How on earth do Dectes survive the winter?

This week we had ANOTHER indoor lab. I'm sorry, I would have rather gone out in the sprinkles (if you could call them that) and looked at anything (dirt, soybean lodged plants from Dectes larvae, jack-o-lanterns, heck even pre-Halloween Christmas decorations). My brain really didn't want to be inside and thinking!

Images from extension.missouri.edu

Dectes larva (left); Dectes adult (right)

So, this weeks assignment was to look at if Dectes texanus (Dectes stem borer) are cold tolerant or cold intolerant (avoid cold). Dectes is a stem borer in soybean; its “original” host is sunflower but in Kansas it is typically found in soybeans (weird, right? another one of entomology's conundrums). For more information on Dectes: http://www.ca.uky.edu/entomology/entfacts/ef149.asp


Cold tolerant insects have “defenses” that keep them alive after they have been frozen at the supercooling point. These defenses include the use of ice nucleating proteins, production of cyroprotectants, and restriction or tolerance of intracellular freezing.

Cold intolerant insects, also, have defenses against cold but often die when they are frozen at the supercooling point. These defenses include migration, hibernation, avoidance, and removal of ice nucleating materials from gut.

For more detail on insect winter survival strategies: http://en.wikipedia.org/wiki/Insect_winter_ecology

We used a “frosty” (seriously that's what they are called) to lower the temperature of 10 thermoconductors with Dectes larvae on them and wrapped in gauze to -15 degrees C and look at survival. Unfortunately, only two of these showed the curve that our teacher is looking for but he reassured us that he has done this several times and that all of the Dectes died. So how do they survive the winter in Kansas?

We then looked at reasons why/how Dectes larvae (overwintering life-stage) would be able to survive in the winter, in Kansas in the two plants (sunflower & soybean). The class came up with several hypotheses:


Tunnel humidity caused by frass cap at end of stem
Stem Size
Chamber Size (width)
Snow and/or soil as insulation against cold
Dectes size (more pith in sunflowers so larger in sunflowers than in soybeans)

The class decided to test chamber size by selecting three representatives from each crop (sunflower and soybeans) with a similar amount of soil around the roots, place the thermocoupler electrodes into the stem to a depth where the Dectes larva would overwinter (so under the soil), and run the experiment until it plateaus out so where there is no more change. So the idea is to see if there is a difference between the two crops. I would say that the hypothesis is that it takes longer for the wider chambers (sunflowers) to reach the supercooling point (-15 degrees C).

For further information on how this experiment was done please look at the teachers class blog: http://ipdmanhattan.blogspot.com/

The below graph is the result of "our" experiment. Each line is a mean of the three plants; xsun is the sunflower, xsoy the soybean, and the control is a measure of the external temperature of the plants (the termocouplers were attached to the outside of the plants).


The first thing to note about this graph is that the control rapidly drops in temperature whereas the plants have a slow descent into the colder temperatures. This indicates that the plants provide some protection for the Dectes larvae overwintering in these plants. The second key point is that the plants plataue above the supercooling point of -15 degrees C which further strengths the thought that the plants are providing protection for the overwintering larvae. The third point is that the sunflowers have a "gentler" slope than the soybeans which may provide more time for overwintering larvae to "adjust" to the colder temperatures. The final point is that soybeans and sunflowers plateau at the same time which indicates that at that point there is no advantage over one plant or the other for the survival of the overwintering larvae which as mentioned earlier is above the supercooling point.

My initial thought about this study was that we might need another control which represented stems without a ball of dirt around them. Now that I've looked at the data I'm undecided about another control (it could help and it shouldn't hurt the data). Setting that aside- I think this data tells us quite a bit and indicates that perhaps just being in the plant keeps the Dectes alive during the winter. However, I think a more important question (given the above data) is why the shift from the original host plant (sunflowers) to soybeans?

Experience rating:

Interesting even though we didn't get to "play" outside. Now that I've looked at the data I have more questions than I did during the class devoted to this lab.