There is concern about the presence of cabbage seed weevils in a number of winter oilseed rape crops.
Teagasc’s head of crop knowledge transfer, Michael Hennessy, said: “It’s very hard to confirm the extent of the problem yet.
“Survey work is ongoing. We are strongly advising rape growers across the country to thoroughly walk rape crops, in order to find out if they have a problem in the first place.
“And it’s important that they go beyond the headlands. Weevils are more likely to show up close to gates and hedges.
“But it’s important for growers to get right into the crop and fully assess the extent of any problems that might exist,” he added.
Cabbage seed weevil
The crops identified with a weevil-related problem are on commercial farms in the south of the country.
In theory, growers can decide to take control measures if more than one weevil is identified per plant.
“But we are not advocating the use of an insecticide in crops that are in full flower,” Hennessy added.
“If weevils are identified, growers should contact their local Teagasc tillage advisor or their own agronomist to get the relevant advice they require.”
Adult cabbage seed weevils are grey and up to 3mm long with a typical weevil long curved proboscis. The larvae are white, legless, up to 5mm with a brown head and are typically ‘C’ shaped.
The adult weevil usually lays a single egg in a seed pod and it has been estimated that an adult cabbage seed weevil can lay as many as 50 eggs in a season. After hatching the larvae feed on the developing seeds.
Damage to crops
The damage created by the developing larvae is relatively minor but the secondary damage caused by pod midges which lay their eggs via the puncture holes is more economically damaging.
Seed weevils invade oilseed rape crops as temperatures increase during May. After about three weeks of feeding, the females begin to lay eggs in the pods and this continues until the seeds are formed.
When fully fed, the larvae leave the pods to pupate in the soil with the adults emerging later in the summer. The cabbage seed weevil has one generation per year.
Larvae feeding in the pods can damage up to a quarter of the developing seeds and where a high percentage of pods has been affected, this can equate to an overall yield loss of 5-10%.
However, additional yield losses may result from brassica pod midge which can exploit feeding damage and egg laying scars to deposit their eggs.