Wheat Midge Degree Days for Predicting Management Needs

Wheat Midge Degree Day Model
for Predicting Emergence, Length of Flight, and Wheat at Risk

Based on data from Canada, the threshold temperature for wheat midge development is 40 F. Observations indicate the following Degree Day accumulations for events in the midge population.

DD	EVENT
450	the midge breaks from the larval cocoon, moves close to the soil surface and forms the pupal cocoon.
1300	10% of the females will have emerged
1475	about 50% of the females will have emerged
1600	about 90% of the females will have emerged.

Field scouting of wheat fields that are heading should continue until midge populations in a region are below economic levels. Typically, the most significant flight period for the entire wheat midge population extends over a 14 to 18 day window of time within a region. Individual adult midge may survive from 3 to 7 days, depending on environmental conditions. Observations over the years in North Dakota indicate that by about 1800 DD, adult numbers decline to the point where field activity is below economic levels. However, in areas where reduced or minimum tillage is common, significant adult activity has been reported and observed up to about 1900 DD.

HRSW planted PRIOR to accumulating 200 DD will head before significant numbers of wheat midge emerge.

HRSW planted FROM 200 to 600 DD will be heading at the time wheat midge are emerging and is at greatest risk to infestation.

HRSW planted AFTER 600 DD will head after peak emergence of midge and should be at lower risk to midge infestation (late planted wheat is at greater risk to cereal aphid infestations, barley yellow dwarf viral infections and exposure to frost injury, however).

We must remember that temperature drives midge development; however, soil moisture must be adequate to allow adult emergence from the soil, and warm temperatures, calm conditions, and adequate moisture during heading are also required by midge in order for economic injury to occur. Soil conditions in reduced tillage situations will vary from normal tillage conditions and may delay some midge emergence in a region. Conditions that favor survival of adult midge may put even later planted wheat at some risk. Scouting until adult midge activity has reached well below economic levels is the best way to avoid unnecessary losses due to wheat midge infestations.

For calculating degree days (DD), it is recognized that growth only occurs within a range of temperatures. The minimum temperature below which no growth occurs is called the minimum threshold. Rate of growth increases with higher temperatures up to a maximum developmental threshold. Thresholds are determined experimentally and are different for each species.

Degree days can be easily calculated by using the average daily temperature minus the minimum threshold temperature. Using the average temperature method we calculate DD for each day, determining the number of degrees above the threshold. DD for a single day are calculated as follows:

Degree days are totaled over a period of days to determine when an insect has reached a certain stage or biological event. The date to start accumulations will differ between species. For wheat midge, degree days are accumulated beginning March 1, when temperatures are likely to begin exceeding the minimum threshold.

Although DD are useful in predicting development of many insect species, these predictions are only estimates. The accuracy of a DD estimate is dependent on the temperatures used in calculating degree days. DD should be calculated with temperatures that represent the environment where insects are developing. Temperatures at one site give only a rough estimate of insect development at another site miles away.

The primary use of DD in IPM is to time scouting for pest species. DD can also be used to predict when an event will occur. By knowing the number of DD accumulated to date, we can estimate future DD by using average maximum and minimum temperatures, such as 5 or 30 year averages. These averages give only an approximation of when an event occurs, but they can be useful in planning our sampling and control activities. By using DD we can eliminate unnecessary scouting, we can avoid missing injurious pest populations, and we can make better management decisions. Thus these techniques help prevent economic losses and excess use of pesticides.


		Department of Entomology, 202 Hultz Hall North Dakota State University, Fargo North Dakota Extension Entomologist: Janet Knodel