Abstract
In recent years, damage caused by Tyrophagus similis Volgin(Acari: Acaridae)to greenhouse spinach increased and it is threatening the stable production of spinach. However, details of the climbing behavior of mites on spinach sprout and influence of soil surface algae on mite behavior in the greenhouse remain unknown. It is known that T. similis colonizes soil surface algae under a high humidity condition and that it shows good population increase after feeding algae. In this study, we investigated proximate factors for T. similis to move to spinach sprout. In 2012, we conducted laboratory tests to observe behavior of T. similis which aggregated on soil surface algae under two aerial humidity conditions(65%RH and 95%RH). The total number of mites, after 24 hs without algae, was largest on spinach sprout at the 65%RH condition and in the soil at the 95%RH condition, respectively. With algae as well as spinach sprout, mite numbers were also highest on spinach sprout at 65%RH but most mites moved to algae at the 95%RH condition. To test the feasibility of our findings, algal occurrence and mite damage were investigated in the greenhouse in Yamaguchi pref. in April 2013. Algae covered all soil surfaces when spinach was young, and then depending on dryness, algal distribution was sporadic at the harvest time. Areas without algae showed a high damaging index of mites showing a significant negative relationship between algal occurrence and mite damage. These results suggested that soil inhabiting mites aggregated on algae in humid condition and dry condition triggered their movement to spinach sprout.
