Early sampling of larvae enables pro-active management of stem borers in Colombian sugarcane

Management of sugarcane stem borers (Diatraea spp.) in Colombia's Cauca River Valley (CRV) relies mainly on biological control. Augmentative releases of biocontrol agents are guided by damage assessments (percentage of internodes bored) at harvest and then scheduled accordingly for the next crop cycle. A more proactive monitoring method conducted during the current crop cycle could improve correspondence between natural enemy releases and actual pest pressure. Between 2013 and 2015, 644 commercial fields were sampled for larvae using a standardized 2 person-hour effort per field less than 3 mo post-emergence. Injury data (% of internodes bored) were also collected at harvest from 535 of these fields between 2014 and 2015, 348 before, and 187 after, larval sampling. The relationship between early larval counts and at-harvest damage was analyzed using geostatistical tools and Median tests. Larval counts ranged from 0 to 48 per unit of sampling effort, whereas damage ranged from 0% to 22% of internodes. The highest larval counts and injury levels occurred in northern and central CRV. Larval counts at 3 mo post-emergence were correlated with at-harvest damage in the previous crop: fields with 4% of internodes bored at harvest had a high probability of exceeding ≥30 larvae per sample in the subsequent crop. These results suggest that early season counts of ≥30 larvae per 2 person-hour sampling effort might represent a useful threshold for initiation of biocontrol augmentation programs.