by Carl Strang
I now have 7 years’ data in which I have noted the first and last dates on which I heard each singing insect species. This year was characterized by a mild winter followed by a warm spring and then a summer of drought. The mild winter and spring apparently were responsible for this year’s early phenology. First song dates were the earliest I have recorded in DuPage County for 17 of the 21 species for which I have 7 years of records. The chi-squared value of 77.33 (with an expected value of 3 species per cell for each rank of earliest to latest) is, of course, statistically significant.
As for last song dates, singing insects generally finished early this year. Of the 20 species for which I have 7 years’ data, 15 had their earliest or second-earliest ending dates, and the chi-squared value was a statistically significant 22.68. This was not a particularly cold or dry late summer and autumn, so the implication is that singing insects have a fixed rate of attrition or duration of song season, so that an early start results in an early finish. The 12 species for which I have the best, most reliable records do have differences in observed song season lengths (ranges for the 7 years, and ranked lowest to highest: 18-42 days for Roesel’s katydid, 16-52 days for gladiator meadow katydid, 52-96 days for the greater angle-wing, 58-96 days for snowy tree cricket, 67-91 days for the scissor-grinder cicada, 64-94 days for the greenstriped grasshopper, 62-109 days for Linne’s cicada, 72-105 days for the dog day cicada, 81-107 days for the common true katydid, 107-139 days for Allard’s ground cricket, 111-141 days for striped ground cricket, and 113-143 days for Carolina ground cricket).
The number of clear, cold nights seemed high enough in November that they might partly explain the early conclusion of common ground cricket songs this year, especially given the recent study by MacMillan et al. (2012) indicating that there is a metabolic cost to recovering from cold-temperature paralysis. However, I found no significant relationship between last song dates and the number of November days with low temperatures below 33F over 2006-2012 for any of the three species (Spearman’s r values 0.51 for Allard’s ground cricket, -0.39 for striped ground cricket, and 0.33 for Carolina ground cricket). It is interesting, though, that I have seen a few red-legged grasshoppers active a couple weeks after the last ground cricket.
This bigger insect may have larger fat reserves to draw upon and so extend its season.