136 Moore and Lefoe: The Effect of a Heat Wave on Urban Tree Pests in Melbourne those managing urban forests is to reduce the nega- tive impacts of uncertainty on effective management (Kendal et al. 2014; McPherson et al. 2018). There are 91 Australian hemi-parasitic mistletoes (mostly Loranthaceae). Many have evolved host mim- icry (crypsis) where their foliage resembles the host’s (Watson 2011), so they often go unnoticed by those managing trees. They are water and nutrient parasites (Calder and Bernhardt 1983; Davidson et al. 1989; Watson 2011), and in large numbers can compromise the host tree’s vigor when branches beyond the point of mistletoe attachment die back, particularly on older trees and in drier seasons (Knutson 1983). When there are greater than 20 mistletoes per tree, both the growth rate and flowering of the host are reduced (Watson 2011). Eucalyptus camaldulensis Dehnh. (river red gum) is a single-trunked eucalypt between 12 and 40 m tall and is widely distributed across Australia (Boland et al. 1984; Nicolle 2006). The period 1999 to 2010 was a period of below average rainfall for southeastern Australia (Bureau of Meteorology 2011) and many remnant river red gums heavily infested with native mistletoes (40 to 50 parasites) died. The mistletoes (Amyema miquelii Lehm. ex Miq. Tiegh. and A. pen- dula Sieber ex Spreng. Tiegh.) are native species important in the ecology of plant communities and to native bird and insect species (Calder and Bernhardt 1983; Watson 2011; Moore 2018). Large English (Ulmus. procera Salisb.) and Dutch (U. × hollandica Mill) elms make a significant contri- bution to the landscapes of Melbourne, where the avenues are among the finest in the world, as Australia is Dutch Elm Disease free (Moore 1990; Dunn 2000). U. procera and U × hollandica are heavily grazed by the elm leaf beetle (ELB), Xanthogaleruca (=Pyrrh- alta) luteola (Muller)(Kwong and Lefoe 1998; Miller 2000; Lefoe et al. 2014). There is usually one life cycle of ELB per season in Melbourne, but there may be two. Infestations are sublethal, but trees suffer major foliage damage by the end of summer, which impacts on their appearance. The life cycle of the ELB is influenced by temperature and grazing occurs earlier and to a higher level under warmer conditions. Peak egg laying occurs at 289 degree-days (DD), peak first instar at 400 DD, and third instar larvae at 551 DD (Dreistadt et al. 1991; Dahlsten et al. 1993; Lawson and Dahlsten 2003; Lefoe et al. 2014). The late instar larvae begin their trunk assent at 500 DD. In Melbourne, egg laying occurs in mid-November, first instar peaks in very late November/early December, ©2020 International Society of Arboriculture and the third instar larvae peak from mid-December to Christmas day (Lefoe et al. 2014). When the value of accumulated DD exceeded 380, effective chemical control with imidacloprid (0.125 g/L) was under- taken (Kwong and Lefoe 1998). The City of Melbourne manages 568 Moreton Bay fig trees, Ficus macrophylla Desf. ex Pers. A native in the states of New South Wales and Queensland up to 50 m tall, with a large flanged and buttressed trunk, it is widely planted in other Australian states and has been planted in Hawaii and New Zealand (Boland et al. 1984). The figs are threatened by the psyllid, Myco- psylla fici (Tryon 1895), which defoliates trees, impact- ing on their health and amenity value by depleting nutrients, potentially vectoring pathogens, and causing early leaf fall. The psyllids form a protective shell-like structure from crystallized honeydew called a lerp. Some trees suffer complete defoliation but damage to that extent is rare in Melbourne (Fromont et al. 2015). Higher temperatures are drivers of higher psyllid num- bers (Hall et al. 2015). The pattern of fig psyllid infes- tation in Melbourne sees a rapid rise in adult numbers from less than 10 to 50 to 90 per leaf in one or two weeks with a peak in late-December or January. Infes- tation levels on Melbourne’s Moreton Bay figs have been determined by randomly sampling a proportion of trees within sites since 2004, and control with imi- dacloprid (0.125 g/L) effectively targeted sites with unacceptable damage (Honan and McArthur 1998). The occurrence of heat waves and record daily high temperature days in Melbourne, Australia, in January and early February 2009 afforded an oppor- tunity to investigate their impact on urban trees and pests in Melbourne. During this period, a heat wave culminating on 7 February with a record temperature of 46.4 °C (115.5 °F) in Melbourne (Figure 1) resulted in the infamous Black Saturday bushfires that killed 173 people and in another 374 heat wave-related deaths. The impact of these heat waves and very high temperature days on the infestations of native mistle- toes on a eucalypt species, the grazing of elm leaf beetle (ELB) on elms, and psyllid infestations of Moreton Bay figs have significance for the manage- ment of the urban forest in Melbourne. An article in Arborist News reported on some aspects of these impacts using smaller and different data sets (Moore 2018), but this paper reports greater detail of these impacts that were detected through observation by arborists and by structured sampling, which detected changes in pest populations that had not been
March 2020
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