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Winter 2009 Insect responses to invasive shrubs in early- successional habitats Johanna L. Fickenscher University of New Hampshire, Durham
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Recommended Citation Fickenscher, Johanna L., "Insect responses to invasive shrubs in early-successional habitats" (2009). Master's Theses and Capstones. 521. https://scholars.unh.edu/thesis/521
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BY
JOHANNA L FICKENSCHER B.A. DePauw University, 2006
THESIS
Submitted to the University of New Hampshire In Partial Fulfillment of the Requirements for the Degree of
Master of Science in Natural Resources: Wildlife
University of New Hampshire December 2009 UMI Number: 1481725
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ProQuest LLC 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106-1346 This thesis has been examined and approved.
Director, Dr. John A. Litvaitis, Professor of Wildlife
tQ*. Dr. Thomas D. Lee, Associate Professor of Forestry
Dr. Paul CT5ehnson, Assoeiate\P#6feKsor of Natural Resources
\^-H-2£X3^ Date ACKNOWLEDGEMENTS
Thank you to my committee members Dr. Thomas Lee and Dr. Paul Johnson, and especially my advisor Dr. John Litvaitis. This project would not have ensued without funding from the USDA's Cooperative State Research, Education, and Extension Service, the UNH Department of Natural Resources, the New England Farm and Garden Association, the UNH Graduate School and the Farrington Fund for travel assistance, or without the permission of New Hampshire Fish and Game for use of public land. Special appreciation for Bonnie Caruthers for her expertise and advice on moth rearing, and for Donald Adams for providing Saturniidae eggs for this project. I am grateful to Dr. Jim Benedix, Dr. Dana Dudle, Dr. Janet Vaglia, and Dee Seketa for their continued enthusiasm and inspiration. Thank you to my family and friends; also Dr. Judith Moyer for her invaluable guidance, and Nathan for encouragement and support during this endeavor.
in TABLE OF CONTENTS
ACKNOWLEDGEMENTS Hi
LIST OF TABLES v
LIST OF FIGURES .....vi
ABSTRACT vii
CHAPTER PAGE
I. INTRODUCTION 1
II. METHODS 7
III. RESULTS 14
IV. DISCUSSION 21
CONCLUSION ..26
LIST OF REFERENCES 27
APPENDICES
A. EXCLUSIVE INSECT FAMILIES FOR FLIGHT-INTERCEPT TRAPPING 33
B. EXCLISIVE INSECT FAMILIES FOR BEAT SAMPLING 34
IV LIST OF TABLES
1. Characteristics of sites used for flight-intercept trapping and individual shrub sampling during the summer months of 2007 and 2008 in southeastern New Hampshire..8
2. Diet trials in larvae choice experiment using both H. cecropia and A polyphemus in 2008 12
3. The 10 most abundant insect families captured through flight-intercept trapping among four sites (dominated by either native or invasive shrubs) in southeastern New Hampshire, both 2007 and 2008 combined 15
4. Rare insect families (< 5 individuals) captured through flight-intercept trapping among four sites (dominated by either native or invasive shrubs) in southeastern New Hampshire, both 2007 and 2008 combined 16
5. Comparisons between native and invasive shrub foliage in captive larvae choice experiment in June through September 2008 in southeastern New Hampshire 20
v LIST OF FIGURES
1. Overall insect abundance (based on flight-intercept traps) among four sites in southeastern New Hampshire 16
2. Average weekly insect abundance found on native and invasive shrubs within sites for sampling seasons in 2007 and 2008 in southeastern New Hampshire 17
3. Average weekly herbivorous insect abundance found on individual shrubs in early - successional sites in southeastern New Hampshire for 2007 and 2008 18
4. Average weekly insect family richness found on native and invasive shrubs, within sites for sampling seasons 2007 and 2008 in southeastern New Hampshire 19
5. Lepidopteran larvae abundance in 2007 (from flight-intercept trap data in early - successional sites in southeastern New Hampshire) with songbird hatchlings from same sites 24
6. Lepidopteran larvae abundance of four sites in 2008 from flight-intercept trapping data in early-successional sites in southeastern New Hampshire 24
VI ABSTRACT
INSECT RESPONSES TO INVASIVE SHRUBS IN
EARLY-SUCCESSIONAL HABITATS
by
Johanna L. Fickenscher
University of New Hampshire, December, 2009
Early-successional habitats are valuable to wildlife species and susceptible to invasive shrubs, possibly negatively affecting biodiversity of whole communities. Insects provide energy links between plants and wildlife, thus, change in insect communities due to invasive shrubs may have detrimental impacts on wildlife - especially nesting songbirds. I gauged how invasive shrubs affect insect abundance, richness, and phenology using flight-intercept trapping and beat/sweeping methods. Lepidopteran larvae were provided a choice to consume native or invasive shrub foliage. Invasive shrubs negatively affected communities with reduced insect abundance and richness on individual shrubs as well as the entire habitat. Lepidopteran larvae preferred native shrub foliage over invasive, and also displayed higher survival rates when consuming native shrub foliage. Insects are either unable or unwilling to consume invasive shrubs, meaning a reduction of food for insect consumers. Nesting songbirds may abandon early-successional habitats or have to expand their territory size in order to find food.
vn I. INTRODUCTION
Exotic plants can become pervasive when introduced in novel habitats (Vitousek et al. 1997, Lockwood et al. 2007). These plants have been introduced through a variety of avenues, both purposeful and unintentional (Crosby 1993). Often, factors of biotic resistance such as competition, native herbivory, and soil fungal communities prevent exotics from establishing (Levine et al. 2004). Yet some invasive plants have been shown to have survived genetic bottlenecks because of multiple and uncontrolled introductions, resulting in novel genotypes and a high adaptive potential in their new range (Lavergne and Molofsky 2006).
After colonizing a community, exotic plants have been shown to replace some native vegetation (Williamson 1996, Mack et al. 2000). This may have drastic environmental and economic costs (Mack et al. 2000, Pimentel et al. 2000, Mooney et al.
2005). For example, the aggressive vine kudzu {Puerarlia lobata) covers 3 million ha in the United States and is estimated to spread at the rate of 500,000 ha per year (Mitich
2000). In the northeastern United States, honeysuckles (Lonicera spp.) have invaded the understory of forests and formed dense patches that reduce native herb species richness as well as the density of tree seedlings (Woods 1993, Collier et al. 2002). Additionally, glossy buckthorn (Frangula alnus) has been shown to have similar effects, dominating forest understories and reducing the growth and survival of tree saplings (Frappier et al.
2003, Fagan and Peart 2004).
The replacement of native plants by exotic species may have more profound consequences than changing local plant communities; they may also affect herbivore populations. For example, phytophagous insects may be deterred by unpalatable exotic
1 plants (Tallamy 2004). The impact of insects on exotic plant fitness has been well-
researched (see review of Keane and Crawley 2002); however, the effects of invasive
plants on the insect community is still unclear (McEvoy 2002, Tallamy 2004). Insect
diversity and abundance can be reduced in habitats invaded by exotic plants (Olckers and
Hulley 1991, Sam ways et al. 1996, Herrera and Dudley 2003, Greenwood et al. 2004; see
reviews in Colautti et al. 2004, Harris et al. 2004). Exotic plants have also been shown to
support different guild-assemblages of insects than native plants (Harris et al. 2004,
Proches et al. 2008). This could result in a greater herbivore pressure on native plants
(Olckers and Hulley 1991, Dietz et al. 2004), which in turn may enhance the ability of
non-native plants to invade.
Previous research on the effects of exotic plants on insect abundance and diversity
provides mixed results. There are examples of non-native plants that establish with no
measurable effects on native insects (Williamson 1996, Davis 2003). Preference and
cafeteria-style experiments, on the other hand, have provided conflicting results. In some
studies, insects prefer exotic plants (Gross et al. 2001, Agrawal and Kotanen 2003,
Frenzel and Brandl 2003, Lankau et al. 2004), whereas others have observed preference
for native plants (White et al. 2008). If there is an insect preference for exotic plant
foliage in a lab setting rather than in the field, it may be due to an inability to recognize
cues of a viable food source as a result of lack of coevolution with these non-native plants
(Lankau et al. 2004).
Why are early-successional habitats vulnerable?
Approximately 90% of inland terrestrial vertebrates found in the northeastern
United States utilize resources found in early-successional habitats (Litvaitis et al. 1999,
2 DeGraaf and Yamasaki, 2000). These habitats were historically abundant in New
England, a consequence of wide-spread clearing of forests for agriculture (Litvaitis 1993,
Foster et al. 2002). Land-use patterns changed during the 19th century and most of these
farms were abandoned (Litvaitis 1993). As these areas matured into second-growth
forests, the abundance of early-successional habitats has declined dramatically (Litvaitis
1993, Foster et al. 2002). Present day land uses may be reducing the amount of early-
successional habitat (Brooks 2003), as management of remaining habitats may make
them vulnerable to invasions of exotic plants (Hobbs and Huenneke 1992, Mack et al.
2000). Early-successional areas are dependent on disturbance, which is the very factor
that facilitates many invasive plants to establish in these areas (Hobbs and Huenneke
1992, Mack et al. 2000). Certain invasive shrubs, such as autumn olive (Elaeagnus
umbellata) and multiflora rose {Rosa multiflora) were also planted in these areas to
enhance food and cover for game animals (Gill and Healy 1974). Because of the
reduction of early-successional habitats in the northeastern United States and the role of
disturbance, these areas are vulnerable to invasion by exotic shrubs.
Implications to food chains
Herbivorous insects are an important component in food webs in that they transfer
energy from plants to higher trophic levels, and are included in the diet of many species
of birds, mammals, reptiles, and amphibians (Wilson 1987, Weis and Berenbaum 1988).
A reduction or change in insect communities as a result of invasive shrubs may have detrimental impacts on organisms of higher trophic levels. For example, Ortega et al.
(2006) demonstrated a negative effect on migratory songbird reproductive success based on a reduced number of insects in a habitat invaded by an exotic forb. Likewise, Williams and Karl (2002) found a reduced abundance of native birds and more exotic
birds in invasive shrub habitats.
Can invasive shrubs induce a change in insect phenology, diversity, and abundance?
Some invasive shrubs leaf out earlier and go through senescence later than native
shrubs (Woods 1993, Trisel 1997). This may alter phenology of insect species that find
invasive shrubs palatable. Early leaf-out of invasive shrubs might cause a surge of
insects earlier in the spring than would otherwise occur due to environmental cues
(Visser et al. 2006). This, in turn, may shift insect life cycles to earlier in the spring and
may result in earlier die-out of these insect species in the fall. Consequently, invasive
shrubs may initially have a greater abundance of insects than native shrubs when birds
arrive and alter where birds feed and nest (Visser et al. 2006). This could result in a food
shortage for fledglings when, later in the summer, insect species die out because of a shift
towards earlier life cycles. This demonstrates one of the potential effects of invasive
shrubs on entire food chains.
Because some invasive shrubs have been shown to overtake habitats and reduce plant species diversity (Woods 1993, Collier et al. 2002, Frappier et al. 2003, Fagan and
Peart 2004, Webster et al. 2006), it is intuitive that they will also change insect species
diversity. Past research has indicated lower insect diversity and abundance on introduced
shrubs than those same shrubs in their own native habitats (Strong et al. 1984, Fenner and
Lee 2001, Hierro et al. 2005, Cripps et al. 2006). Invasions will favor those phytophagous insect species that find those non-native shrubs palatable and leave most
other species without food-plants, (often these insects are generalist herbivores that may in turn aid in exotic shrub invasions, Parker et al. 2006). This would reduce the insect
4 diversity in those habitats, resulting in greater numbers of a few insect species
(generalists) and reduced numbers of other insect species that are unable utilize invasive
shrubs (specialists).
Early leaf-out and late senescence in invasive shrubs also have been shown to
affect native plants through competition effects, such as reducing growth rates, size,
fecundity, and possibly survival of individual native plants (Woods 1993, Trisel 1997,
Miller and Gorchov 2004). This may affect herbivorous insects depending on native
plants for a food source. Because native insects did not evolve with non-native plants
(Strong et al. 1984), there may be a lack of cues that would signal those plants as being a
palatable food choice for native insects (Agrawal and Kotanen 2003). Also, if there are
fewer native plants because of competition from invasive plants, there would be a lower
abundance of insects that feed only on those native plants. If there are fewer insects
overall in the community, there is less food for other organisms of higher trophic levels
(such as small mammals, reptiles, and especially birds).
Given the importance of early-successional habitats for many species, and how
vulnerable these areas can be to invasion, more information is needed to determine how
these invasions by exotic shrubs are affecting wildlife communities. The first step in
understanding how invasive plants affect wildlife communities is to begin with the
primary consumers, specifically insect herbivores (McEvoy 2002, Tallamy 2004).
Preliminary observations in southeastern New Hampshire (B. Clifford and J. Panaccione,
unpublished reports) indicated that invasive shrubs seemed to support fewer insects, and this may affect reproduction of songbirds in early-successional habitats. My research will attempt to bridge the information gap between invasive plants and wildlife responses
5 through both observation and field experiment. The goal of this study is to determine how exotic shrub invasions are affecting early-successional communities by examining how insect phenology, family richness, and abundance are affected by invasive shrubs.
Specific objectives are:
1. Determine if insect abundance, family richness, and phenology are affected by
exotic shrub invasions at the site level.
2. Examine insect abundance and family richness on individual native versus
invasive shrubs.
3. Investigate insect preference for native shrubs or exotic shrubs using feeding trials
with captive moths.
6 II. METHODS
Study Sites
Southeastern New Hampshire is characterized by hemlock-beech-oak-pine forests
(Tsuga canadensis-Fagus grandifolia-Quercus alba-Pinus strobus) (Sperduto et al.
2004). The climate is humid continental, with variable weather patterns and a large seasonal temperature variance (Climate Change Research Center (CCRC) 1998).
Summers are warm and humid, with average July daytime highs of 24-28°C (CCRC
1998). Winters are cold and wet, with average annual snowfall ranging from 150 cm to over 250 cm, and average daytime temperatures in January around 1°C (CCRC 1998).
There is uniform precipitation year round. Because southeastern New Hampshire borders the Atlantic Ocean, climate is moderated and usually mild and wet in comparison to inland areas (CRCC 1998).
Satellite images (using Google Earth) were utilized to locate four early- successional old-field sites in Rockingham and Strafford counties of New Hampshire.
All sites were abandoned agricultural fields that were managed by infrequent mowings for over 30 years. Sites ranged from 1 to 2.5 ha, contained 40% - 70% shrub coverage, and a varying percentage of invasive shrubs (Table 1). Two sites were dominated by native shrubs (< 20% invasive shrubs) and two sites were dominated by invasive shrubs
(> 60%o invasive shrubs). Vegetation was sampled by a line-intercept method (Bonham
1989) in the summer of 2007. Transects were set up systematically placed 50 m apart, and ran north to south beginning with the southwest corner of each site. All vegetation was recorded to type (grass/forb/fern/woody) and woody vegetation was identified to species along alternate 10 m increments (Johnson et al. 2006). General weather patterns were recorded each sampling day, as well as monthly precipitation for each sampling month, according to the National Oceanic and Atmospheric Administration (NOAA
2009). Native-dominated site #1 (N#l) was privately owned land located in the town of
Lee. Surrounding land was largely secondary forest and the site had 1.3% invasive shrub cover. Native-dominated site #2 (N#2) was also privately owned plot in the township of Rollinsford with 18.1 % invasive shrub coverage. The surrounding land was used for agriculture, mostly hay fields. Invasive-dominated site #1 (I#l) was state-owned and located in the city limits of Portsmouth with 91.1 % invasive shrub coverage. The plot was occasionally used for recreational purposes and surrounded by secondary- growth forest and other abandoned agricultural fields. Invasive-dominated site #2 (I#2) was in the city limits of Dover and part of state-owned lands used for recreation. The site had 63.6% invasive shrub coverage and was surrounded by abandoned agricultural fields.
Table 1. Characteristics of sites used for flight-intercept trapping and individual shrub sampling during the summer months of 2007 and 2008 in southeastern New Hampshire.
„. Total Shrub Native Shrub Invasive Shrub Grass/Forb/ Cover (%) Cover (%) Cover (%) Fern (%) N#l 100.0 98.7 1.3 48.2 N#2 64.4 81.9 18.1 100.0 I#l 77.3 8.9 91.1 100.0 I#2 56.5 36.4 63.6 43.5
Common native shrubs and young trees included arrowwood viburnum
(Viburnum dentatum), gray dogwood (Cornus racemosd), silky dogwood (C. amomum), black cherry (Prunus serotina), and quaking aspen (Populus tremuloides). Common invasive shrubs included autumn olive (Elaeagnus umbellata), glossy buckthorn (Frangula alnus), common buckthorn (Rhamnus cathartica), honeysuckle (Lonicera spp.), multiflora rose (Rosa multiflora), and Japanese barberry (Berberis thunbergii).
Objective 1: Characterizing insect abundance, richness, and phenology
I examined how insect abundance, richness, and phenology varied with the degree of invasive shrub abundance, based on volumetric shrub coverage. To characterize the overall insect communities (abundance and richness), ten flight-intercept traps (British
Museum of Natural History 1974) were deployed at each site. This method captures insects that drop down when they collide with the intercept mesh. Traps were distributed systematically 35m apart, and insects were retrieved every seven days during June 18 through September 6, 2007 (eleven weeks) and from May 5 through September 6, 2008
(sixteen weeks). The second sampling season started in early May to catch shrub leaf- out, because some invasive shrubs have been shown to leaf out earlier than native shrubs
(Woods 1993, Trisel 1997), possibly causing a surge in insects earlier in the season
(Visser et al. 2006). Sampling occurred over two seasons because I anticipated natural year-to-year fluctuations in the data collected.
All insects were transferred from water-filled bins to plastic sandwich bags that were filled with isopropyl alcohol and refrigerated until identification. Insect and arthropod identification and nomenclature was according to Bland (1947), Triplehorn and
Johnson (2005), and Wagner (2005) for Lepidoptera larvae. Identification was only completed to the family level because of volume of insects involved. Phenology of each site was compared by summarizing insect totals among the more complete dataset of the second year.
9 Abundance and richness were summarized by week for each site and compared to the invasive shrub coverage of sites using analysis of variance (ANOVA), followed by
Student-Newman-Keuls multiple comparisons (SNK) if the ANOVA was significant (P <
0.05). Insect family richness data were compared using ANOVA to show differences between the four sites based on amount of invasive shrub coverage over each summer of sampling. This was followed by SNK multiple comparisons to further show how the sites differed. Sites were also grouped into two sites of low invasion (< 20%) and two sites of high invasion (> 60%), and analyzed for insect abundance and richness using a two-sample t-test. Sampling seasons (years) were analyzed separately.
Objective 2: Insects affiliated with individual shrubs
Five invasive shrubs and five native shrubs were sampled at each of the four sites
(10 shrubs per site, 40 total). Weekly sampling occurred from 2 July to 6 September,
2007, and from 5 May to 6 September, 2008. Beat-sampling was used because it captures insects that do not, or are unable to fly (e.g. beetles, plant bugs, and larvae,
British Museum of Natural History 1974). Each shrub was 1.5 - 2m tall and 1 - 1.5m wide, had a similar leaf-density, and sampled by beating a lm section ten times.
To identify which shrubs supported the most insects and the greatest familial richness within sites, I used generalized linear mixed-effects models. The sites were nested within shrub type (invasive/native-dominated), and this was used as the fixed effects. The dependent variables were total number of insects and total number of families for the respective mixed models analyses. Insects and arthropods were also organized into feeding guilds based on their general niche - herbivores, predators, and scavengers/detritovores (Triplehorn and Johnson 2005). After identifying insects to
10 guild, herbivorous insect abundance for each site was compared using ANOVA then
SNK multiple comparisons. Herbivorous insects are an important energy link between plants and higher trophic levels (Wilson 1987), and were used in this portion of the study to tease out the effects that other non-herbivorous insect groups might have on the results.
Herbivorous insect abundance of the sampled shrubs was ranked using an index of production for insect consumers. The shrub species with the highest average herbivorous insect abundance per week was assigned a 1.0 and others are based on the relative productivity to 1.0. The shrubs that were ranked were only those shrubs used in earlier parts of this study: highbush blueberry {Vaccinium corymbosum), silky dogwood, quaking aspen, autumn olive, glossy buckthorn, and multiflora rose.
Objective 3: Moth preference for native or exotic shrubs
Saturniidae moths were utilized to examine foraging preferences and success on individual plants. The three species used were luna (Actias luna), polyphemus
{Antheraeapolyphemus), and cecropia {Hyalophora cecropia). All three are characterized as generalist herbivores that are known to consume many species of woody vegetation (Collins and Weast 1961, Wagner 2005). They have five instars in the larval form and can reach sizes up to 7.5 cm in length when acquiring an abundance of nutrients
(Tuskes et al. 1996). The three moth species have one brood per year throughout New
Hampshire (Tuskes et al. 1996).
Eggs from the three Saturniidae were obtained from a captive breeder who used moths caught in the wild and mated in semi-captivity. Luna moth eggs were deposited in early June of 2008 and had an incubation period of 7 to 14 days. Polyphemus eggs were
11 deposited in late June and early July had an incubation period of around 14 days.
Cecropia eggs were laid in mid and late July and had a 10 - 14 day incubation period.
Caterpillar feeding preference
Caterpillar preference for native or invasive shrubs was tested through a choice
experiment. Native shrubs used for this portion of the study were silky dogwood, quaking
aspen, and black cherry. Invasive shrubs used were glossy buckthorn, autumn olive, and
multiflora rose. Caterpillars were reared from eggs, and once hatched, were placed in an
outside 10 x 10cm enclosure containing the leaves of one native shrub and one invasive
shrub (methods modified from Agrawal and Kotanen 2003). Actual comparisons are
listed in Table 2, with 42 replicates. Larvae were left to wander the container and taste
both shrubs. Leaves were replaced daily. When larvae reached the 2nd instar, they were
assumed to have chosen their food plant, because typically caterpillars continue
consuming the same food plant they have chosen in the 1st instar (Stone 1991, Tuskes et
al. 1996). After the 2n instar, choice was over and larvae were fed their chosen shrub
until death or pupation. Plant preference among caterpillars was determined by direct
observation.
Table 2. Diet trials in larvae choice experiment using both H. cecropia and A. polyphemus in 2008. QA = quaking aspen, CH = black cherry, DW = silky dogwood, AO = autumn olive, MR = multiflora rose, GB = glossy buckthorn.
QA vs. AO CH vs. AO DW vs. AO
QA vs. MR CH vs. MR DW vs. MR
QA vs. GB CH vs. GB DW vs. GB Caterpillar survival
Larvae of A. polyphemus and H. cecropia were placed on either a native or an invasive shrub directly from hatching (no choice). Hatchlings were placed in 10 x 10 cm outside containers with shrub foliage (N = 133). Foliage was replaced with fresh leaves and containers cleaned daily. Upon reaching 3rd instar, caterpillars were placed within garden-cloth sleeves in the field until pupation. Twice a week sleeves were emptied of frass and once a week caterpillars were moved to another branch on the same shrub for fresh foliage. Survival in number of days of caterpillars on native vs. invasive shrubs was determined. Larvae survival was compared using ANOVA for each food plant and each moth species.
13 III. RESULTS
Objective 1: Characterizing insect abundance, richness, and phenology
Flight-intercept trapping yielded 30,642 insects for 2007 and 42,315 insects for
2008. These compromised 23 orders and 138 families. Number of families may have been greater because certain groups weren't identified below order or class1, and some insects were unidentifiable.
Insect abundance
Insect abundance sampled via flight-intercept traps did not differ among sites
(2007: P = 0.39, F = 1.03; 2008: P = 0.778, F = 0.37). However, among families,
Lepidopteran larvae were less abundant in invaded sites (2007: P = 0.002, F = 7.922;
2008: P < 0.001, F = 18.102). For combined sampling years, site N#l had 449, N#2 had
382, I#l had 238, and I#2 had 254 larvae total. There is a clear outlier where site I#2 had a much higher insect abundance than all sites any other week. This is likely due to a carrion beetle, Necrophila americana, found in high abundances throughout the summer for that site (4945 individuals, Table 3), and especially high numbers that week of sampling (487 individuals). The beetle emits a pheromone to attract each other to a carcass to mate. The results were run without this outlier, by taking the average abundances of the week before and the week after, and this did not change the result.
' Classes Acari (mites, ticks), Chilopoda (centipedes), Diplopoda (millipedes), as well as the orders of Araneae (spiders), Blattaria (cockroaches), Collembola (springtails), Ephemeroptera (mayflies), Isopoda (pill bugs), Opiliones (harvestmen), Psocoptera (booklice), and Thysanura (thrips) were not identified to family. 14 Table 3. The ten most abundant insect families captured through flight-intercept trapping among four sites (dominated by either native or invasive shrubs) in southeastern New Hampshire. Numbers are total captive for 2007 and 2008 combined.
Invasive shrub site #1 Native shrub site #1 Homoptera: Cicadellidae 5136 Coleoptera: Staphlinidae 2437 Diptera: Tabanidae 2956 Diptera: Drosophilidae 2235 Diptera: Muscidae 1046 Hymenoptera: Formicidae 2028 Hymenoptera: Formicidae 983 Coleoptera: Silphidae 1292 Araneae 909 Diptera: Muscidae 1245 Homoptera: Aphididae 786 Diptera: Tabanidae 882 Hemiptera: Miridae 616 Diptera: Culicidae 696 Hemiptera: Cicadellidae 456 Unknown Diptera 682 Diptera: Culicidae 435 Araneae 659 Diptera: Drosophilidae 418 Opiliones 347
Invasive shrub site #2 Native shrub site #2 Homoptera: Cicadellidae 5445 Homoptera: Cicadellidae 4243 Coleoptera: Silphidae 4945 Araneae 2960 Diptera: Muscidae 1712 Diptera: Culicidae 1026 Diptera: Tabanidae 1091 Diptera: Muscidae 902 Araneae 1063 Diptera: Drosophilidae 883 Hymenoptera: Formicidae 969 Unknown Diptera 880 Diptera: Culicidae 921 Coleoptera: Scarabidae 806 Thysanoptera 837 Coleoptera: Staphlinidae 542 Diptera: Drosophilidae 810 Thysanoptera 493 Coleoptera: Staphlinidae 613 Opiliones 477
Family richness
Abundance of exotic shrubs was associated with insect richness in 2007 (P =
0.022, F = 5.68), with average richness for the two low-invasion sites with 35.8 families per week versus 29.0 families among the sites with abundant invasive shrubs. However, family richness was not affected by exotic shrub abundance among sites for 2008 (P =
0.23, F = 1.48). When both years were combined, there is a clear difference in the richness of insect families. There were almost twice the number of rare (< 5 individuals) insect families represented in native sites (13 families) than invasive sites (7 families;
Table 4), and more than double the amount of families exclusive to native sites (35 families) than invasive sites (16 families; see Appendix 1).
15 Table 4. Rare insect families (< 5 individuals) captured through flight-intercept trapping among four sites (dominated by either native or invasive shrubs) in southeastern New Hampshire, both 2007 and 2008 combined.
Site Order Family Common Name Feeding Guild Invasive #1 Coleoptera Dytiscidae diving beetle predator Invasive #1 Diptera Dolichopodidae long-legged fly predator Invasive #1 Hymenoptera Ceraphronidae cerephronid wasp parasitic Invasive # 1 Hymenoptera Perilampidae chalid wasp parasitic Invasive #2 Coleoptera Erotylidae pleasing fungus beetle fungivore Invasive #2 Hemiptera Alydidae broad-headed bug herbivore Invasive #2 Neuroptera Mantispidae mantidfly predator Native #1 Chilopoda Chilopoda centipede predator Native #1 Coleoptera Meloidae blister beetle herbivore Native #1 Diptera Pipunculidae big-headed fly parasitoid Native #1 Diptera Sepsidae black scavenger fly detritovore Native #1 Ephemeroptera Baetidae small minnow mayfly detritovore Native #1 Hemiptera Nepidae waterscorpion predator Native #1 Hymenoptera Cephidae stem sawfly herbivore/palynivore Native #1 Hymenoptera Diprionidae conifer sawfly herbivore Native #1 Trichoptera Trichoptera caddisfly detritovore/palynivore Native #2 Diptera Cecidomyiidae gall midge herbivore Native #2 Diptera Therevidae stiletto fly predator Native #2 Homoptera Nabidae damsel bug predator Native #2 Odonata Macromiidae cuiser dragonfly predator
Phenology
Insect phenology was not affected by increased levels of invasion by exotic shrubs. There was no clear trend of insect abundance among sites during the first month of sampling (Fig. 1).
2000 i 2007 2008
1500 ^
1000
500 fc^#2^ 0
'als^SSfSf >>>,>,3;3GGG •3"3 3 SO W) 60 ^ < < <
Figure 1. Overall insect abundance (based on flight-intercept traps) among four sites in southeastern New Hampshire.
16 Objective 2: Insects affiliated with individual shrubs
Beat sampling yielded 2,408 insects for 2007 and 2,251 for 2008, that comprised
of 21 orders and 80 families of insects. Insect abundance on individual shrubs was
different among sites in 2007 (P < 0.001, F = 991.64) as well as 2008 (P < 0.001, F =
530.49). Greater abundances of insects were found on native shrubs within sites having
lower abundances of exotic shrubs in 2007, whereas the least insect abundances were
found on invasive shrubs within sites of high exotic shrub abundance (Fig. 2). All sites in
2008 had lower abundances of insects than 2007. Site N#l had a lower overall insect
abundance than other sites in 2008, but especially on invasive shrubs. There were no
known differences (in terms of management) for this site between sampling years, so this
may be a seasonal fluctuation.
60
% 50 N #1 N #2 I#l I #2 N#l N#2 I#l I #2 2007 2008
Figure 2. Average weekly insect abundance found on native and invasive shrubs within sites for sampling seasons in 2007 and 2008 in southeastern New Hampshire.
In 2007, herbivorous insect abundance was reduced in sites of high invasion for
insects caught on individual shrubs (P < 0.001, F = 18.13; Fig. 3). The pattern reversed in 2008, where shrubs in sites of higher exotic shrub abundance had a higher herbivore
17 load (P < 0.001, F = 10.608). This could be offset by site I#2, which had more average herbivorous insects than all other sites that sampling year. Flatid planthoppers
(Homoptera: Flatidae), were very abundant on multifiora rose and quaking aspen during the July weeks of sampling for site I#2. There were also more phalacrid beetles
(Coleoptera: Phalacridae) found on quaking aspen during July, which is likely the reason there were more herbivorous insects captured for this site than other sites.
50 - 45 - •~\ * 40 - V s 35 - IS s 30 - S X! 25 - <•-4) 20 ' o 15 - i-bi v n 10 - 5 -
o • Site N#l N#2 I#l Site
Figure 3. Average weekly herbivorous insect abundance found on individual shrubs in early-successional sites in southeastern New Hampshire for 2007 and 2008.
Based on the abundance of herbivorous insects captured on average for each shrub species, quaking aspen was found to be the most productive plant in terms of herbivorous insects. Glossy buckthorn was found to carry the least amount of herbivorous insects, with 1.0 captured on average per week. Quaking aspen was assigned a ' 1.0' for being the highest producing plant with 3.47 herbivorous insect individuals captured on average per week, and all other species were ranked in productivity relative to 1.0: Populus tremuloides (1.00, 3.47 insects) > Vaccinium corybosum (0.69, 2.38
18 insects) > Rosa multiflora (0.59, 2.06 insects) > Cornus amomum (0.54, 1.88 insects) >
Elaeagnus umbellata (0.47, 1.63 insects) > Frangula alnus (0.29, 1.00 insects). The
native shrubs tended to carry the highest amount of herbivorous insects on average,
whereas the invasive shrubs tended to carry the least.
Family richness was different on individual shrubs among sites in 2007 (P <
0.001, F = 1919.31) as well as 2008 (P < 0.001, F = 775.57, Fig. 3). Native shrubs within
sites of low invasion had the highest average insect family richness and invasive shrubs
within sites of high invasion had the lowest average family richness (Fig. 4).
25
g 20
EVD
2S 15 1 Native 10 f 1 Invasive to 5 0 +-1 N#l N#2 I#l I #2 N#l N#2 I#l I #2 2007 2008
Figure 4. Average weekly insect family richness found on native and invasive shrubs, within sites for sampling seasons 2007 and 2008 in southeastern New Hampshire.
Objective 3: Moth preference for native or exotic shrubs
Feeding Preference
When provided with a choice, all polyphemus and cecropia moth larvae prefered the native shrub foliage over the invasive shrub foliage (Table 5). Luna larvae were not used for this portion because there were no survivors from the first experiment. Because moth larvae only chose native shrub foliage in every trial, suggesting larvae either were
19 unable to recognize invasive shrub foliage as a food source or unable to consume
invasive shrub foliage.
Table 5. Comparisons between native and invasive shrub foliage in captive larvae choice experiment in June through September 2008 in southeastern New Hampshire. QA = quaking aspen, CH = black cherry, DW = silky dogwood, AO = autumn olive, MR = multiflora rose, GB = glossy buckthorn; N= 42.
Native Invasive Choice shrub shrub QA 5 0 AO QA 4 0 MR QA 4 0 GB CH 6 0 AO CH 5 0 MR CH 4 0 GB DW 5 0 AO DW 5 0 MR DW 4 0 GB Total == 42 larvae Total = 0 larvae
Survival
Polyphemus and cecropia moth larvae survived at a higher rate when consuming
native shrub foliage over invasive shrub foliage. All cecropia larvae (100%) survived on
native shrub foliage and had a 12.5% survival rate on invasive shrub foliage (N = 65).
Polyphemus larvae had a survival rate of 55.5% on native shrubs and no survival on
invasive foliage (N = 68). There were no luna moth larvae survivors for either shrub type.
20 IV. DISCUSSION
Exotic shrubs in New England early-successional habitat negatively impact insect communities, both on the individual shrub scale as well as the entire habitat. Insect abundance was reduced and loss of certain insect taxa, specifically herbivorous insects, has occurred in habitats with greater levels of invasion. Insects collected from shrubs individually mimic this pattern to a greater extent, also displaying drop out of insect herbivore taxa on invasive shrubs, especially in Lepidopteran species. Lepidopteran larvae chose native shrub foliage to consume over invasive shrub foliage in every trial, for every species combination. Larvae also had a higher rate of survival when raised on foliage from native shrubs. Therefore, Lepidopteran larvae are unable or unwilling to consume foliage from exotic shrubs, which may be facilitating the overall reduced insect abundance and diversity found in highly invaded habitats.
Effects on insect biodiversity
Invasion by exotic shrubs may cause a marked loss in insect diversity. Some insect taxa have been shown to increase in abundance with invasion by exotic shrubs, whereas others decrease (e.g., Donnelly and Giliomee 1985, Breytenbach 1986, and
Samways et al. 1996), causing certain insects to become more rare (Greenslade and New
1991, New 1993). This may be because invasive shrubs have been shown to increase the abundance of few generalist insect taxa and decrease many specialist taxa (Strong et al.
1984). Similar to the findings of Elias et al. (2006), where a higher abundance of blacklegged ticks {Ixodes scapularis) were associated with invasive shrubs, my study suggests that a greater level of invasion causes mosquitoes (Culicidae), muscid flies
21 (Muscidae), deer flies (Tabanidae), and carrion beetles (Silphidae) to increase in abundance (Table 3). There was also some loss to rare species found in native sites
(Table 4). Almost twice the amount of rare families were found in native sites than invaded (13 rare families in native sites, 7 rare families in invasive sites). Among these, only one herbivorous family was rare in invaded sites, broad-headed bugs (Alydidae).
Four herbivorous families were only present in native sites: blister beetles (Meloidae), stem sawfiies (Cephidae), conifer sawflies (Diprionidae), and gall midges
(Cecidomyiidae), echoing Proches et al. (2008), who also found fewer herbivores on exotic plants. More insect families were exclusive to native sites: 35 insect families were found only in native dominated sites and 16 families were found only in invaded sites
(Appendix A). This change in insect taxa results in a lower diversity in sites with greater invasion of exotic shrubs, which leads to loss of certain families (and therefore loss of insect species) in the areas affected.
Among the insect families discussed above, a difference in Lepidopteran larvae was observed for sites with high levels of invasion. The larvae of Lepidoptera have been shown to have a strong connection with their habitat because of larval-host associations
(Tuskes 1996, Wagner 2005). They have been used as indicators of habitat quality in urban and fragmented areas as well as indicators of effects of land management (e.g.
Hammond and Miller 1998, Kerr et al. 2000, Rosch et al. 2001). The abundance of
Lepidopteran larvae was reduced in the two invaded sites (I#l and I#2) for both years
(Fig. 5 and 6). Two Lepidopteran families were present only in the native sites (N#l and
N#2), white/sulfur butterflies (Pieridae, 35 individuals) and giant silkworm moths
(Saturniidae, 101 individuals). This follows the findings of Tallamy and Shropshire
22 (2009), where Lepidopteran richness was higher among native woody plants than exotic.
Lepidopteran species diversity was also negatively correlated with invasive wetland plant abundance (Schooler et al. 2009). Lower Lepidopteran diversity may be an indicator of poor habitat quality in early-successional areas highly invaded by exotic shrubs.
Implications to insect consumers
Lower insect diversity caused by invasive shrubs in early-successional habitats may have negative consequences for insect consumers. Because a lower familial richness was observed in areas with high amounts of invasive shrubs, consumers dependent on those groups of insects may have to work harder to find a food source. This could entail leaving the habitat in search for another with a greater availability of food or staying in the area and increasing territory size, ultimately expending more energy to find a food source. This is especially the case for nesting songbirds, as lower Lepidopteran larvae abundance may imply that there is less food in those habitats (Marshall et al. 2002).
Songbird reproductive success has been negatively affected (Ortega et al. 2006) as well as bird abundance (Williams and Karl 2002) in areas with high levels of an invasive plant and reduced abundances of insects. Using data from a companion study (J. Panaccione, personal communication) conducted in the same sites, it is apparent that Lepidopteran larvae were much reduced in sites of high invasion by exotic shrubs, and was not in synch with the hatch dates of certain nesting songbirds (Fig. 5, Fig. 6). Common yellowthroats (Geothlypis trichas) for example, nest earlier in the summer and might be directly affected by lower caterpillar abundances on highly invaded sites. These birds apparently increased their territory size in response to the abundance of invasive shrubs
(especially autumn olive, J. Panaccione, personal communication).
23 ACSWA
XCOYE 60 4:5 • GRCA o 3 U + RBGR X 2V, -YWAR 1 60 1 a o 0W
-•—N#l -•— N#2 -•A—-I#l ~X-~I#2
Figure 5. Lepidopteran larvae abundance in 2007 (from flight-intercept trap data in early-successional sites in southeastern New Hampshire) with songbird hatchlings from same sites. CSWA = chestnut-sided warbler, COYE = common yellowthroat, GRCA = gray catbird, RBGR = rose-breasted grosbeak, YWAR = yellow warbler.
3*^ 5 vi 60 • BBCU [-4^ o ABWWA 3 ea K XCOYE 2% XGRCA 'Jo 1 SO ' a o • INBU L0M -YWAR
•N#l •N#2 •I#l
Figure 6. Lepidopteran larvae abundance of four sites in 2008 (from flight-intercept trapping data in early-successional sites in southeastern New Hampshire) with songbird hatchlings. BBCU = Black-billed Cuckoo, BWWA = Blue-winged warbler, INBU = Indigo bunting.
24 How do conservationists respond?
Many early-successional habitats are public lands dedicated to wildlife and managed by some form of disturbance. To remove all invasive shrubs is impractical and nearly impossible, due to limited funding and the widespread nature of invasions. A reasonable answer may be to focus on some of the worst offenders of invasion: the shrubs that are of little known use for wildlife - whether for food or cover. The herbivorous insect rankings of individual shrubs (above) are useful in determining how to distribute resources for management of certain invasive shrubs. Based on these rankings, it is evident that both autumn olive and glossy buckthorn attract less herbivorous insects than the other shrubs surveyed in this study (1.63 herbivorous insects and 1.0 herbivorous insects, respectively). Multiflora rose was ranked among the native shrubs (2.06 herbivorous insects), and could potentially be excluded from removal regimes.
Narrowing the focus on the shrubs that aren't utilized by herbivorous insects may allow for more aggressive plans for their management.
Another consideration is how shrubs are used as important structural features of habitat by wildlife. Some invasive shrubs are now utilized by rare or endangered wildlife, and the removal of certain shrubs may be detrimental to their success. The invasive shrub Ulex europaeus in New Zealand is valuable to many native insects (Harris et al. 2004), most importantly the few threatened species of giant weta (Orthoptera:
Anostostomatidae) (Gibbs 1999). In the United States, invasive salt cedar (Tamarix ramossisima) is valuable structural feature for the endangered southwestern willow flycatcher {Empidonax traillii extimus) (Sogge et al. 2006). The threatened New England cottontail (Sylvilagus transitionalis) uses invasive shrubs as cover (Litvaitis et al. 2003; I.
25 Hanley, unpublished report), sparking a current debate among regional conservationists
about the management of areas with high abundance of invasive shrubs. A possible
solution may be to focus on the removal of those invasive shrubs that do not provide
much cover for cottontails, such as glossy buckthorn. Both multifiora rose and autumn
olive form large crowns with thorns, forming dense thickets that can deter predation on
cottontails, whereas glossy buckthorn typically forms thin branches and remains sparse.
Factoring in the amount of herbivorous insects found on these invasive shrubs and the
structure of these plants, it might be best to focus funds and efforts only on the shrubs
that aren't providing either of these aspects important to wildlife, like glossy buckthorn.
This would free up time and expenses to try and eradicate one plant while still leaving
food and cover for other taxa.
Conclusions
This study provides evidence that insect communities in early-successional
habitat in New England are negatively affected by invasions of exotic shrubs. Loss of
insect abundance and insect taxa have been shown at both the site level as well as on
individual shrubs. Generalist native Lepidopteran larvae may be better adapted to
consume native shrubs than invasive. Herbivorous insect preference for native shrubs
species could have implications for the management of early-successional areas with
invasive shrubs. This study was limited to two field seasons, only a small glimpse of the
overall effects that invasive shrubs can have on wildlife. Long term effects of invasive
shrubs on wildlife of higher trophic levels should be monitored, and comparisons should be made on insect herbivory between local native and introduced congeneric shrubs
species.
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32 Appendix A. List of insect families exclusive to either native or invasive shrub dominated sites using flight-intercept trapping for both seasons combined.
Site Order Family Number l#l Coleoptera Dytiscidae l i#l Coleoptera Scolytidae 1 l#l Dermaptera Dermaptera 26 l#l Diptera Dolichopodidae 1 l#l Diptera Tephritidae 28 l#l Hymenoptera Ceraphronidae 1 l#l Hymenoptera Eulophidae 1 l#l Hymenoptera Perilampidae 1 l#l Odonata Libellulidae 2 l#l Orthoptera Tridactylidae 2 I#2 Diptera Tachinidae 1 I#2 Hemiptera Alydidae 1 I#2 Hemiptera Mesoveliidae 4 I#2 Hymenoptera Chalcididae 6 I#2 Neuroptera Mantispidae 1 I#1,I#2 Dermaptera Forficulidae 28 N#l Chilopoda Chilopoda 1 N#l Coleoptera Bruchidae 14 N#l Coleoptera Hydrophilidae 1 N#l Coleoptera Meloidae 1 N#l Diplopoda Diplopoda 7 N#l Diptera Calliphoridae 2 N#l Diptera Empididae 1 N#l Diptera Lauxaniidae 3 N#l Diptera Otitidae 3 N#l Diptera Pipunculidae 1 N#l Diptera Sepsidae 1 N#l Ephemeroptera Baetidae 1 N#l Hemiptera Nepidae 1 N#l Homoptera Miridae 1 N#l Hymenoptera Cephidae 1 N#l Hymenoptera Diprionidae 1 N#l Hymenoptera Scelionidae 28 N#l Hymenoptera Tenthredinidae 7 N#l Lepidoptera Lasiocamidae 9 N#l Odonata Anisoptera 9 N#l Odonata Gomphidae 63 N#l Trichoptera Trichoptera 1 N#2 Diptera Cecidomyiidae 1 N#2 Diptera Therevidae 1 N#2 Homoptera Issidae 43 N#2 Homoptera Nabidae 1 N#2 Odonata Macromiidae 1 N#1,N#2 Hymenoptera Andrenidae 13 N#1,N#2 Hymenoptera Braconidae 7 N#1,N#2 Hymenoptera Megachilidae 12 N#1,N#2 Lepidoptera Notodontidae 3 N#1,N#2 Lepidoptera Nymphalidae 3 N#1,N#2 Lepidoptera Pieridae 35 N#1,N#2 Hemiptera Pyrrhocoridae 9 N#1,N#2 Lepidoptera Saturniidae 101 Appendix B. List of insect families exclusive to shrub species, beat sampling, both seasons combined.
Site Shrub Order Family N#l AO Coleoptera Cleridae N#l AO Coleoptera Lycidae N#l AO Diptera Cecidomyiidae N#l AO Diptera Scatopsidae N#l AO Diptera Simuliidae N#l AO Diptera Stratiomyidae I#2 AO Hemiptera Membracidae N#2 AO Homoptera Cixiidae N#l AO Homoptera Coccidae N#l AO Homoptera Dictyopharidae I#2 AO Lepidoptera Nolidae N#l AO Orthoptera Ensifera I#l GB Diptera Psychodidae I#l GB Odonata Coenagrionidae I#2 MR Hemiptera Acanaloniidae N#2 MR Hymenoptera Vespidae N#2 MR Lepidoptera Arctiidae I#2 MR Lepidoptera Saturniidae N#l AS Coleoptera Cerambycidae N#l AS Coleoptera Dermestidae N#l AS Coleoptera Melandryidae I#2 AS Coleoptera Meloidae I#2 AS Diptera Tipulidae I#2 AS Hemiptera Aleyrodidae I#2 AS Hemiptera Largidae I#2 AS Hemiptera Nabidae I#2 AS Hymenoptera Apocrita N#l AS Hymenoptera Heloridae I#l AS Lepidoptera Libytheidae N#2 AS Neuroptera Mantispidae I#l AS Tricoptera Tricoptera N#2 BB Coleoptera Cryptophagidae N#l BB Diptera Conopidae N#l BB Diptera Tabanidae N#2 BB Hemiptera Hydrometridae N#1,N#2 BB Homoptera Lygaeidae I#l BB Homoptera Miridae N#l BB Hymenoptera Braconidae I#l BB Hymenoptera Cynipidae I#l BB Hymenoptera Mutillidae N#l BB Isopoda Isopoda N#l BB Lepidoptera Lymantriidae N#2 DW Homoptera Lygaeidae N#2 DW Hymenoptera Cimbicidae N#2 DW Hymenoptera Encyrtidae