Response of Forest Insects and their Natural Enemies to Experimental Ice Storms in a Northeastern Forest

Final Report to the Edna Bailey Sussman FoundationWendy Leuenberger

State University of New York College of Environmental Science and Forestry

Introduction

Concomitant with warming temperatures, climate change will increase the frequency and

intensity of extreme weather events. Ice storms are one such event that can have major

environmental, societal, and economic consequences as they can encompass large areas and

affect both natural ecosystems and human infrastructure and commerce. A novel experiment at

Hubbard Brook Experimental Forest (HBEF) in New Hampshire simulated ice storms of varying

severity this winter by spraying water into the canopy under freezing conditions to develop an

empirical understanding of their effects on forests. I interned with HBEF investigating the effects

of these experimental ice storms on avian and caterpillar communities. I also developed and

disseminated a lesson plan inspired by my research.

Methods

Ten plots were chosen for ice storm manipulation at Hubbard Brook Experimental Forest in New

Hampshire, USA. Plots encompassed mixed hardwood stands co-dominated by American beech

(Fagus grandifolia), sugar maple (Acer saccharum), and yellow birch (Betula alleghaniensis)

with an age of approximately 100 years. Fixed area plots (20 m × 30 m; approximately

basketball court-sized) were assigned to one of five treatments, with two plots in each treatment

(Fig. 1). Icing in winter 2016 occurred between January 18th and February 11th. Pilot methods for

application of ice storms were described in Rustad and Campbell (2012) and updated methods

will be published by these authors and additional investigators.

Figure 1. Experimental process of simulating ice storms: application (a), ice accretion (b), branches breaking (c), and downed branches (d) (Rustad and Campbell 2012).

Figure 2. The author measuring ice accretion in a high treatment plot (¾ inch/19 mm ice accretion). Photo courtesy of the Hubbard Brook Research Foundation.

Figure 1. Terrain (LiDAR) map of ice storm experiment plots with colors representing the target amount of experimentally-applied ice accretion. Hubbard Brook is in the northern part of the figure and the main road is south of the plots. Plots 1 and 8 will be iced in both 2016 and 2017 while all other icings were applied only in 2016. The plots were located within Hubbard Brook Experimental Forest in New Hampshire.

In addition to these ten ice storm plots, I sampled six additional untreated plots (n = 16

plots total) to address the role of edge effects. Untreated plots were located 100 m away (n = 3)

and 250 m away (n = 3) from the nearest ice storm plot (Fig. 2). Plots were located in areas with

comparable vegetation species and structure to the manipulation plots. For point counts, I also

sampled an additional location between manipulation plots 1 and 2 (plot ISEa), for a total of 17

point count locations.

Fig 2. Location of plots for the ice storm experiment at Hubbard Brook Experimental Forest in New Hampshire. Ice storm treatments are located within ISE1-ISE10. Additional control plots include ISEa and plots A100-C250. Circles represent the 15 m point count radii. I visited each point count location six times in 2015 and 2016.

.I assessed avian communities using point counts, where I recorded birds during six 10-

minute intervals per year in the ice storm plots. I restrained my analyses to only include foliage

gleaning birds such as warblers and vireos. These species search leaves and branches for food,

such as caterpillars.

I used plasticine caterpillars to estimate relative predation rates and identify predators of

Lepidopteran larvae. Plasticine is a type of non-drying clay commonly used for stop-motion

photography. In comparative studies, these simple models suffer levels of attack similar to real

larvae, and thus provide a useful comparison of predation intensity, causation, and impact across

treatments. I created and deployed 50 light green geometrid (inchworm) models in each plot for

six days. Caterpillars were glued to leaf petioles or next to leaves in life-like typical day time

inchworm poses (Fig. 3, Howe et al. 2009). Caterpillars were checked every other day to look for

evidence of predation and removed if predated (Bereczki et al. 2014). Remaining caterpillars

were removed after six days (Bereczki et al. 2014). All caterpillars were examined under 1.75×

magnification for evidence of predation that could have been missed in the field (Howe et al.

2009). ‘Wounds’ on retrieved caterpillars were ascribed to birds, invertebrate predators, or small

mammals based on characteristic damage (Low et al. 2014).

Figure 3. (a) Plasticine caterpillar (left) next to a notodontid caterpillar (right) at Hubbard Brook Experimental Forest, NH, in June 2015. (b) Caterpillar model with clear evidence of bird predation.

I reared gypsy moth caterpillars (Lymantria dispar dispar) in the lab to determine

whether treatment affected caterpillar growth rates. Caterpillars were fed sugar maple or beech

leaves for three days. Caterpillars and leaves were weighed before and after the experiment to

determine relative growth and consumption rates.

I expected that caterpillar predation and avian use of treated areas would increase due to

additional structural heterogeneity resulting from downed limbs and other ice storm-related

damage. I predicted that caterpillars would grow better on leaves from trees in the higher

treatment plots as there is more light reaching the leaves, resulting in more nutritious leaves.

Results

I recorded observations of 125 birds from 13 species during point counts. Of these, 99

observations from eight species were foliage gleaners with more observations in 2016 (n = 76)

than in 2015 (n = 23). I recorded between two and 13 observations of foliage gleaners at each

plot over the course of two years. The most common species (> 10 observations) were red-eyed

vireos (Vireo olivaceus; n = 42), black-throated blue warblers (Setophaga caerulescens; n = 16),

and black-capped chickadees (Poecile atricapillus; n = 10). The index of abundance for birds is

greater in the high treatment plot than in plots without treatment. No other differences among

treatments were found.

Fig 4. Detection-corrected index of avian abundance with 95% confidence intervals for foliage gleaning birds in response to experimental ice storm treatments. Letters indicate significant differences among treatments (α = 0.05). Icing was applied to two 20 m × 30 m plots per treatment during January and February, 2016 at Hubbard Brook Experimental Forest in New Hampshire.

In 2015, 202 caterpillars were predated by birds (n = 63), invertebrates (n = 123), and

small mammals (n = 23). In 2016, 177 caterpillars were predated by birds (n = 59), invertebrates

(n = 99) and small mammals (n = 23). I recovered 792 out of 800 caterpillars per year, for a

recovery rate of 99%. The main model explaining caterpillar predation by birds is that predation

rates were lower at the additional control plots than the ice storm experiment plots. This result

indicates that predation rates are linked more to the density of caterpillars than to the effects of

ice storm treatments, as birds learn that plasticine caterpillars aren’t food and stop trying to eat

them over time. Small mammal and invertebrate predation was primarily driven by whether or

not the caterpillars came unglued and fell to the ground where they were more available to these

predators.

Caterpillar growth rates increased with ice storm treatment for both sugar maple and

birch. Growth rates were higher on sugar maple than on beech, as sugar maple leaves are more

nutritious than beech leaves.

Fig 5. Relative growth rates of gypsy moth caterpillars on American beech and sugar maple leaves from ice storm treatment plots. Treatments include control (0 mm), low (6 mm), medium (13 mm) and high (19 mm) of ice accretion. Icing was applied to two 20 m × 30 m plots per treatment during January and February, 2016 at Hubbard Brook Experimental Forest in New Hampshire.

bab

a

c

b

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c

Implications

Birds are spending more time in the higher treatment plots. While there is no evidence of

changes in predation rates by birds, caterpillars grow faster on leaves from the higher treatment

plots. Therefore, my conjecture is that birds are attracted to the canopy gaps caused by the

experimental ice storms to search for more or higher quality food.

Lesson plan

I worked with Jackie Wilson from the Hubbard Brook Research Foundation to improve the

lesson plan I piloted last year. She aligned the lesson plan with Next Generation Science

Standards for middle and high school students, which ties the lesson into public school

curriculum to make it easier for teachers to implement the plan. The lesson plan is now publicly

available on the HBRF’s webpage

(http://hubbardbrookfoundation.org/classroom-resources/coming-soon-plasticine-caterpillar-

experiment/).

Future Directions

I will be investigating the role of leaf quality (carbon to nitrogen ratio) and leaf defenses

(phenolics) in the caterpillar growth to mechanistically determine why the caterpillars grew

better on leaves from the treated plots. I continue to work with the two pilot schools and both

used the lesson plan again this year. We are working on evaluating the effectiveness of this

lesson plan as a teaching aid regarding the scientific method. These results will be submitted to a

peer-reviewed teaching journal to bring the lesson plan to a wider audience. The work included

in my Sussman internship forms the basis of my Master’s thesis. I will be writing up the bird and

caterpillar studies into two peer-reviewed journal articles.

Communications

I presented my research at several conferences to share my results and receive feedback. My

initial results were presented at Hubbard Brook’s annual meeting in July, 2016. We were also

able to hire a Research Experience for Undergraduates (REU) student to assist with my research,

and she co-presented with me, gaining valuable presentation experience. At the Ecological

Society of America’s annual meeting in August, I gave an invited presentation on this research as

part of an organized oral session on disturbance. I also presented a poster regarding the lesson

plan, which allowed me to talk with educators and others involved in outreach to get feedback

and spread the word about the lesson. I also presented a poster about my research at the

International Congress of Entomology in Orlando, FL in September. The Edna Bailey Sussman

Foundation is credited in the lesson plan and all presentations or posters.

Literature Cited

Bereczki, K., P. Ódor, G. Csóka, Z. Mag, and A. Báldi. 2014. Effects of forest heterogeneity on

the efficiency of caterpillar control service provided by birds in temperate oak forests.

Forest Ecology and Management 327:96–105.

Howe, A., G. L. Lövei, and G. Nachman. 2009. Dummy caterpillars as a simple method to assess

predation rates on invertebrates in a tropical agroecosystem. Entomologia Experimentalis

et Applicata 131:325–329.

Low, P. A., K. Sam, C. McArthur, M. R. C. Posa, and D. F. Hochuli. 2014. Determining predator

identity from attack marks left in model caterpillars: guidelines for best practice.

Entomologia Experimentalis et Applicata 152:120–126.

Rustad, L. E., and J. L. Campbell. 2012. A novel ice storm manipulation experiment in a

northern hardwood forest. Canadian Journal of Forest Research 42:1810–1818.