Pea and Bean Weevil infestations result in far more damage than the relatively superficial visible leaf notching would suggest, warns Syngenta Field Technical Manager, Pete Saunders. It is the weevil larvae feeding on root nodules that will hit crop yield and limit the valuable residual nitrogen.
Speaking at the latest PGRO/Syngenta Pulse Roadshow, Mr Saunders highlighted that last year’s excellent bean seed yields made it one of the most profitable crops for many growers. Furthermore, its value in the arable rotation, for control of difficult grass weeds and the residual nitrogen available to following crops, could be worth as much again to overall farm profitability.
However, whilst crop nitrogen fixation can be as high as 210 kg/ha, field studies have shown that in some instances it may be as low as 40kg/ha - with much of the shortfall attributed to weevil activity destroying the nitrogen fixing root nodules.
Early intervention at the first signs of adult weevil activity to stop female weevils laying eggs is key to protecting this nitrogen fixing capability and yield, advised Mr Saunders. He advocated initial HALLMARK ZEON application as early as two-leaf pairs where there is any sign leaf notching.
“Whilst the characteristic leaf notching of adult weevil feeding may not look serious, and the crop can quickly outgrows the effects, the weevils lay many hundreds of eggs in the soil around the base of the stems,” reported Mr Saunders. “When they hatch the larvae move down to the roots and develop inside the root nodules.”
The activity limits the plant’s ability to create its own nutrients - which inhibits plant vigour, hits the physical yield of the crop and reduces root mass that makes it more susceptible to drought, stress and disease.
Reported trials in Cambridgeshire had demonstrated two early sprays of HALLMARK ZEON at a fortnight interval from the two-leaf stage, followed by a third spray of cypermethrin, effectively reduced notching by 30% on new growth. But more importantly, the programme resulted in a yield increase of 0.5t/ha.
Syngenta field research last year had also shown the significant improvement in bean yield, seed size and seed number from control of weevil, along with a huge visible increase in root mass and nitrogen fixation nodules
Application challenge
Mr Saunders pointed out that targeting spray applications at adult weevils sheltered under bean plant leaves and on the soil surface is a difficult challenge, especially if seedbeds are cloddy or uneven.
“Syngenta application research has shown using angled nozzles, alternated forwards and backwards along the spray boom, has proven most effective at directing spray under the leaves, both sides of the plants and all-round on the seedbed,” he advised.
Application during the warmest parts of the day, when weevils are most active, could further improve targeting of the pests. “HALLMARK ZEON also has a residual repellent activity that would help to reduce further migration into the field from margins and hedgerows where the adults overwinter,” he added. Furthermore, with the Syngenta formulation reducing breakdown in sunlight, it gives the most reliable performance.
Root attraction
The ability of legume plants to fix nitrogen relies on a healthy soil microflora, containing the essential rhizobia soil bacteria. PGRO Plant Pathologist, Dr Kerry Maguire, described how the roots of pea and bean seedlings exude compounds that attract beneficial rhizobia to the plants; roots then grow around the bacteria which create the nitrogen fixing nodules.
It is a complex symbiotic relationship, whereby the plant provides essential growth nutrients to the bacteria, but gets the all-important nitrogen fixation in return. “Soil health and conditions are even more imperative for legumes than other arable crops to enable this to work effectively.
“Anaerobic conditions from water logging or compaction are extremely detrimental to the soil bacteria activity,” she warned. “Soil organic matter helps to create a more diverse population and can beneficially change the balance of microbes.”
However, Dr Maguire also highlighted that there are some bad bacteria and microbes in most soils. Downy mildew, for example, is likely to be found in most soils where pulses have grown in the past. The pathogen has adapted to the follow the same compounds put out by the roots to attract rhizobia, which raises the risk of infection.
Seed treatment with WAKIL XL can provide seedlings with crucial protection from primary Downy mildew infection and reduce the incidence of other damaging soil-borne root disease issues.
Dr Maguire pointed out that Aphanomyces euteiches can remain in the soil for 10 years after an affected pulse crop prevent the plant attracting rhizobia and result in reduced root growth and damage - which also creates an entry point for a plethora of Fusarium species in the soil microflora.
“A third of samples received by the PGRO Plant Clinic are for root rots, primarily caused by different species of Fusarium pathogen,” she reported. “DNA investigation has shown that there are a wider number of species involved than previously thought, which are especially troublesome where roots have been damaged or stressed plants.” Affected plants with damaged rooting are more susceptible other secondary problems, including drought, wind rock, diseases and weevil infestation.
Dr Maguire advised that growers should pay greater attention to soil microbial health and avoiding damage during cultivations and drilling wherever possible. PGRO trials and experience has repeatedly shown drilling should be governed by soil conditions, rather than calendar date.
