Caterpillars can damage Brassicas from feeding, and they can also be contaminants. It is important to monitor your plants for any signs and carefully tailor your treatments to the pests’ life cycles.
The caterpillars of various species of moths and butterflies can pest Brassica plants (see table).
Insect caterpillars of horticultural Brassicas:
| species | Activity periods | importance | Monitoring methods |
| Small white butterfly (Pieris rapae) |
May / June and late summer – more often in late summer |
Can be harmful and difficult to use see on plants |
Yellow water trap or sticky trap |
| Big white butterfly (Pieris brassicae) |
May / June and late summer – more often in late summer |
But can usually be harmful affects a small number of plants in great numbers and harm is generally obvious |
Yellow water trap or sticky trap |
| Cabbage moth (Mamestra brassicae) |
May / June and late summer – more often in late summer |
Localized pest – can be difficult to control see on plants at a young age |
Pheromone trap |
| Garden pebble moth (Evergestis forficalis) |
May / June and late summer – more often in late summer |
Localized pest – difficult to see on plants |
Pheromone trap |
| Diamondback moth (Plutella xylostella) |
Mainly migrant species and can arrive at any time – usually from June |
Can be very harmful and difficult to use see plants when they are small |
Pheromone trap |
| Silver Y moth (Autographa gamma) |
Migrant and can arrive at anyone Time from early spring |
Rarely does significant damage on Brassicas |
Pheromone trap |
| Beet moth (cut worm) (Agrotis segetum) |
End of May – beginning of July, sometimes a second Generation later in summer |
Rarely does significant damage on Brassicas |
Pheromone trap Forecast available |
Life cycles
With the exception of the diamond-backed moth and the silver Y-moth, which are migrants, all species overwinter across the UK, in the pupal stage or as larvae (beet moth). There is increasing evidence that small populations of diamond moths can overwinter in warm locations (e.g. in the south west of England), but this species does not have a specific wintering stage. All but the migrant species have two generations per year (see table). The diamond moth can complete two or more generations depending on when it arrives in the UK. The silver Y-moth will complete at least a generation.
damage
All caterpillar species cause direct feed damage and can be contaminants in harvested produce (see table above for details).
Monitoring and forecasting
Walking with the harvest regularly is important to spot pest problems in your crop. The timing of treatment is particularly critical for the diamond-backed moth, whose life cycle can be very fast.
Pest traps
Like the cabbage root fly, adult small white and large white butterflies can be monitored with yellow water traps or sticky traps. Adult males of all moth species can be caught in certain pheromone traps. Male moth catches in the traps indicate when female moths are likely to lay eggs in the crop.
forecast
For the beet moth (Cutworm), data from traps caught in Wellesbourne in 2005-2007 were used to estimate a D ° sum for the start of flight activity. This was 340D ° over a base of 7 ° C as of Jan 1st. The Cutworm model uses weather data to predict the rate of development of beet moth eggs and caterpillars. It also predicts the extent of rain-induced mortality among the caterpillars in the early stages. The cutworm model was programmed into the MORPH decision support software and can be used to make forecasts using the weather data used to make the forecasts for cabbage root flies and pollen beetles. Consult the AHDB Pest Bulletin for your current forecast.
Non chemical control methods
There are no established non-chemical methods of reducing the overall population of these pests. Fine mesh nets used to rule out cabbage root flies also exclude adult butterflies and moths. However, it is likely that some species, particularly the diamond moth, lay eggs on the net where it touches the crop so that the larvae have access to the foliage.
usefull links
Read more about pest insects in Brassica crops: Fly pests
Read more about pest insects in Brassica crops: beetles
Read more about pest insects in Brassica plants: aphids
Read more about pest insects in Brassica crops: cabbage whitefly
Read more about the whole plant IPM strategy for pests in Brassica plants
Read the Brassica Crop Walkers’ Guide
If you would like to order a hard copy of the Brassica Crop Walkers’ Guide, please contact publications@ahdb.org.uk or call 0247 799 0069.
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