Friday July 5th, 2013
This week marked the beginning of our experiment about the use of spider silk in agricultural productions to reduce arthropod herbivory. In order to see if the presence of silk can affect plant productivity, we are using kale as a test crop. Chlorophyll content, height of plant, number of leaves as well as fresh weight measurements will help us determine if there are differences in plant productivity between plants in the open field treated with spider silk and ‘control’ plants that are untreated.
This Tuesday, we officially started the experiment by planting red russian kale in the field. A 150 feet long bed at the horticultural centre has been made available to us by the MSEG initiative at McGill University. MSEG has also started the kale plants in the greenhouse and gave us numerous advice regarding kale planting and harvesting. MSEG members are always around the fields and available to talk so they provide a very precious help for the well being of this research.
At the time we went to the field to plant the kale, we already had about fifteen spiders (genus Tetragnatha) in the lab; we had previously collected them from the McGill Bird Observatory (MBO). The spiders we aim to milk (to get silk from them) on a weekly basis for this experiment are mainly Tetragnatha extensa; we collected them sweeping the vegetation surrounding a watershed at the MBO. We picked spiders from the genus Tetragnatha to do the experiment because of Chris Buddle’s previous experience with them while working on ‘Spider silk reduces insect herbivory’. Also, the fact that those spiders are present in the surrounding ecosystems make it easier for us to obtain them and imagine possible interactions with local arthropod species.
The day kale was planted, we processed to SPAD measurements on every leaf of all plants. We aim to do a longitudinal study of the chlorophyll content of kale leaves to see if their productivity is affected by the presence of spider silk. After we planted the kale and applied chicken manure to the bed, we watered the plants and let them thrive in their new environment. On the Wednesday, we went back in the field, watered the plants again (which were under the ‘transplant shock’ as some leaves showed yellowish, soft bottom leaves) and took height measurements. Wednesday was the first day on which we applied silk to randomly picked plants among the two first rows of the kale bed. On that day we refined our ‘milking’ techniques and used plastic frames to collect the silk from Tetragnatha spiders. We let the spiders wander around the plastic frame and as they fell from on of its sides (while being connected to the frame by a silk strand), we started rotating the frame to coil the silk around it. This technique works well while not consistently, and it comes with a cost: the spiders often fall from the frame directly to the ground and often get injured. Also they are easy to lose in the field, so conditions for milking spiders in an open field are not ideal, especially when there is wind. When we had about 10 strands of silk per frame, we let the spider go back into the vial and passed the frame from the tip of the desired plant’s leaves to the ground, letting the silk strands stick onto the kale leaves. It was sometimes difficult to see the silk strands on the frame; counting the strands was most of the time impossible, therefore we trusted the number of turns we rotated the frame with the spider hanging down. In one afternoon, as a 3-people team, we succeeded in measuring all 150 kale plants and applying silk on twenty of them. On the Thursday morning, we went back in the field and applied silk on an additional 16 plants. This time, spiders cooperated more throughout the milking process; we hypothesized that maybe the cooler temperature or again the fact that we didn’t feed them beforehand might be factors affecting the spiders’ cooperative behaviour (we will see if those factors are significant throughout the research).
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