Molly Hassett
Edna Bailey Sussman Foundation Final Report
“Availability of sugar resources to emerald ash borer parasitic wasps at
New York release sites”
Rationale
In the United States, emerald ash borer (EAB) has killed millions of native ash
trees. Ash is used widely commercially and as urban street trees. Urban communities
spend billions of dollars to treat, remove, and replant infested ash trees. Ash trees are also
important in native forests. Limiting the spread of EAB has been unsuccessful, so in 2007
the United States Department of Agriculture (USDA) released three parasitic wasps,
which have larval stages that eat EAB larvae and eggs, in an attempt to manage EAB
population growth. In its native habitat, EAB is limited by these wasps. However,
monitoring at release sites in the United States by USDA-APHIS has shown that the
wasps have had variable success at different sites. Juli Gould and others at USDA-APHIS
need to know why the wasps have been unsuccessful at some sites to control EAB in
native and urban forests. As adults, parasitic wasps consume nectar and honeydew from
plants and aphids. These non-prey food resources may limit the success of released wasps
and account for site differences. However, these alternative food resources have not been
quantified at release sites.
It was proposed to:
1) carry out measurements of tree density, percent canopy cover, and basal
area to outline overall habitat characteristics at sites
2) count the number of flowers on shrubs and plants within each site to
determine flower availability and
3) collect honeydew droplets on filter papers to determine the abundance as it
falls from the canopy.
All of these will be used to determine whether the differences in site resources correlate
with wasp success to better help understand what habitats the wasps need to be successful
at lowering emerald ash borer numbers.
Work Completed
By May 14, 2014, all ten New York release sites were set up (Figure 1). I
collected tree, shrub, flower, and honeydew data biweekly at each site from May 19,
2014 to September 20, 2014.
Figure 1. Location of 10 New York State parasitoid release sites used in this study.
At each site, we found tree density/ ha and basal area (m2/ha), shrub density/ha,
and percent canopy cover to outline overall habitat characteristics at sites. We measured
shrub and herbaceous flower density/m2 and honeydew drops/m2 at sites biweekly.
The most abundant species found at our ten study sites was ash, although some
study sites had low densities, comparatively – such as at H3 and S2 (P<0.001) (Figure 2).
Sites with lower densities had larger trees (P=0.072, Adjusted R2=26.84). This is
important because the parasitoids lay their eggs on ash trees infested with emerald ash
borer and often cannot lay their eggs in trees with thicker bark. Sites with a lower density
of ash also may have lower densities of emerald ash borer. Sites with fewer emerald ash
borer could potentially have lower parasitoid establishment as it would be more difficult
to find emerald ash borer larvae to feed on. Also, ash trees have extrafloral nectaries on
the underside of their leaves, so a higher density of ash trees could also help parasitoid
success by providing sugar.
900
Number trees/ha
800
700
Other
600
Buckthorn
500
Black Cherry
Juniper
400
Maple
300
Ash
200
100
0
H1
H2
H3
H4
R1
R2
R3
R4
S1
S2
Figure 2. Mature tree (>10cm dbh) density/ha of the most common species at our
ten study sites.
Shrub stem density varied at our sites, with the highest density of shrubs at R4
(Figure 3). Many of our sites had a high number of invasive species (privet, multiflora
rose, Japanese barberry and honeysuckle). These species, with the native species
increased the amount of sugar available to the ecosystems by flowering and/or producing
extrafloral nectar (viburnum, privet, honeysuckle). Although R4 had the greatest number
of shrubs, it had few flowers compared to most of the other sites (P=0.001) (Figure 4).
80000
Number shrub stems/ha.
70000
Viburnum
60000
Raspberry/Blackberry
50000
Privet
40000
Multiflora rose
Japanese barberry
30000
Honeysuckle
20000
Dogwood
10000
0
H1 H2 H3 H4 R1 R2 R3 R4 S1 S2
Figure 3. Shrub stem density/ha of species with flowers and/or extrafloral nectaries
at our ten study sites.
140.00
Number shrub flowers/m
2
120.00
100.00
80.00
60.00
40.00
20.00
S2
S1
4
R
3
R
2
R
1
R
4
H
3
H
2
H
H
1
0.00
Figure 4. Shrub flower density/m2 at our ten study sites.
Herbaceous flower density varied among our sites, with the highest density of
flowers at H1 (P<0.001) (Figure 5). Honeydew drop density also varied among sites
(P<0.001) (Figure 6). Honeydew and flowers both are potential sugar resources for
parasitoids and parasitoids could potentially utilize both; however some sites with a high
number of flowers had low honeydew production – such as H1 and R2.
Average number flowers/m
2
350.00
300.00
250.00
200.00
150.00
100.00
50.00
Figure 5. Herbaceous flower density/m2 at our ten study sites.
S2
S1
R4
R3
R2
R1
H4
H3
H2
H1
0.00
9000
8000
Drops honeydew/24hrs/m
2
7000
6000
5000
4000
3000
2000
1000
0
H1
H2
H3
H4
R1
R2
R3
R4
S1
S2
Figure 6. Drops of honeydew collected/24 hours/m2 within each of our study sites
over the 18 week study.
It is also important to look at when these sugar resources are available for use.
Figure 7 shows when the different sugar resources are available among all sites for the
parasitoids. If the parasitoids use sugar resources, it is when they are adults. Although
some parasitoids are adults in late May, the majority become adults from mid-June until
early August. This could mean that honeydew and herbaceous species that flower during
that time are important to the parasitoids, however, parasitoid success and establishment
data are still being processed.
0.8
Shrub Flowers
0.7
Herbaceous Flowers
0.6
Honeydew
0.5
0.4
0.3
0.2
0.1
8/
14
9/
4
25
/1
4
8/
8/
11
/1
4
7/
28
/1
4
14
/1
4
7/
6/
30
/1
4
16
/1
6/
2/
14
6/
5/
19
/1
4
0
Figure 7. Proportion of total flowers or honeydew drops counted by species during
weeks. This shows when herbaceous plants and shrubs were flowering and when
honeydew was being produced the most.
Future plans
Currently, I am processing biomass data to create allometric equations to predict
the number of leaves on trees and shrubs at the sites to predict the number of extrafloral
nectaries at sites.
Once parasitoid success and establishment data are processed, I will use stepwise
regression to show what at the sites serve as predictors for parasitic wasp success to help
increase the success of future releases.