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Early sowing offers the best protection for carrots - Carrot psyllids can destroy up to a third of a crop

22.01.2008

A doctoral dissertation produced at MTT Agrifood Research Finland reveals that early sowing of carrots is the most effective way to prevent the damage caused by carrot psyllids. If the carrot’s number one enemy gets at the seedlings at the cotyledon stage, it can rapidly destroy up to 35% of the crop.

In her dissertation, Research Scientist Anne Nissinen examined ecological methods of fighting carrot psyllids. The study was conducted in a greenhouse where the pests were allowed to feed and lay eggs on 15 different carrot varieties and a wild carrot in cages.

Nissinen examined the carrot psyllid host plant selection and the severity of the damage caused by the pest at different stages of root and shoot growth. She also assessed the possibility to utilize the push-pull strategy in management carrot psyllids.

Small, Ugly And Bearded Carrots

Carrot psyllids are a major problem in Finland’s most important carrot production regions. The psyllids overwinter in spruce trees and fly to carrot fields from early June onwards. They instantly begin to suck nutrients from carrot leaves, which quickly renders the shoots damaged.

Carrot psyllid attacks usually peak either a week before or a week after the summer solstice. In terms of the crop, it is critical that the shoots have grown enough by that time to survive the pest attack.

If the carrots have only been sown in the end of May, the pests may catch the shoots at the cotyledon stage when the carrots are most vulnerable to damage. After a pesticide treatments, the shoots may recover and look fine on the surface, but reveal small, ugly and bearded carrots at harvest, Nissinen describes.

More Research Needed Into Lure Plants

Nissinen also examined whether a particular carrot variety was more attractive to psyllids than others, which would allow it to be used as a trap crop to keep the pests from attacking the cultivated variety. Of the varieties studied, an old non-hybrid variety, which can still be found on a Swedish seed list, turned out to be the most attractive.

Nissinen believes that the use of a trap crop could be an efficient way of reducing the numbers of psyllids on carrot crops. She does, however, concede that she dare not recommend the use of trap crop in practice based on this study alone.

The effectiveness of a trap crop should always be tested in comparison to the cultivated variety in question, Nissinen concludes.

Psyllids Unfazed By “Bad Smell”

The study also included an experiment where carrot psyllids were exposed to a volatile compound: limonene. Limonene can be found both in carrots and in the psyllids’ winter host, the spruce.

A study conducted in Sweden during the last decade found that limonene repelled carrot psyllids in the field. In Nissinen’s greenhouse experiments, however, limonene failed to shoo psyllids away. neither when sprayed directly onto the carrot crop nor when released from a carrier substance.

The contradiction between the research findings may be because greenhouse conditions differ from field conditions and the psyllids are unable to carry out the change of host plant as they would in the course of their normal live cycle. Nissinen explains.

She adds that in her opinion it is important that future experiments involving potential repellents or attractants should take place in field conditions.

The dissertation of Anne Nissinen, M.Sc. (Agriculture and Forestry), is in the field of environmental science and is titled “Towards ecological control of carrot psyllid (Trioza apicalis)”. The dissertation will be reviewed on 25 January 2008 at the University of Kuopio, Finland. Nissinen’s opponent will be Doctor Robert Glinwood from the Swedish University of Agricultural Sciences, and Professor Jarmo Holopainen from the University of Kuopio will be acting as her supervisor.