Willow Sawfly Management Trials - Weeds Australia

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Willow Sawfly Management Trials - Weeds Australia

High willow sawfly numbers were also found at the Baranduda site in both 2007/08 and 2008/09,although tree defoliation only occurred in 2007/08 (Ede et al. 2009). In the 2009/10 season,willow sawfly numbers were low throughout the season (Table 8) and no tree defoliation wasobserved (Table 9).In contrast, after three seasons of very low populations, willow sawfly numbers at Happy Valleyincreased to high levels (Table 8), and caused considerable tree defoliation (Table 9). Themonitoring site at this location is situated in a corridor of willows that stretches for about onekilometre along Happy Valley Creek. Defoliation of willows was first noticed at the western(downstream) end of this corridor in late December 2009, and as the season progressed, the waveof defoliation spread eastwards (upstream) along the creek. The trees that are monitored wereamongst the last to be defoliated.Willow sawfly population levels at Happy Valley were low throughout the 2008/09 season (Edeet al. 2009) and there was no indication that numbers would increase in 2009/10 as dramaticallyas they did. However, whole tree defoliation has been evident in previous seasons on willows inneighbouring sites particularly on the Ovens River nearby, so willow sawfly populations in thevicinity have been high for the last two seasons. This may explain the increase in numbers at theHappy Valley monitoring site in 2009/10.272.3.2 Willow Tree DefoliationSubstantial whole tree defoliation of willows did not occur at any site until the latter part of theseason, although some limited leaf and branch damage as a result of willow sawfly activity wasobserved during the December assessments at Everton and Kergunyah.The highest levels of tree defoliation across most taxa at Everton were observed in January(Table 9) which followed the peak in willow sawfly number in December (Table 8). Thecanopies of the black willows recovered substantially later in the season. However, there waslittle recovery of crack willow canopies as regrowth was consumed by new generations of willowsawfly. The regrowth of golden willows canopies also remained incomplete for the rest of theseason. The purple osiers were not substantially defoliated by willow sawfly (Table 9) despitehigh willow sawfly numbers in December (Table 8). However, the overall health of these shrubswas not good.At Kergunyah, the foliage present on one tree in January was almost all regrowth, indicating thatfull tree defoliation had occurred prior to the assessment. By March, most of the foliage on alltrees was regrowth. The most severe defoliation was evident in April (Table 9), which followedthe peak in willow sawfly numbers at the site observed in March (Table 8).As described above (Section 2.3.1), whole tree defoliation across the entire Happy Valley Creeksite was first noticed in late December. Defoliation of the trees in the monitoring area was verylimited in January but increased as the season progressed (Table 9). In March, defoliation wasmore apparent on the river-side of the trees monitored than the landward side, but by Aprildefoliation was severe across the entirety of tree canopies.

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