Grant A Herron

Evidence of superclones in Australian cotton aphid Aphis gossypii Glover (Aphididae: Hemiptera)

BACKGROUND Aphis gossypii is an important pest of cotton that has developed resistance to many chemicals used for its control. Any lack of understanding of its genetic structure, resistance status and host plant specialisation hampers effective management. RSULTS Eight microsatellite markers were genotyped for a collection of Australian A. gossypii field isolates from 55 plant species from major Australian cotton-producing regions. The aphids pirimicarb resistance status linked to the ACE1 (acetylcholinesterase) S431F mutation was determined by PCR-RFLP. Overall, the genetic diversity was low and there were only 13 multilocus genotype (MLG) groups found in a total of 936 aphids, suggesting asexual reproduction. Three MLGs (Aust-01, Aust-02 and Aust-04) represented 78% of all aphids tested. MLGs Aust-01 (41%) and Aust-02 (18%) were linked to the ACE1 S431F mutation and found on cotton and a range of hosts. Aust-04 (19%) hosted mainly on cotton (but also Asteraceae and Malvaceae) was predominantly susceptible to pirimicarb. Given their abundance and widespread occurrence, these three clones were considered to be superclones. CONCLUSION The study demonstrated that any strategy to control A. gossypii and manage pirimicarb resistance should target A. gossypii strains of all MLG types residing on any plant species and not just cotton

A Potter spray tower bioassay of two petroleum spray oils against adult female Panonychus ulmi (Koch) and Tetranychus urticae Koch (Acari: Tetranychidae)

A Potter spray tower was used to determine the susceptibility of adult female European red mites (Panonychus ulmi (Koch)) and two-spotted spider mites (Tetranychus urticae Koch) to suffocation by petroleum spray oils (PSOs). The LC95 values calculated for P. ulmi against C23 Ampol D-C-Tron NR and C21 Caltex Lovis were 104 and 165 μg cm-2, respectively. For T. urticae the values were 169 and 207 μg cm-2, respectively. The results are consistent with established relationships between PSO efficacy and increasing PSO molecular weight. The results are discussed in relation to previously reported bioassays on spider mites and to concurrent evaluations of PSOs for use in pome fruit integrated pest management programmes in Australia.

Clofentezine and fenbutatin oxide resistance in the two-spotted spider mite, Tetranychus urticae Koch (Acari: Tetranychidae) from deciduous fruit tree orchards in Western Australia

The resistance status of Tetranychus urticae Koch to the miticides clofentezine and fenbutatin oxide was investigated in deciduous fruit tree orchards in Western Australia. Mites were collected from 30 orchards across three fruit growing regions and subjected to discriminating–dose assays. There was a significant relationship between clofentezine (=;0.64) and fenbutatin oxide (V=;0.44) use and the proportion of resistant mites in each sample. Improved clofentezine models were obtained when the Perth Hills region (V=;0.90) was modelled separately from Donnybrook and Manjimup (V=;0.80). Clofentezine could be expected to fail in the latter regions after four sprays and in the former region after only one spray. The relatively low pest status of T. urticae in Western Australia did not delay the clofentezine resistance. For any given number of spray applications the response of T. urticae to fenbutatin oxide was more variable than it was to clofentezine.

Investigating the Effect of Invasion Characteristics on Onion Thrips (Thysanoptera: Thripidae) Populations in Onions with a Temperature-Driven Process Model

ABSTRACT A temperature-driven process model was developed to describe the seasonal patterns of populations of onion thrips, Thrips tabaci Lindeman, in onions. The model used daily cohorts (individuals of the same developmental stage and daily age) as the population unit. Stage transitions were modeled as a logistic function of accumulated degree-days to account for variability in development rate among individuals. Daily survival was modeled as a logistic function of daily mean temperature. Parameters for development, survival, and fecundity were estimated from published data. A single invasion event was used to initiate the population process, starting at 1–100 d after onion emergence (DAE) for 10–100 d at the daily rate of 0.001–0.9 adults/plant/d. The model was validated against five observed seasonal patterns of onion thrips populations from two unsprayed sites in the Riverina, New South Wales, Australia, during 2003–2006. Performance of the model was measured by a fit index based on the proportion of variations in observed data explained by the model (R2) and the differences in total thrips-days between observed and predicted populations. Satisfactory matching between simulated and observed seasonal patterns was obtained within the ranges of invasion parameters tested. Model best-fit was obtained at invasion starting dates of 6–98 DAE with a daily invasion rate of 0.002–0.2 adults/plant/d and an invasion duration of 30–100 d. Under the best-fit invasion scenarios, the model closely reproduced the observed seasonal patterns, explaining 73–95% of variability in adult and larval densities during population increase periods. The results showed that small invasions of adult thrips followed by a gradual population build-up of thrips within onion crops were sufficient to bring about the observed seasonal patterns of onion thrips populations in onion. Implications of the model on timing of chemical controls are discussed.

Efficacy of two thiamethoxam pre-germination seed treatments and a phorate side-dressing against neonicotinoid- and pirimicarb-resistant cotton aphid, Aphis gossypii (Hemiptera: Aphididae)

In a glasshouse trial with potted cotton plants grown from thiamethoxam‐treated seed, neither 2.76 g a.i./kg seed (Cruiser®) nor 5.52 g a.i./kg seed (Cruiser Extreme®) protected plants from neonicotinoid‐resistant Aphis gossypii Glover, 1877. Against susceptible A. gossypii each treatment was highly effective, providing control of >90% for 42 days. Continued use of either thiamethoxam treatment against resistant A. gossypii will select for resistant phenotypes and probably restrict the useful life of the neonicotinoid insecticides against this pest. In a separate trial, side‐dressing of cotton seed with phorate 200 g/kg (Thimet®) effectively provided plants with protection from susceptible A. gossypii. The insecticidal activity of phorate‐treated plants against pirimicarb‐resistant A. gossypii was not statistically different to untreated plants (P > 0.05). To maintain the effectiveness of pirimicarb in the Australian cotton integrated pest management strategy, the use of phorate as an alternative pre‐germination treatment to thiamethoxam for aphid control must be managed. We recommend that the first foliar spray applied to cotton treated with phorate at planting should not be pirimicarb or any other insecticide affected by insensitive cholinesterase (ACE1) type resistance.

The stability of tebufenpyrad resistance in two-spotted spider mite (Acari: Tetranychidae) under laboratory conditions.

The successful use of chemical rotations to manage insecticide resistance requires reversion between alternate chemical applications. We tested a tebufenpyrad resistant population of Tetranychus urticae Koch after some 55 months laboratory culture without pesticide selection and found LC50 level resistance had dropped from 63.29- to 2.41-fold. However, the population was still heterogeneous with LC99 level resistance at 38.03-fold. It is likely that a lack of reversion contributed directly to the initial tebufenpyrad control failure.

A significant fitness cost associated with ACE1 target site pirimicarb resistance in a field isolate of Aphis gossypii Glover from Australian cotton

The aphid Aphis gossypii Glover is an important pest of Australian cotton and has developed resistance to many chemicals used for its control. Its resistance management is partially based on chemical rotation that relies on insecticide resistance being associated with fitness costs. Therefore, understanding fitness costs associated with insecticide resistance is critical to its sustainable resistance management. We studied the fitness cost of pirimicarb resistance in A. gossypii caused by a single mutation in the acetylcholinesterase gene ACE1 by mixing different ratios of susceptible and resistant aphids. This was achieved by establishing A. gossypii populations of a known starting allele frequency in aphid proof cages and measuring allele frequency change over time via qPCR. Unlike traditional cohort fitness studies, we used competitive fitness as a measure of relative fitness of resistant versus susceptible aphids in the same environment. We demonstrate that competitive fitness measured in this study is an accurate predictor of overall relative fitness. We found that pirimicarb resistance had a significant fitness cost in the presence of susceptible aphids in the absence of insecticide pressure and that the fitness cost was related to the initial resistance allele frequency. By using the competitive fitness measure and knowing the initial allele frequency, it is possible to predict the likely time from resistant to an essentially susceptible population. As resistance was stable in the absence of susceptible competition, we recommend the use of resistance management tactics that do not completely eliminate the susceptible genotype such as complimentary integrated pest management.

Baseline susceptibility and cross‐resistance in Aphis gossypii Glover (Aphididae: Hemiptera) to phorate and sulfoxaflor

Susceptible discriminating doses of phorate (0.2 g/L) and sulfoxaflor (0.01 g/L) against cotton aphid Aphis gossypii Glover were determined by laboratory bioassay where aphids were sprayed with insecticide with the aid of a Potter spray tower. All of the populations tested were susceptible to sulfoxaflor, and only a pirimicarb resistant strain had cross‐resistance to phorate. If phorate is used as a side dressing in Australian cotton for insect control, neither pirimicarb, or any other chemical associated with insensitive acetylcholinesterase type one resistance, should be used as the first foliar spray for any subsequent aphid control.

The Management of Insect Pests in Australian Cotton: An Evolving Story

The Australian cotton industry progressively embraced integrated pest management (IPM) to alleviate escalating insecticide resistance issues. A systems IPM approach was used with core principles that were built around pest ecology/biology and insecticide resistance management; together, these were integrated into a flexible, year-round approach that facilitated easy incorporation of new science, strategies, and pests. The approach emphasized both strategic and tactical elements to reduce pest abundance and rationalize decisions about pest control, with insecticides as a last resort. Industry involvement in developing the approach was vital to embedding IPM within the farming system. Adoption of IPM was facilitated by the introduction of Bt cotton, availability of selective insecticides, economic validation, and an industry-wide extension campaign. Surveys indicate IPM is now embedded in industry, confirming the effectiveness of an industry-led, backed-by-science approach. The amount of insecticide active ingredient applied per hectare against pests has also declined dramatically. Though challenges remain, pest management has transitioned from reactively attempting to eradicate pests from fields to proactively managing them year-round, considering the farm within the wider landscape.

Can resistance management strategies recover insecticide susceptibility in pests?: a case study with cotton aphid Aphis gossypii (Aphididae: Hemiptera) in Australian cotton

Cotton pest management in the Australian cotton industry was highly pesticide reliant from its inception in the early 1960s until the late 1990s. Aphids were controlled effectively over this period either co‐incidentally by compounds applied against Helicoverpa spp. or by targeted applications of dimethoate/omethoate or pirimicarb in the late 1990s. By cotton season 1999–2000, after this prolonged period of selection, the cotton aphid Aphis gossypii Glover showed resistance to pirimicarb and cross resistance to dimethoate/omethoate as well as resistance to older organophosphates (profenofos, chlorpyrifos‐methyl, chlorpyrifos‐ethyl) targeted against Helicoverpa spp.. No new chemical controls were available so effective control required development of an insecticide resistance management (IRM) strategy incorporated with integrated pest management (IPM) strategies designed to maximalise any known selective disadvantage for resistant individuals. The IRM component emphasised rotation between insecticide mode of action (MOA) groups and limited the number of applications of any MOA. The IPM component incorporated farm hygiene to reduce overwinter hosts for resistant aphids, conserved natural enemies and incorporated effective use of pest sampling and thresholds. The aphid management strategy was fluid, evolving and adapting as new knowledge, new resistance and further IPM or IRM tactics became available. For instance, in 2007–2008 resistance was detected in aphids to the neonicotinoids resulting in field failures. Detection of neonicotinoid resistance required modifications to the aphid management strategy to address prolonged selection by both neonicotinoid seed treatments and foliar applications. The strategy also considered the need to manage concurrent pest species to mitigate the risk that insecticides sprayed against those pests would coincidently cause aphid outbreaks or select for resistance in aphids. The integrated strategy has now almost completely recovered susceptibility to IPM friendly pirimicarb and resistance to neonicotinoid insecticides has declined dramatically.