(Spruce Bud Moth) on Food Resource Availability on Picea Glauca (White Spruce) in Subsequent Years

UNIVERSITY OF BRITISH COLUMBIA DEPARTMENT OF FOREST AND CONSERVATION SCIENCES FRST 498 B.Sc. Thesis in Forestry Impact of Simulated Mechanical Defoliation caused by Zeiraphera canadensis (Spruce Bud Moth) on Food Resource Availability on Picea glauca (White Spruce) in Subsequent Years MAY ANNE, THEN SUPERVISORS DR. ALLAN CARROLL DR. YOUSRY EL KASSABY APRIL 2015 Abstract Zeiraphera canadensis (spruce bud moth) larvae feed on newly burst buds of Picea glauca (white spruce). It has been observed that Z. canadensis herbivory on apical shoots leads to loss of apical dominance and the release of dormant buds in subsequent years due to shoot damage. It has also been observed that there is greater success of larval colonization in years following herbivory. This paper explores whether Z. canadensis herbivory increases the amount of food resource available for subsequent generations by simulating mechanical defoliation in a controlled experiment. The results rejected our original hypothesis and total buds produced was found to decrease with increasing herbivory. We did not observe positive resource regulation feedback in the spruce bud moth and white spruce system. Key words: Zeiraphera canadensis, Spruce Bud Moth, growth compensation, herbivory, food resource feedback, mechanical defoliation, Picea glauca, White Spruce May Anne Then April 2015 1. Introduction Native insect folivores are important actors in complex forest ecosystems, with important roles in water and nutrient cycles, as agents affecting successional changes, as plant growth stimulants and other ecosystem processes (Schowalter and Lowman 1999, Trumble et al. 1993). It is known that folivore populations can not only cause small scale changes (at the tree level) as well as larger effects, such as reducing host plant density and productivity at the landscape level during population eruptions (Schowalter 2006). Host-plant interactions of herbivorous insects can also pose serious implications for forest management, especially when populations reach damaging levels as ‘pests’ due to their significant impacts on tree growth and wood quality (Schowalter 2006). The spruce bud moth (SBM) and white spruce complex is a well studied system, with many studies since the 1990s focussing on the temporal and spatial patterns of the insect-host relationship and on SBM survivorship. The spruce bud moth (SBM) larvae, Zeiraphera canadensis Mut. & Free. (Lepidopptera: Tortricidae) is a native phyllophagous webworm that feeds on foliar buds. Its preferred host is the white spruce, Picea glauca (Moench) Voss, though it also feeds on black spruce trees (Picea marinara). Its native distribution matches that of white spruce and black spruce in Canada and populations have been associated with eruptive spruce budworm levels in the Eastern most provinces (Natural Resources Canada 2011). An individual insect’s fitness is determined by its response to environmental conditions (Schowalter 2006). In return, feeding can alter future habitat structure and resource distribution spatially and temporally, either by causing damage to plants after excessive feeding and biomass loss or by inducing growth (Romoser and Stoffolano 1998). Previous studies have shown that the !1 of !24 May Anne Then April 2015 susceptibility of white spruce to SBM may not related to nutritional quality but related to tree physiology (Quiring 1992). Due to the small time window that buds remain suitable for spruce bud moth colonization, the amount of resource available following herbivory can be important for future generations (Quiring 1993). Studies have shown that SBM fitness and survivorship is related to food resource availability; spatially and temporally. Larvae first emerge in May and feed on newly flushed foliar buds until June-July, eventually spinning a characteristic silk bud cap to protect itself from predators (Natural Resources Canada 2011). Over the past two decades, research has demonstrated that there is a close relationship between bud burst phenology and SBM emergence (Quiring and McKinnon 1999; Quiring 1994), with developing buds remaining suitable for first- instar larvae for only a few days after bud burst, after which survivorship decreased significantly (Quiring 1992). Spatial and temporal variation in trees may be an important mechanism for reducing herbivory (Quiring 1993). However, there evidence to show that an insect which has been exposed to constitutive or induced tree defence mechanisms repeatedly over time can develop adaptations against them (e.g. Karban and Niiho 1995; Carroll & Hoffmann 1980). Plant architecture is often altered following heavy levels of herbivory, which has impacts on future plant growth and susceptibility to herbivores (Schowalter 2006). Bud-feeders like the SBM have the potential to kill developing shoots and induce growth of lateral shoots (e.g. Clark and Clark 1985). Carroll and Quiring (1993) also observed that a history of herbivory lead to shorter shoots and greater scarring, with multiple leaders created in following year after herbivory. These impacts were greater on terminal branches than on proximal branches, had greater effect on shoot length reduction and defoliation (Carroll and Quiring 1993). Herbivory was also seen to cause large reductions in vertical growth (Schowalter 2006). Volume increment !2 of !24 May Anne Then April 2015 was found to be influenced, though this was only significant after a few consecutive years of heavy damage (Carroll et al. 1993; Piene 2003). Significant height growth reductions coupled with continued large radial growth increments by heavily damaged trees during the first few years of attack by Z. canadensis caused distinctive stem growth patterns evident in oblique sequence (Carroll et al. 1993b). This is confirmed by anecdotal observations which identify the morphological difference between non- infested trees which are generally conical in shape as compared to SBM infested trees which have a more shrub-like structure, due to ramified branching patterns as multiple leaders fight for dominance (Natural Resources Canada 2011). Heavily damaged trees also reported with greater growth losses between high and low damage categories (Carroll et al. 1993b). However, white spruce appears to be very tolerant to tissue loss in forested stands, where canopy closure causes rapid decline of SBM populations (Carroll et al. 1993b). Timing of bud burst influenced by previous herbivory (Carroll 2003). Though there are studies suggesting a decrease in fitness and reduction in incremental growth post herbivory, not much is known about the quantitative food resource post herbivory. There are also seemingly competing hypotheses with regard to plant response. Tree Growth Response to Herbivory Further studies have revealed a more complex web of relationships between herbivory and plant productivity, which traditionally has been viewed as a process that reduced primary plant production (Schowalter 2006). The health of the plant, its developmental stage and intensity of herbivory can affect survival, productivity, and growth form differently. !3 of !24 May Anne Then April 2015 There are a few theories regarding the way trees with determinate growth, such as white spruce, could respond to the loss of plant tissue due to herbivory. Defence mechanisms could reduce tree damage by producing less food resources following herbivory to reduce herbivore population or alternatively tolerate herbivory by compensating with growth or reproduction (while employing other mechanisms) to maintain plant vigour which can have stabilizing effects no herbivory (Strauss and Agrawal 1999). If a plant employs tolerance against herbivory as a strategy, a positive feedback cycle of increased herbivory and increased growth may commence until the plant no longer has the resources needed for compensatory growth. At this point, a negative feedback loop may begin instead, which would have negative effects on insect population (Paul 2010). It is well documented that Z. Canadensis herbivory can lead to loss of apical dominance and release of dormant buds. Most apical buds damaged from Z. Canadensis feeding on elongating shoots, lead to the release of dormant buds, mainly proximal ones (Carroll 2003). Damaged shoots had more active buds near previous year’s bud scales and there was more successful colonization of basal bud that were on damaged shoots (Carroll 2003). Buds on damaged shoots flushed earlier than on intact shoots and earlier bud burst best synchronized with egg hatch (Carroll 2003). One theory of plant response to herbivory is the overcompensation theory or herbivore optimization hypothesis whereby that primary production is maximized at low to moderate levels of herbivory (Bast and Reader 2013; Pedigo et al. 1986) where greater regrowth may be observed post herbivory. The plant regrowth due to injury may exceed that of normal growth of a non-injured plant (Bast and Reader 2013). This theory was formed under the assumption that more dormant meristems are released to replace the injured dominant meristems given that there !4 of !24 May Anne Then April 2015 is sufficient nutritional to sustain the development and growth of these meristems (Bast and Reader 2013). This study showed that the absence of overcompensation in Black Spruce was mostly due to a relatively small supply of dormant meristems on the trees that were tested (Bast and Reader 2013). The resource regulation hypothesis suggests that the generalized plant response to herbivory is to produce more of the herbivore’s preferred resources

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