We’ve gotten a lot of wet weather the past few weeks which has greatly affected the vegetable growers in the region. Soil preparations and field plantings have been delayed, putting everything a couple weeks behind schedule. With the delays in crop host establishment, it seems like pests may also be delayed in the colonizing of crop fields. One way of determining whether this is the case is to use degree day models.
In pest management, degree days are a measure of heat which can be used to track development or manage insect pests. Organisms have development temperature thresholds, below and above which development is arrested. For example, with San Jose scales Quadraspidiotus perniciosus the lower threshold is 51ºF and the upper threshold is 90º. As long as the ambient temperature is between these two thresholds, San Jose scales continue to develop. For onion thrips, the lower threshold is 52.7ºF.
Degree days are the accumulated product of time and temperature between temperature thresholds. One degree days is one 24 hour period in which the temperature is one degree above the lower development threshold. Of course, temperature varies throughout a 24 hour period in field situations. This is taken into account when constructing a degree day model. Degree days can also be accumulated throughout a growing season, which is how emergence predictions are calculated.
In the simplest model, the equation for a degree day calculation for one 24 hour period looks something like this:
(min temp +max temp)/2 – (min threshold)
Summing the above term for the number of days would give you the degree day accumulation. Other calculations model temperature fluctuation in more complicated ways, which may provide closer estimates of actual degree days accumulated.
So let’s check on onion thrips in Elba, NY. Cornell has degree day information here, with 147.6 degree days accumulated since Jan 1. This calculation is based on 50ºF as the minimum threshold. The UC IPM site states 140.4 degree day accumulations are necessary for egg development. Given that onion thrips have a slightly higher threshold than the Cornell model, they’re not hatching. In Penn Yan, degree day accumulation is 182.0. Slightly higher, which is better for onion thrips. But on account of the rain, which demolishes thrips populations, I still didn’t see any thrips out and about when I was in the field yesterday. So until we get some sun and it warms up, I may just be sitting pretty.