Effect of hairiness in brassica lines on the abundance, feeding and oviposition behavior of flea beetles, DBM and Aster leafhopper
Term: 3 years, ending 2021
Status: Complete
Researcher(s): Chrystel Olivier, Dwayne Hegedus, Tyler Wist, Meghan Vankosky, AAFC
SaskCanola Investment: $260,130
Total Project Cost: $390,000
Funding Partners: ACPC
Grower Benefits
Leaf damage (% fed leaf area) at the cotyledon stage was similar between hairy (DOS-2) and non-hairy (AC Excel) canola cultivars. At the 4-leaf stage, leaf damage was higher on AC Excel than DOS-2.
Diamondback moth larvae avoided hairy leaves and exhibited difficulty mining the leaves.
Brassica napus, even hairy B. napus does not produce hairs on cotyledons because it is embryonic tissue. The cotyledon stage is when a large portion of flea beetle damage occurs.
Development of hairy canola is still a work in progress. Hairy Brassica lines that have been developed are not canola quality, so field testing of plants should be considered very preliminary.
Project Summary
Flea beetles (both Crucifer and Striped), diamondback moths and aster leafhoppers are major pests of canola, all feeding on the plant at different times throughout the growing season. The outbreaks of each of these insects are difficult to predict year to year and currently there are no resistant varieties available, leaving insecticide application as the only control option. In recent research AAFC scientists have identified natural lines of Brassica napus and the related Brassica villosa species that exhibited high levels of hairs (trichomes) on their leaves and stems. A transgenic “hairy” canola at the seedling stage had demonstrated resistance to flea beetle feeding and this research was the inspiration for this project. The current project studied the effect of the hairiness of the hairy Brassica lines on the feeding and oviposition behaviour of flea beetles, diamondback moth and aster leafhoppers in the field and lab bioassays.
Brassica species and lines used included Brassica villosa, an extremely hairy species (>3000 trichomes per leaf), and Brassica napus DOS-2, a moderately hairy, double haploid line developed at AAFC Saskatoon capable of producing >300 trichomes per leaf. Also used was the B. napus line “Hairy canola” that produces (800 trichomes/leaf) hairs on its leaves, petioles and stems, and AC Excel, a mildly hairy, commercially available cultivar that produces 75-100 trichomes per leaf. Trichomes were observed using the Canadian Light Source synchrotron. Spectral differences along the length of the trichomes were observed which indicate structural differences within trichomes. Observations revealed the presence of various metals/metalloids in trichomes and their distribution throughout the trichomes. This will help to enhance knowledge on plant trichomes and their role in defense against insects.
Field trials were conducted over 2 years with naturally hairy B. napus lines and B. villosa to assess feeding damage from flea beetles, diamondback moth and aster leafhoppers. In all project years, cereal crops and canola were planted to track the populations of each of these species.
Flea beetle and leafhoppers were monitored with sweep nets and yellow sticky cards. In 2020, enough seed from these diverse Brassica lines was available for a small field trial. Assessment of flea beetle damage showed that % damaged area at the cotyledon stage was similar between hairy (DOS-2) and non-hairy canola (AC Excel) at 75%. At the four-leaf stage though, damage was higher on AC Excel than DOS-2 (Figure 1 – light green bars) suggesting that hairs were deterring flea beetle feeding. At the end of the season, the flea beetle damage was higher in DOS2 due to late maturity, lack of bolting and because it was greener than AC Excel. In 2021, the drought negatively affected the striped flea beetle population, the second (overwintering) population was dominated by crucifer flea beetles (Figure 2). In 2020, no diamondback moth larvae or damage was observed in any of the plots and none of the plants showed symptoms of aster yellows infection. In 2021, trials were compromised due to drought. Diamondback moth pheromone traps were used to assess the migration of adult moths. No diamondback moths were caught in the traps in 2020 or 2021 but there were larvae found in the field in 2020.
Figure 1. The average % leaf surface damaged by flea beetle in the small field trials.
Figure 2. Total number of flea beetles (Phyllotreta cruciferae and Phyllotreta striolata) and diamondback moth larvae (Plutella xylostella) collected in 50 sweep samples from 2018-2021.
Aster leafhopper populations increased in the cereal crops, mainly in barley in 2018 and oats in 2019. This indicates that the aster leafhoppers were reproducing in the cereal crops, but mostly stayed in the cereals through August with the exception of a few that moved into canola. A subsample of leaves were taken from DOS-2 and AC Excel plants that were grown adjacent to a canola field and tested negative for presence of aster yellows phytoplasma which means that no transmission had occurred during aster leafhopper feeding.
Choice and no-choice striped flea beetle bioassays were conducted in small cages, at different temperatures and moisture levels. In 2018, most of the flea beetles collected were striped, so all bioassays were done with that species. Striped flea beetle damage was higher in warm temperatures, and they tended to avoid areas with trichomes and fed more readily on trichome-free areas, such as stems and petioles. They tended to avoid and not feed on B. villosa and were located mainly on stems and petioles of DOS-2. On AC Excel, the flea beetles were evenly distributed on the plant. Striped flea beetles tended to avoid the non-hairy cotyledons of B. villosa and DOS-2, suggesting there is a deterrent or compound (other than trichomes) in the cotyledons of these hairy lines.
Diamondback moth oviposition bioassays were conducted by depositing 20 females and 5 males into cages containing plants at different stages. The number and location of eggs were recorded every 48 hours. Diamondback moth larval movement and feeding bioassays were completed by placing late instar larvae in cages with plants at different stages. Larval movement, location and plant damage were recorded several hours and days later. Diamondback moth can lay eggs on AC Excel, DOS-2 and B. villosa, with a preference for B. villosa stems and DOS-2 leaves over AC Excel. However, the first instar larvae had difficulties mining the leaves of B. villosa as their tunnels were short, and larvae crawled between trichomes, or tried to cross trichomes by producing silk mats. This indicates that trichomes reduce feeding activity.
For the aster leafhopper feeding, oviposition and transmission of aster yellows bioassays, different stages of plants were tested, then sampled at the end and tested by PCR for presence of phytoplasma. Leafhoppers fed and survived on hairy and glabrous B. napus, but mortality was high regardless of plant growth stage. No differences were found in mortality between moderately- and less-hairy plants. No eggs were observed so leafhoppers did not reproduce on any plants. Tests indicated that leafhoppers can transmit aster yellows phytoplasma to the DOS-2 and AC Excel at low frequency as 1/54 plants tested positive.