Inter-seasonal variation in Sitona discoideus Gyllenhal (Coleoptera: Curculionidae) larval damage to lucerne in Canterbury and the economics of insecticidal control

Abstract

In four out of five seasons at Darfield, Canterbury, Sitona discoideus Gyllenhal larval damage significantly reduced lucerne ( < 5 years old) production in at least one cut. Of all the yield losses, a maximum of 46.5% was measured at the time of the second cut in a 1-year-old stand during the 1986 – 87 season. Levels of damage similar to those at Darfield were also recorded in Marlborough and North Canterbury in the 1985 – 86 season and were observed qualitatively in the Timaru and Ashburton areas. An economic analysis using 85% dry matter (DM) 25 kg bales valued at either $2.50 or $4.50 each showed that in four out of five seasons, there was distinct economic benefit derived from applying insecticide. Recommended insecticide use did not incur a significant economic loss on any occasion. Such results indicate that prophylactic sprays are justifiable, at least in areas comparable to Darfield. Each season's maximum percent yield loss correlated significantly with a rainfall index. Data collected also suggested that, contrary to earlier opinion, some stands of > 4 years old may in fact show higher sensitivity to S. discoideus larval feeding than their younger counterparts. This may be the result of the lucerne undergoing progressive nodulation failure of an undetermined nature. S. discoideus damage significantly increased the foliar percent DM. Affected plants were generally chlorotic and shorter with reduced axillary buds and smaller leaves.

Keywords:

• Sitona discoideus
• lucerne
• seasonal crop losses
• insecticidal control
• economic return
• pest insects