The Importance of the Flaxseed Count in Predicting the Actual Fly-Free-Date

The period of emergence of either the fall or spring brood of Hessian flies, Phytophaga destructor Say, varies from year to year with bioclimatic conditions. On this account "fixed" or "approved" dates are unreliable and the seeding date each year should be correlated with the season and with the fall brood of flies. In Iowa, the flaxseed count has been the only constant and reliable factor to use in predicting accurately the actual f1y-free-date. One hundred or more flaxseeds were collected at each station each day (average about 1,000 per day at Monona county station) from old stubble dug up at random in heavily infested fields. These flaxseeds were then classified as parasitized, empty and viable. The viable flaxseeds were then dissected to determine the per cent of larvae, of pupae and of parasitized larvae. Hence, the flaxseed count gave the daily rate of transformation from the larva to the pupa and to the adult state, as well as the rate of approach of the fly-free-date. When this count yields very few pupae and only a small per cent of larvae in contrast to a large number of empty flaxseeds, it ;s proof that the actual fly-free-date is at hand and that the few remaining pupae will immediately issue as flies, lay their eggs and perish before wheat drilled at this time will come up. Thus it is possible to predict the f1y-free-date so that farmers can begin drilling wheat six or seven days before the end of the egg laying period of the fall brood of flies. The records of the Iowa Observation Stations for 1922 and 1923 are given in the form of a summary.