Willow Sawfly Management Trials - Weeds Australia

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30Key Points: <strong>Willow</strong> sawfly population levels and extent of tree defoliation were monitored onwillows of various taxa at nine sites across Victoria throughout the 2009/10 season; After maintaining low population levels at Happy Valley Creek for three seasons,willow sawfly numbers exploded in 2009/10, resulting in extensive tree defoliation; Population levels at four other sites remained low, while levels decreased from high tolow at two sites, and remained high at two sites in 2009/10 in comparison with 2008/09; Tree defoliation occurred at Everton, Happy Valley and Kergunyah, with the mostextensive defoliation occurring approximately a month after a peak in population levels; Two grey sallow plants were extensively defoliated by willow sawfly at Happy Valley; The ratio of live: dead material on willow trees decreased slightly at Kiewa in 2009, butwillow sawfly does not yet appear to have killed any willow trees in Victoria.Defoliation of grey sallow atHappy Valley January 2010.Photo: DPI Victoria
2.4 Synthesis of Monitoring ProgramOver the four seasons of monitoring, there has been considerable variation in willow sawflypopulations both within sites and between sites (Ede et al. 2007, 2009).Low willow sawfly population levels have been maintained for four seasons at some sites,including on the crack willows at Healesville and the grey sallow at Beechworth. And for thethree seasons prior to 2009/10, willow sawfly numbers were low at Happy Valley as well. It ispossible that willow sawfly numbers increased at this site in 2009/10 as a result of highpopulations in neighbouring willow stands, whereas there are no stands of willow supportinghigh willow sawfly populations nearby to either the Healesville or Beechworth sites.At some sites, willow sawfly populations have been sufficiently high in previous seasons tocause defoliation, but in later seasons numbers have declined and defoliation has not occurred.This pattern has been observed at both the Tarago site and the Campaspe River management trialsite, where willow sawfly numbers were very high in the 2006/07 and 2007/08 seasons, but lowin 2008/09 and 2009/10. Tree defoliation was extensive when population levels were high, buthas not occurred in the past two seasons at these sites.This same pattern became evident at the Kiewa site in 2009/10, after four seasons of high willowsawfly populations and tree defoliation. In 2009/10 the population did not develop to high levelsand no defoliation was observed.In contrast, willow sawfly populations at the other two sites in the Kiewa Valley – the Kergunyahmonitoring site and Kiewa River management trial site – were high sufficiently high in 2009/10to cause extensive tree defoliation. Like the Kiewa site, willow sawfly had been active at theKergunyah site for the previous four seasons. High willow sawfly populations also developedagain at the Everton site in 2009/10, for the third season in succession, with extensive treedefoliation observed on three willow taxa at this site.There are no clear explanations for these observed patterns in willow sawfly populations overtime and between sites, as several biological and physical factors can affect population dynamics(Ede et al. 2009).The purpose of monitoring willow sawfly over several sites and several seasons was to determineif the insect was affecting willows and potentially their management. The most obvious impact ofhigh willow sawfly levels is whole tree defoliation, but in most instances, defoliated willows areable to regrow new foliage. Over time, dieback of both large and small branches has beenobserved on willow trees which have been repeatedly defoliated, but no willow tree deathsappear to have been caused solely by the activity of willow sawfly.Apart the very obvious impact of tree defoliation, there are no other visible or obvious signs thathigh willow sawfly populations affect willow tree health. The new foliage that regrows duringthe season is generally healthy, and at the outset of each new season, trees which had beendefoliated the previous season develop full canopies that appear to be as healthy as those on treesunaffected by defoliation. It is possible that reproductive capacity, root health and overall growthare all compromised by repeated defoliation, but these factors have not been investigated.31
Page 5 and 6: Table of ContentsSection 1: Managem
Page 7 and 8: 1Section 1: Management Trials1.1 In
Page 9 and 10: willows limit the light which is av
Page 11 and 12: At the Happy Valley and Campaspe Ri
Page 13 and 14: Planting and sowing were undertaken
Page 15 and 16: 91.3 Results1.3.1 Willow Sawfly Pop
Page 17 and 18: 50% - 100% between December 2009 an
Page 19 and 20: The tallest plants at this site in
Page 22 and 23: 161.3.6 Direct Seeding1.3.6.1 Happy
Page 24 and 25: 18Key Points: Willow trees were def
Page 26 and 27: 20B) BrowsingAt both Happy Valley a
Page 28 and 29: 22Plant growth is limited under the
Page 30 and 31: 24Section 2: Monitoring Willow Sawf
Page 32 and 33: 262.3 Results2.3.1 Willow Sawfly Po
Page 34 and 35: 28Table 9: Willow Tree Defoliation
Page 38 and 39: 32Section 3: Willow Sawfly Across A
Page 40 and 41: 343.2 Western AustraliaThe first re
Page 42 and 43: 36Appendix 1: Details of Statistica

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