2.2. Leaf disc bioassay
Wecompared predation and oviposition rates of N. cucumeris and
A. swirskii on a diet of S. dorsalis, using modification of the method
described by van Houten et al. (1995). The bioassay arena consisted
of a single 2 cm diameter leaf disc removed from a 3–4 week cotton
plant floated upside down on saturated cotton wool inside a 9 cm
Petri dish. In ‘no choice’ tests, either 15 second instar or 15 adult
thrips (80% female) were added to each leaf disc using a fine paintbrush
and a dissecting microscope. In ‘choice’ tests, 8 larval and 8
adult thrips were added to each disc. A single gravid female mite selected
at random from the colony (unknown age) was immediately
added to discs along with no mite controls. A small piece of bran
placed in the middle of each disc served as a refuge and oviposition
substrate and reduced problems of mites (especially N. cucumeris)
exiting discs and drowning. Dishes were fitted with a screened lid
and incubated at L16:D8, 26 ± 1 C and 60–70% r.h. Dishes were inspectedat24
hintervals for4days fornumbers of liveanddeadthrips
and mite eggs. Mites were transferred to a new arena (containing a
fresh leaf discs and thrips) daily and any discs with missing mites
were not scored. There were 3 replicate dishes per treatment and
the study was repeated 5 times for both choice and no choice tests