Arboriculture & Urban Forestry 46(2): March 2020 permanent wilting before host trees, which resulted in their deaths, while none of the host trees died. There is a hormonal involvement in the response of Amyema miquelii and A. pendula and hosts, such as E. camaldulensis, to hot dry conditions. The host trees produce abscisic acid (ABA) in response to low- ered soil moisture levels, which induces stomatal clo- sure. It is part of a more general involvement of ABA in stress responses (Moore 1998). Mistletoes lack ABA and so their stomata remain open, which leads to wilting (Fisher 1983). The main damage caused by mistletoes to mature host trees is through their exces- sive transpirational water loss. The hypothesis that mistletoes wilted before their hosts is supported by the observation that nearly all of the mistletoe on the extremities of the canopy died while those that sur- vived seemed to be on older, larger branches within the canopy, closer to the trunk. Changes to fire regimes and the clearing of native vegetation are considered causes of increased mistle- toe infestation (Liddy 1983; Jurskis et al. 2005; Bowen et al. 2009; Start 2011; Watson 2011). Heat waves and very hot days may play a role similar to fire in limiting mistletoe numbers, which could prove useful especially in urban and peri-urban areas where the occurrence of fire is rare or the risks of fire are too great to allow naturally occurring fires to burn. The surveys undertaken after Saturday, 7 February 2009 indicated that the mistletoe numbers remained low for a period of 5 years after the heat events and that establishment of new mistletoes was very low. Elms and ELB Warm weather, as experienced in November 2009, usually favors the growth and development of ELB larvae, increasing the potential for significant elm leaf beetle damage (Clair 1986). However, average elm leaf beetle damage remained low throughout the 2009/2010 season, despite the potential for increased damage. The large number of ELBs killed by the high temperatures on the hottest day on record in the pre- vious summer appeared to significantly affect ELB numbers and grazing in the following year. The tem- perature of 46.4 °C is consistent with the general range of lethal temperatures for insects, where tem- peratures of 44 to 47 °C for durations ranging from a few minutes to several hours killed insects in hot air (Richcigl and Richcigl 2000). The duration over which the heat is experienced is critical to the effect 143 that high temperatures have on insects. Insect death occurs in minutes at temperatures of 55 °C, but can take hours at temperatures of about 45 °C (Mech et al. 2018). Grazing of elms was observed to be lower for five years following 7 February 2009. It is difficult to establish a causal relationship between the killing of ELBs and the heat waves and high temperature, as there may have been confounding effects. However, there is little doubt that the large number of ELBs killed during the heat wave and on the record hot day contributed to reduced grazing in the following spring and summer and so eliminated the need for ELB con- trol measures for a period of about five years. Figs and Psyllids Given that adult psyllid emergence usually ceases in early autumn (April)(Honan and McArthur 1998), there are three likely explanations for the absence of psyl- lids in surveys across Melbourne after mid-February 2009: host plant condition, parasitoid survival, and high temperature. Host plant condition involves drought stress favoring psyllid outbreaks (White 1969; Morgan 1984), but more recent work con- cluded that sap feeders exhibited enhanced perfor- mance when plants were intermittently but not continuously water stressed (Huberty and Denno 2004), which is supported by data showing reduced herbivory if drought curtailed new leaf production in eucalypts (Gherlenda et al. 2016). Under the weather conditions experienced in Melbourne during 2008 and 2009, this would suggest that a rise in psyllid number might have been expected. Parasitoid sur- vival depended on accurate targeting of sprays against psyllids favoring parasitoid build-up (Honan and McArthur 1998). However, Victoria had been sub- jected to prolonged drought for ten years prior to 2008 and 2009, when psyllid numbers remained high and improved spraying had been in place for over a decade with no evidence of impact. Furthermore, Meineke et al. (2013) found that temperature did not differentially affect parasitoids, which is supported by the finding that parasitoids had little impact on psyllid outbreaks in a eucalypt woodland (Hall et al. 2015). High temperature remains the most likely cause of psyllid decline, as adult longevity can be reduced by high temperatures; from 8 days at 30 °C, to 2 or 3 days at 35 °C. Moreton Bay fig psyllid emergence ©2020 International Society of Arboriculture
March 2020
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