Determining the biology of rhopaea canegrub in the New South Wales sugar industry : SRDC final report BSS201
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Rhopaea canegrub (Rhopaea magnicornis Blackburn) is the major insect pest of the New South Wales sugar industry with about 25% of Condong farms affected. Rhopaea has also been reported as a pest in the Broadwater and Nambour canegrowing areas. At the time this project commenced little was known about the ecology and population dynamics of the pest and no commercially viable control measures existed.Contrary to previous beliefs, some female beetles were observed to fly prior to mating. However, female beetles were also observed that emerged and did not fly, but mated on the soil surface before burrowing back into the soil where they laid their eggs. It is not yet known which behaviour is the most common. The average egg batch was found to be 21.3 eggs.The distribution of the life stages of rhopaea canegrubs was determined using both naturally occurring populations at three sites and an introduced population at a fourth site. First and second instars are shallow feeders; they were found at an average depth of about 10 cm. The third-instar rhopaea larvae not only move deeper into the soil (average depth about 15 cm) but also become more focused around the sugarcane stool and row centre. Finally, as pupation approaches, rhopaea grubs again move up in the soil profile, where they were found to pupate at an average depth of only 6.5 cm below the soil surface. Rhopaea is a comparatively shallow-living grub in contrast to some other canegrub species.Green cane trash blanketing was tested on six farms and ?stool rolling? on five farms. Neither practice was shown to be effective but this may be due to the relatively low pest populations that prevailed during the three years of field work.Fallowing as a control measure was tested with both naturally occurring populations and in a replicated trial where plots were seeded with 20 second instar larvae. A grass fallow was found to increase pest numbers in subsequent crops compared to replanting. In the replicated experiment, all fallow treatments reduced grub numbers by similar amounts four months after the larvae were introduced.Field counts made after land preparation for replanting showed that cultivation can reduce pest numbers by close to 100%. However, such large decreases may also disrupt the disease cycle which is essential in suppressing pest numbers.Disease studies showed that rhopaea is infected by a different species of Metarhizium fungus (M. flavoviride) than other canegrub species which are infected by M. anisopliae. The fungus affects all stages (egg-adult) of the pest so that the ?window of control? is very wide. Some field observations showed over 50% of a female?s eggs could be killed by metarhizium infections. Beauveria bassiana was another prominent fungal disease identified in the population. This fungus has only been rarely observed in other more northern canegrub species. Both fungi have potential as biological control agents.Other diseases found include milky disease (Paenibacillus popilliae), three minor fungal diseases and one incidence of a rare microsporidian disease (Nosema sp.).