Our eri silkmoth (Samia ricini) larvae have all finished spinning cocoons so we collected mass data.
And another brood finished. The last of the larvae finished spinning, so like always, we collected all of them in a tub and weighed them. There are 20 in total, and this time they are quite large compared to previous rearings. This is our fourth rearing of Samia ricini; summary stats are listed below for this brood, as well as two previous broods (the third brood of winter 2016 was never recorded, but was very small). Note that this brood was reared on Liquidambar while the other two were on Prunus, and the fall 2016 brood was of a different lineage.
Summary Statistics for Samia ricini Cocoon Mass:
Mean Std Dev Median Min Max N
Fall 2016 2.221 0.356 2.1 1.6 2.8 14
Spring 2017 1.773 0.337 1.7 1.2 2.5 33
Summer 2017 2.218 0.294 2.19 1.78 2.86 20
The majority of cocoons this time were 2 g or larger which is quite good, and masses were less variable than in previous broods. Of the three broods, both the fall 2016 and summer 2017 broods were significantly larger in mean mass than the spring 2017 brood (p=0.0002 and p<0.0001, respectively), and between those two larger broods, there was no significant difference in mean mass.
Shown above are this brood's cocoons compared to the fall and spring cocoons. The spring are visibly much smaller, and the silk was thinner and and more papery, rather than thick and fluffy. The fall cocoons look about the same size visually as this time's cocoons, but as they were of a different lineage, the shape of the cocoons were flatter and less round.
Most likely the spring rearing produced small cocoons because of much more severe overcrowding in the final instar compared to in the other two and the food quality was not as good. Or, maybe it was the host plant species that made the difference, as this time we used Liquidambar for the first time rather than Prunus. Perhaps Liquidambar was a better host than Prunus, and the reason why the fall brood was on par with the summer brood because that lineage was larger or utilized Prunus more efficiently.