Three species of Tetranychus spider mites are found in Australian cotton crops. Spider mites cause damage to cotton by feeding on individual leaf cells using their chelicerae to pierce and remove the cell contents. Tetranychus urticae (Koch) (two-spotted spider mite: TSM) has been extensively researched in cotton and can reduce photosynthetic capacity, stomatal conductance and ultimately lead to decreased yield and fibre quality in cotton. However, no research into the damage potential or ecology of Tetranychus ludeni (bean spider mite: BSM) and Tetranychus lambi (strawberry spider mite: SSM) has been conducted. This research aimed to compare the relative damage to cotton caused by each spider mite species and to investigate spider mite ecology in cotton.

Cultures of each mite species were established in a glasshouse. Glasshouse studies were performed to compare the damage caused by each species on potted cotton plants, using a leaf damage index (LDI). TSM caused twice the level of damage than BSM and three times the damage of SSM. By week five, 33% of leaves on plants infested with TSM had defoliated. No defoliation was observed for either BSM or SSM. In addition, the damage caused by SSM was never significantly different from control plants where mites were excluded.

Laboratory studies found that average development of BSM (12.3 days) and SSM (13.7 days) from egg to adult on leaf discs was significantly slower than TSM (11.39 days). However, there was no significant difference in the mean number of eggs laid per day for TSM (6.3 eggs) and BSM (5.3 eggs). The implications of life history traits and how the result might change at higher temperatures are discussed.

Competition between mite species on single potted cotton plants was investigated in two glasshouse experiments. After two weeks the total number of females and distribution on plants was compared for each species alone or in the presence of another species. Findings from the first experiment at 30.1 ± 4.5°C suggest TSM suppressed the BSM female population and displaced SSM females from individual nodes of the plant. The second experiment at 26 ± 1.1°C included an additional cotton cultivar containing Bollgard 3 traits. Significantly higher numbers of TSM females were recorded in each pairing with SSM compared with the number of females in SSM populations alone. This was an unexpected result as TSM is considered to be declining in incidence across cotton growing regions and suggests environmental factors are contributing to the changing mite species complex.

Field surveys were conducted in northern New South Wales to determine the distribution of SSM within the cotton canopy to assess the relevance of the current mite sampling protocol for this species. Surveys indicated that the current sampling protocol would be reliable for late season sampling as mite abundance was very similar throughout the cotton canopy. The survey also detected BSM for the first time in recent years in cotton and in the same fields as an established population of SSM.

The results of this study recommend modification of current practices in cotton mite management to ensure accurate identification of the species present. This may help avoid unnecessary insecticide applications for mite species that may never reach economically damaging levels. They also suggest that there are other factors in the cotton landscape contributing to the changing mite species complex. This research provides basic research to support the sustainable management of a changing mite complex in Australian cotton.

For further information, contact Chris Shafto.

Email: chris.shafto@dpi.nsw.gov.au