Earth mites are a complex of related
species introduced into Australia
in the early part of last century which cause economic damage to crops,
especially canola, and to pastures. A general method of chemical control aimed
at interrupting the life cycle at the most effective stage, by spraying in the
spring before mites enter diapause, has recently been adopted for controlling
earth mites and particularly the red legged earth mite, Halotydeus destructor (Tucker). Here we use paired paddock
treatments and field enclosures to test its effectiveness on pest populations
of H. destructor as well as other
pests through two seasons at multiple pasture sites in Victoria.Halotydeus destructor numbers
at the start of the winter growing season were suppressed by up to 85%, but
this control level was not sustained. By spring, numbers of H. destructor and other pests are
similar or higher than those in unsprayed areas. Spring spraying is therefore a
useful means of suppressing H. destructor
in early autumn but its effects disappear relatively quickly during the season.
This strategy also has little impact on the control of the blue oat mite, Penthaleus major (Duges), and other
phytophagous invertebrate pests.The
reasons why earth mite populations respond in this way are not clear.Predator organisms may be affected by the
chemical either directly by the toxin or indirectly by the crash of the prey
populations.Alternatively, earth mite
populations in sprayed areas may surge through lack of either competition or
resource limitations.Trials using
different chemicals known to be less harmful to ‘beneficial’ organisms, and
different spray timings were carried out and the results are being
assessed.The effect of repeated spring
spraying on timing of onset of diapause will be assessed in the coming mite
season, as will possible effects on earthworm populations.
