Population increase and damage by three species of mites on wheat at 20 degrees C and two humidities

Twenty bisected grains of wheat infested with five pairs of the three commonest British grain-storage mites, Acarus siro L., Glycyphagus destructor (Shrank) and Tyrophagus longior (Gervais), were examined every week for 20 weeks. Mite populations, the resulting damage to germ and endosperm, and visible fungal growth were observed at 20 degrees C and relative humidities (r.h.) of 90% and 75%. At 90% r.h., A. siro populations reached nearly 14,000 per test-tube before slowly dropping to 5000. The mites ate the germ before the endosperm, leaving an impenetrable layer of crushed endosperm cells between these regions. The G. destructor population reached only 800 before declining to 300; these mites ate over 75% of the germ and small amounts of endosperm. Tyrophagus longior populations rose to 2200 mites before crashing at week 12 to the initial population level; these mites ate over 75% of the germ and small amounts of endosperm. At 75% r.h., both A. siro and T. longior populations were lower than at the higher r.h., peaking at 3000 and 1000 respectively and decreasing to 500 and 600 mites respectively. Glycyphagus destructor did markedly better than at 90% r.h., reaching 1500 before falling to 400. The damage at this humidity was slower to occur but was similar to that at 90% r.h. at the end of 20 weeks. At both humidities visible fungus was always less abundant on infested grain that uninfested grain.