Vladimir Ovtsharenko (American Museum of Natural History, NY)
Andrei Tanasevitch (All-Russian Research Institute for Nature Protection (Moscow)
||A field study|
||Spiders and Hemlock Woolly Adelgid|
||Evaluation of spider diversity in Hemlock forest|
||Density of spider population in Hemlock forest|
||Opiliones as an additional group of predators|
Fieldwork for this project will be conducted in Black Rock Forest (BRF) in Orange County, NY, a 1500-hectare scientific reserve located in the Hudson Highlands of Upstate New York. This proposal has been developed in consultation with the BRF Director, and as Columbia University and AMNH are members of the BRF managing consortium, we have received enthusiastic support to conduct these studies at the BRF.
Spiders and hemlock woolly adelgids (HWA)
As an initial step it was necessary to evaluate the role of spiders as major predators of insects in the hemlock forest and to assess trophic connections that might exist between spiders and hemlock woolly adelgids in Black Rock Forest. As a result some specific questions were formulated. How diverse are spiders in the hemlock forest? How is the density of spiders distributed along the experimental transect? Which groups of spiders are potential consumers of hemlock woolly adelgids and which families and species are dominant as predators of adelgids? Does the superabundance of HWA enhance the diversity and abundance of the main foliar predatory spiders in that ecosystem? Are there additional predators of adelgids on hemlock trees? This research was conducted for the first time as part of this project in the Black Rock Forest.
Evaluation of spider diversity in the hemlock forest
Using different collection techniques we collected spiders at all levels in the hemlock forest, including soil, leaf litter, under bark of hemlock trees, on trunks, branches, twigs, and leaves including tops of small hemlock trees. The list of spiders related with hemlock forest is completed and includes 109 species belonging to 15 families. Spiders were divided into two groups in respect to their predatory behavior. The first group includes vagrant hunters. They never make hunting webs and hunt their prey on leaves, branches and trunks. Spiders of families Thomisidae, Philodromidae and Salticidae are dominant among vagrant hunters in the hemlock forest. They are very common on hemlock trees, and do feed on adelgides. They could be natural predators for hemlock woolly adelgids. In total eight families of vagrant hunters occur in the hemlock forest: Anyphaenidae, Corinnidae, Gnaphosidae, Liocranidae, Lycosidae, Thomisidae, Philodromidae, and Salticidae.
The second group of spiders in the hemlock forest are the sedentary hunters. Spiders in this group build various catching snares and are found in, or near their webs. These sedentary spiders catch and consume the flying stages of adelgids. Sedentary hunters in the hemlock forest represent 8 families: Agelenidae, Amaurobiidae, Araneidae, Dictynidae, Hahniidae, Linyphiidae, Tetragnathidae and Theridiidae.
We also estimated the spiders related specifically with hemlock
trees, especially those found on branches, twigs, and leaves. This list
includes 24 species of spiders belonging to nine families. Linyphiidae, Dictynidae
and Theridiidae are of particular special interest because these spiders construct
small webs precisely where adelgids are found, and their webs are able to catch
all flying stages of adelgids. Spiders of the family Theridiidae (cobweb weavers)
are usually sedentary, hanging upside-down in the center of an irregular cobweb
between hemlock branches. Dominant in Theridiidae are two species: Argyrodes
trigonum (Hentz, 1850) and Theridion murarium Emerton, 1882. Some
hemlock branches and leaves are very densely covered with cobweb spider webs.
These webs are able to catch adelgids detached from branches and leaves. Two
additional sedentary hunter families Araneidae and Tetragnathidae build their
webs mostly between the tree branches. Younger spiders of these families can
catch small flying adelgids and consume them. Mature spiders, being considerably
larger, probably do not consume small flying adelgids. However, they do construct
large webs with extremely sticky threads which catch flying adelgids leading
to the additional elimination of these stages. Therefore, five families of spiders
could in theory eventually capture all flying adelgids on hemlock tree branches.
Vagrant hunters on the branches are represented by three families of spiders: Philodromidae, Thomisidae and Salticidae. The dominant spiders on hemlock branches are Philodromidae. They catch their prey on leaves, branches and trunks. Very common are two species, Philodromus placidus Banks, 1892 and Philodromus exilis Banks, 1892 these are potential predators of hemlock woolly adelgids.
Spider population density in the hemlock forest
We studied spiders in three equal areas of the experimental Black Forest transect; brook or upper region, middle, and lower parts of the transect. Spiders were collected from tree branches by means of a sweep net. In each part of the transect we took 5 samples, each of them containing approximately 50 tree branches. After collecting spiders from branches they were transferred into a plastic bag and all samples from the 50 branches were deposited into the same bag. Samples were sorted in the laboratory. We collected spiders three times: June 16, June 20 and June 23, 2000.
The total number of spiders in the lower brook area was 78.0, 80.0, 92.5 specimens per 100 branches; in middle brook area 50.0, 54.0, 76.7 per 100 branches; in upper brook area 64.0, 76.0, 90.0 spiders per 100 branches. The total calculation shows that an approximate number of spiders on one branch of a tree varies between 0.5 spiders and 0.93 spiders per branch. According to our data, the richest diversity of spiders is found in the lower part of the transect: 83.5 spiders per 100 branches. Whereas 76.7 spiders per 100 branches was found in the upper transect (brook), the lowest was the middle transect brook area with 60.2 spiders per 100 branches.
Does the superabundance of HWA enhance the diversity and abundance of the
main foliar predatory spiders in that ecosystem?
As our preliminary study has shown, the diversity of spiders is much higher on hemlock trees than on foliage trees. Sweeping the branches of foliage trees along the same transect has shown that spiders are essentially absent on the branches of these trees. At the same time, sweeping hemlock trees produces a great abundance of spiders as well as opilionids. It appears that both these groups of predators are especially attracted to hemlock trees.
An additional group of predators of Hemlock Woolly
Adelgids occurs on hemlock trees
During our research we were paying particular attention to finding new groups of predators of hemlock woolly adelgids. Opiliones, (harvestmen or daddy-long-legs) are of special interest. Some hemlock branches were completely covered by harvestmen. They were also located on the bottom of leaves, exactly where adelgids are located. Harvestmen feed mainly on living insects, sometimes on dead animals or even plant juices. Consequently, including adelgids in their diet appears to be a distinct possibility. Our experimental observations are also supported by opilionid experts (J. Cokendolpher, pers. comm.). This leads us to the conclusion that Opiliones are natural predators of adelgids and could probably be used as a natural regulator of hemlock woolly adelgid density in eastern hemlock communities.
| General Information on the Project | List of Spiders | Key to Spider Families & Genera |
| Biotopical Distribution of Spiders |