Last week of the experiment...

These two past weeks concluded our field experiment: we did the last silk application last week and took the last measurements on kale this week.  

We let all our spiders free

 ...and harvested the kale.

The last measurements done on kale include:
-Height
-Chlorophyll content
-Above ground biomass
       -fresh weight
       -dry weight
-Average leaf damage

With those measurements, we aim to evaluate if there is a difference regarding plant productivity between kale plants treated with spider silk and the control plants.

It was a good time to finish the experiment because the kale leaves grew much bigger and thicker which made it harder to apply silk on the plants without damaging them.  Doing damage on the plants is counter-productive because we aim to evaluate the level of damage as an indicator of plant productivity.

Next steps:

-use statistics methods to evaluate if there is a difference in the different productivity measurements among our two samples

-graph our results

-evaluate the changes made to our initial experimental design

-consider a possible 'lab component of the experiment' with more controlled variables

-discuss possible improvements and provide advice for further investigation in the same topic

Third week of the experiment

Thursday, July 18th, 2013

Third week of the experiment; about halfway through!

When we came back in the field this Monday, we noticed some drastic changes among the kale plants:

Most of them had almost doubled their leaf number and size and we also noticed a much reduced presence of flea beetles.  The new kale leaves were not only less attacked by herbivores, but also much thicker and more resistant to other pests.

In order to adapt our treatment as kale was growing, we decided to apply more silk per plant (about 30 strands per plant).

As a consequence, the silk strands were much more visible both on the frame and on the treated plants on the day of the application.  Thus, we became interested in seeing for how much time was the silk remaining on the treated plants.  Generally, it was very difficult to see the silk again on the treated plants one day after we applied it on them.  Also, after we finished the silk treatment for the 36 plants, a heavy rain started to fall and winds started to blow, probably reducing silk’s persistence on the leaves.

Herbivory

In order to have an idea of the herbivory level at the stage of the experiment, we decided to create a system of visual reference for us to evaluate the overall damage on each leaf of each plant.  We decided to work with this ‘visual approach’ because damage and leaf shape were generally not regular, making an approximation of the ‘missing parts’ difficult.  Also, having a reference table to estimate the level of herbivory for each leaf made our task easier and less time-consuming than more extensive calculations.

Using this table (part of it shown here), we proceeded to an estimation of an average of total damage for each plant from each leaf’s percentage damage value (1 corresponding to 0-10% damage, 2 to 10-20% damage, up to 9, corresponding to 80-90% damage).

This method contains both advantages and disadvantages:
Advantages
-easy to use
-time-efficient
-quick, time-efficient technique giving a general idea of the state of the experiment; very helpful for troubleshooting
Disadvantages
-subjective to the person using the table
-less accurate than direct measurements
-difficult to perceive the difference between close categories (e.g.[50%-60%[ and [60%-70%[)
-most new leaves show low levels of herbivory and older ones show high levels of herbivory: those extreme results 'cancel' themselves out in the calculation of the average damage for each plant

Preliminary results using the reference table

First of all, it is important to mention that different sizes of samples are available to compare the results between the control and treated plants. Therefore, comparing groups of different sizes creates a bias in our experiment.  
The following graphs show a very general overview of the results for the level of herbivory at this stage of the experiment. From this, we can see that most of the data is very concentrated below the [30%-40%[ interval. While being visually easy to understand, those 'box and whisker' diagrams do not show clearly the differences between herbivory on spider silk treated and control plants.

The results of our preliminary statistical analysis do not give us confidence to conclude regarding the level of herbivory for treated and non-treated plants.  A similar analysis shall be made at the end of the experiment along with an analysis of variance to see if we can or not perceive difference regarding the level of herbivory among both treatment groups.

How to apply silk on kale plants

1. Milk a Tetragnatha extensa using a plastic frame; let the spider hang from her silk strand and rotate the frame to extract silk from the spider.

2. Count the number of silk strands that are on the frame (depending on the stage of the experiment, this can vary between 10 and 20 strands per frame)

3. Apply silk through passing the frame over a kale plant.

First week of the experiment

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).

On Thursday, we already noticed the presence of flea beetles on kale leaves!  We  had initially decided to do the experiment in an open field to have an idea of a realistic setting for future conclusions regarding this type of integrated pest management.  While we cannot control for the interactions between herbivores and treated plants in the field, open field herbivory gives a wide range of possible interactions between different species of herbivores and spider silk.

Our next steps for this experiment will be to return in the field and re-apply silk on a weekly basis on the same plants.  Height, SPAD, leaf number measurements as well as potentially level of insect herbivory will be measurements done throughout the duration of the experiment.

Introduction to the experiment

How could spider silk and agricultural yield be potentially connected...

Who?

Margot Charette, undergraduate student at McGill University in Environment

Yifu Wang, undergraduate student at McGill University in Environmental Biology

Christopher Buddle, Associate Professor at McGill University and supervisor of this research

in collaboration with Macdonald Student-run Ecological Gardens (MSEG)

What?

We aim to study the effect of spider silk on plant productivity in agricultural ecosystems using kale as a test crop.

When?

July and August 2013

Where?

McGill University Macdonald campus, Saint-Anne-de-Bellevue, Qc.

Why?

Because it has been previously shown in many studies that spiders and cues left by spiders have an effect on arthropod's herbivore behavior and we aim to push the specific  research about silk a step further to know if spider silk could be an interesting component of efficient integrated pest management systems.

How?

The following blog posts will answer that question!