Maize growers must weigh up the risks posed on their farm when considering whether or not to use Sonido treated seed.
Wireworms are the larvae of the Click Beetle. Butter yellow to golden-brown in colour and up to 25mm in length, they have a thin body and three pairs of legs behind the head. Established grassland allows wireworm numbers to increase and when swards are cultivated the pest can pose a significant risk to following crops for up to three years.
The larvae may feed on the germ of maize kernels or completely hollow out the seeds, leaving only the seed coat. This is most often evidenced by gaps in the rows of plants. Additionally, the larvae may bore into the stalk at or below soil level, resulting in affected plants turning purple, wilting and often breaking at the level of the soil surface.
March-May is a peak period of wireworm activity, which of course coincides with the sowing period for maize. Damage is intensified when maize is planted early and the weather turns cold (as in 2013), slowing germination and establishment. Infestations are worse in areas of a field that stay moist for long periods of time. As soil temperatures warm, wireworms move deeper in the soil and eventually a point is reached where they are no longer a threat to growing maize. There are no insecticidal sprays approved for wireworm control in maize; seed treatment and cultural control are the only tools available.
OPTIONS FOR WIREWORM CONTROL IN MAIZE
Given that wireworm pose greatest threat for the first two years after an old ley is cultivated, cultural control techniques are centred on avoiding growing maize in these situations. An alternative strategy would be to grow winter cereals for a couple of years (perhaps for wholecrop silage?) and rely on the cereal seed treatment Deter to offer a measure of wireworm control.
Neonicotinoid insecticidal seed treatments (such as Poncho) offered a measure of wireworm control but amidst fears over bee safety, this chemistry was removed from the market for a two year period (we are now in the second year).
A replacement for Poncho was found in Sonido, which is available on a limited range of maize varieties. Sonido is slightly less effective against wireworms than Poncho (in trials in France and Germany). Also, significantly, Sonido cannot be applied in conjunction with Mesurol, which acted as an efficient bird repellent. DuPont Pioneer offer Korit as a bird repellent which is co-applied with Sonido on some of their varieties. Limagrain have adopted a different approach and announced that higher rates of the fungicide Thiram appear to deter birds. For the 2015 season they offer Sonido plus Thiram.
Both Poncho and Sonido have been found to affect the germination and viability of treated seed. This effect appears to be intensified when seed is sown into cold, wet seed-beds and therefore special care should be taken to ensure sowing conditions are adequate.
Maize growers must weigh up the risks posed on their farm when considering whether or not to use Sonido treated seed. Consideration should be given to the age of the sward which preceded the maize crop and also, how many crops of maize have been grown since the grass was ploughed-out.
Written by: Dr Simon Pope – Crop Protection Product Manager
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