This data is from a planting date study done at the UK-Research and Education Center in Princeton, KY. This work was done in the mid 80's but the same principle holds every year. The acturaly dates will change depending on the season. NOTE: The large increase in aphids in the first planting date. The third planting date contained almost no aphids.

We can make this relationship with temperature work for us by using planting date, our best tool. Usually, earlier plantings have more aphids earlier, for a longer period of time. Additionally, earlier planted wheat will be infected at a younger stage and virus replication will be more rapid over a longer time period, resulting in more seriously damaged plants. Later plantings will have smaller aphid populations, which begin later and are present for shorter periods. it's simple, earlier plantings face more days of warm weather than do later plantings.

We can convert this idea of a number of aphids over a period of time to a graphic demonstration of relative risk. If you multiply the number of aphids present each day by the number of days present and add them over the season, you see graphically how much more exposure the earlier planting gets to aphids than does the later planting.The relative risk of being exposed to aphids carrying BYDV is illustrated by the area under each line. You can see that the first planting date has much greater risk than either of the othe two. Earlier plantings get aphids sooner the populations grow faster so more aphids are present, and because they start earlier they are present for a longer time.