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Cotyledons are embryonic and not plant tissue, and this situation poses a problem with no trichome expression on cotyledons. Cotyledons therefore, cannot be protected by defensive trichomes, but another potential form of deterrence seems to be in action on the transgenic, DOS-02 and B. villosa lines that warrants further study.
Project Summary
Overview
Crucifer flea beetles (CFB), Phyllotreta cruciferae, and striped flea beetles (SFB), Phyllotreta striolata (both from the order Coleoptera and family Chrysomelidae), diamondback moths (DBM) and aster leafhoppers are major insect pests that threaten canola production each year. The project aims to determine the effects of trichomes (hairs) on the feeding and/or egg-laying behaviour of three canola pests: striped flea beetles, diamondback moths (DBM) and aster leafhoppers.
The objectives of the project were to conduct lab bioassays and field trials with naturally-hairy B. napus lines and B. villosa to assess:
1) feeding damages of flea beetles and DBM
2) feeding and oviposition behavior of flea beetles, DBM and aster leafhoppers
3) AY transmission by aster leafhoppers
Results
In laboratory assays, striped flea beetles were used because they were plentiful to catch in the field early season and the crucifer flea beetles were not. Also, striped flea beetles emerge from overwintering two weeks before crucifer so they are the first flea beetles to challenge young canola seedlings on the farm.
Hairy leaves of the DOS2 brassica were less fed-upon by SFB than glabrous AC Excel which the very hairy B. villosa was not preferred by SFB at all. More stem clipping by flea beetles however, occurred on the DOS-02 plants whose petioles and stems are not hairy, which suggests the need for breeding for trichome-covered petioles and stems as well as leaves.
Cotyledons of DOS2 and B. villosa were also less preferred for feeding than those of AC Excel even though they are as hairless as AC Excel canola cotyledons and this phenomenon warrants further attention. In lab bioassays, DBM larvae overwhelmingly avoided the hairy leaves, with the first instars having difficulty mining the hairy canola leaves. Trichome-covered leaves bode well as a DBM deterrent as this trait is expressed on leaves throughout the life of the plant, especially under drier environmental conditions.
In the lab bioassays, aster leafhopper did not exhibit feeding deterrence or preference for hairy or non-hairy canola plants (similar average number of aster leafhoppers in all plants), but laid very few eggs on all crucifer plants. In the 2020 field plots, there were very few aster leafhoppers. All plants tested negative for the presence of aster yellows phytoplasma, AYp, the cause of aster yellows disease.
In 2020, a small scale field trial was conducted at the Saskatoon site and showed that leaf damages (% of fed leaf areas) at the cotyledon stage were very similar between hairy (DOS2) / non-hairy canola cultivars (AC Excel), but DOS2 cotyledons are not hairy. However, at later stage (4 leaf stage), leaf damages were higher on AC Excel compared to DOS2, and at this time, DOS2 leaves expressed the hairy trait on all leaves. In 2021, heavy flea beetle pressure coupled with hot and dry conditions overcame all of the experimental seedlings that were planted at the experimental farm. In the 2020 field plots, there were very few diamond back moths (larvae and adults), and no DBM damage could be estimated.
Preliminary results of this project were provided in this June 2019 GCIRC presentation.
Conclusions and recommendations
Development of hairy canola lines is still a work in progress. Hairy Brassica lines that have been developed are not Canola quality and so field testing of plants should be considered as only very preliminary. Deterrence of DBM larvae is promising, while the destruction of early-season seedlings by flea beetles in 2021 is not. The cotyledons are embryonic and not plant tissue, and this situation poses a problem with no trichome expression on cotyledons. Cotyledons therefore, cannot be protected by defensive trichomes, but another potential form of deterrence seems to be in action on the transgenic, DOS-02 and B. villosa lines that warrants further study.
