inc.) was used to measure the surface area of the cotyledons and true leaves. Biomass was calculated from seedling counts and seedling fresh weights. The four centre rows of each subplot were harvested at maturity with a small-plot combine to determine seed yield.

RESULTS AND DISCUSSION

Flea beetle damage and agronomic attributes of spring-seeded canola varied significantly from year to year (Table 1). Damage was higher in 1996 than in 1995 or 1997, averaging 86%, 57% and 56%, respectively. As a result of higher damage, seedling counts, seedling fresh weight, cotyledon-leaf area, biomass and seed yield were lower in 1996 than in 1995 or 1997.

Table 1. Flea beetle damage and agronomic attributes of canola in 1995, 1996 and 1997 ^a

Year	Flea beetle damage (%)	Seedlings per row	Cotyledon-leaf area (mm²)	Fresh weight (mg)	Biomass (g/row)	Seed yield (g/m²)
1995	57a	38.3a	177.3b	108.1c	4.3b	228.2b
1996	86b	32.0a	101.7a	59.4a	2.1a	150.6a
1997	56a	51.0b	170.3b	85.3b	4.6b	174.7a

^a means in each column followed by the same letter are not significantly different (LSD, p=0.05)

Flea beetle damage and agronomic performance differed in the four cultivars (Table 2). Damage ranged from 62% in Cyclone to 69% in AC H102. Seedling counts were lower in AC H102 than Cyclone, AC Elect and Profit. Fresh weights differed in the four cultivars ranging from 77.2mg in Profit to 90.0mg in AC H102. Cotyledon-leaf areas and biomass were similar in all entries. Seed yields were higher in Cyclone and Profit than AC Elect and AC H102.

Table 2. Flea beetle damage and agronomic attributes of cultivars in 1995-1997^a

Cultivars	Flea beetle damage (%)	Seedlings per row	Cotyledon-leaf area (mm²)	Fresh weight (mg)	Biomass (g/row)	Seed yield (g/m²)
AC Elect	68b	41.3b	155.0a	85.5ab	3.8a	174.3a
Profit	66ab	44.0b	147.2a	77.2a	3.6a	191.0b
Cyclone	62a	44.0b	138.9a	84.4ab	3.9a	201.6b
AC H102	69b	32.3a	158.0a	90.0b	3.3a	171.1a

^a means in each column followed by the same letter are not significantly different (LSD, p=0.05)

Flea beetle damage and agronomic attributes of spring-seeded canola varied markedly depending on the size of seed planted (Table 3). Damage to the cotyledons declined as seed size increased. Damage was highest in seedlings grown from small seed and lowest in seedlings grown from large seed. Bodnaryk and Lamb (1991) had similar results in choice and no-choice feeding tests in the laboratory. In the current study, year by size, size by cultivar and year by size by cultivar interactions on damage were not significant (p=0.48, 0.78 and 0.24, respectively). Interactions indicated that differences in flea beetle damage in seedlings grown from small, medium and large seed were consistent from year to year and from cultivar to cultivar. On average, damage was 8% lower in seedlings grown from large seed than seedlings grown from small seed.

As described in other Brassica species (Heather and Sieczka 1991; Shanmuganathan and Benjamin 1992), seedling establishment, seedling growth and yield improved as seed size increased. The positive association between seed size and seed yield in B. napus contradicts previous findings (Major 1977; Mendham et al. 1981). Compared to small seed, medium-sized seed improved seeding counts, cotyledon-leaf areas, seedling fresh weight, biomass and seed yield 1.5, 1.2, 1.2, 1.7 and 1.2 times, respectively. Corresponding improvements with large seed were 2.0, 1.5, 1.6, 3.0 and 1.5 times, respectively. Results indicated that seedlings grown from medium or large seed are agronomically superior and more tolerant to flea beetle damage than seedlings grown from small seed. Year by size interactions on seedling counts, biomass and seed yield were highly significant (p=0.0001, 0.002 and 0.0001, respectively). Differences in agronomic attributes of plants grown from small, medium and large seed were greater in 1996 when flea beetle damage averaged 86% than in 1995 when flea beetle damage averaged 57%. Differences in agronomic performance of small, medium and large seed were also pronounced in 1997 when dry conditions prevailed during seedling establishment and early seedling growth. Size by cultivar and year by size by cultivar interactions on seedling counts, cotyledon-leaf area, seedling fresh weight and seed yield were not significant (p=0.05-0.81). Interactions indicated that seed-size influences on agronomic performance were similar in open-pollinated, doubled haploid and hybrid cultivars. In cultivars of each varietal type, seedlings grown from large seed were agronomically superior and more tolerant to flea beetle damage than seedlings grown from small seed or medium seed.

Table 3. Flea beetle damage and agronomic attributes of canola grown from small, medium and large seed in 1995-1997

Seed size^a	Flea beetle damage (%)	Seedlings per row	Cotyledon-leaf area (mm²)	Fresh weight (mg)	Biomass (g/row)	Seed yield (g/m²)
small	70	27.1	120.4	65.5	1.9	147.1
medium	66	39.3	144.7	81.4	3.3	182.9
large	63	54.8	184.2	106.1	5.8	223.6
contrast
linear	*	***	***	***	***	***
quadrate	ns	ns	ns	ns	*	ns

^a***, ** and * contrast significant at p=0.001 and p=0.01 and p=0.05, respectively; ns, not significant

ACKNOWLEDGEMENTS

We thank Larry Mann and Mike Robinson for their technical assistance. This work was supported by a grant from the Canola Agronomic Research Program and was funded by the Saskatchewan Canola Development Commission and Alberta Canola Producers Association.

REFERENCES

Bodnaryk, R.P. and Lamb, R.J. 1991. Influence of seed size in canola, Brassica napus L. and mustard, Sinapis alba L., on seedling resistance against flea beetles, Phyllotreta cruciferae (Goeze). Canadian Journal of Plant Science 71: 397-404.

Heather, D.W. and Sieczka, J.B. 1991. Effect of seed size and cultivar on emergence and stand establishment of broccoli in crusted soil. Journal of the American Society of Horticultural Science 116: 946-949.

Lamb, R.J. 1989. Entomology of oilseed Brassica crops. Annual Review of Entomology 34: 211-229.

Major, D.J. 1977. Influence of seed size on yield and yield components of rape. Agronomy Journal 69: 541-543.

Mendham, N.J., Shipway, P.A. and Scott, R.K. 1981. The effects of seed size, autumn nitrogen and plant population density on the response to delayed sowing in winter oil-seed rape (Brassica napus). Journal of Agricultural Science, Cambridge 96: 417-428.

Shanmuganathan, V. and Benjamin, L.R. 1992. The influence of sowing depth and seed size on seedling emergence time and relative growth rate in spring cabbage (Brassica oleracea var. capitata) L. Annals of Botany 69: 273-276.