The developmental threshold forTetrastichus julis(Walker), a larval parasite ofOulema melanopus(L.), was determined to be 48°F (8.89°C). This threshold can be used to determine emergence rates under field conditions. However, soil surface cover significantly influences soil temperature and was found to be a controllable factor in manipulating spring emergence. The relationship between soil temperature and air temperature is sufficiently correlated to allow emergence rate predictions forT. julisfrom standard air temperature measurements. This prediction is critical to a control program which would allow for pesticide control of the cereal leaf beetle while having minimal effect on theT. julispopulation.Emergence of the overwintering generation ofT. julisis well synchronized with the presence of susceptible hosts in the spring. The summer generation of the parasite is poorly sychronized and many hosts have pupated before the parasites emerge. These late hosts are critical since they are the means by whichT. julisoverwinters.