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Improving the management of Kelly’s Citrus Thrips in citrus: Summary, conclusions and recommendations of the 2000-04 Kelly's citrus thrips research project.

Summary

The Problem

Kelly’s citrus thrips (KCT), Pezothrips kellyanus (Bagnall), is a serious pest of navel and Valencia oranges, grapefruit and lemons in the Riverland-Sunraysia region, and to a lesser extent in Western Australia and the Riverina.  The feeding of KCT on young and mature fruit causes scurfing (or halo) marking and rind bleaching, respectively. These blemishes reduce fruit quality, thereby reducing the packout of export quality fruit and rendering some fruit unsaleable.  A 2003 survey reveals that Riverland navel orange producers alone lose around $9+ million per annum from KCT.

Given the lack of biological information about this new pest, KCT management has been limited to the use of several insecticides, which often provide poor results.  This project has built on findings of CT97007 to improve KCT insecticidal control options for citrus growers, and, in light of the CT97007 finding that KCT pupate in the soil, has set about to determine whether a sustainable IPM system based upon the biological control of KCT in citrus orchard soils is achievable.

The Project Science

The research focused in a number of key areas:

• Studies to determine the significance of resistance in Riverland populations of KCT to insecticides commonly used for their control.
• Efficacy trials to assess several new candidate insecticides for KCT control, and to generate data to assist in the registration of the effective compounds.
• Assessment of the impact of the commonly used and promising new KCT insecticides on citrus beneficials.
• Biological control studies to determine the potential impact of soil predators on KCT abundance in citrus orchards, and the orchard management factors that favour or harm these beneficials.
• Assessment of a strain of Metarhizium fungus as a mycoinsecticide for KCT control.
• Studies of KCT population movement (important for improving pest and resistance management strategies), and of the role of colour in attracting KCT to flowers (basic knowledge needed for the design of a mass trapping system).

The Key Research Findings, Extension Highlights and Industry Outcomes

• A survey has revealed a 30% increase in KCT sprays per Riverland citrus orchard over the past five years, and that an increasing number of growers are now substituting methidathion for chlorpyrifos when controlling KCT.
• These changes reflect the increasing difficulty experienced by more and more growers to adequately control KCT.
• Insecticide bioassays have confirmed that some populations of KCT in Riverland-Sunraysia citrus have substantial levels of chlorpyrifos (and to a lesser extent methidathion) resistance . The higher levels of chlorpyrifos resistance that were recorded would almost certainly be causing field control failures.
•  Baseline susceptibility levels and ‘discriminating doses’ for chlorpyrifos, methidathion, methomyl and several new candidate insecticides for KCT control have been established using a susceptible strain of KCT collected from Adelaide. This allows quick diagnosis of any shifts in susceptibility, and judgement whether field control failures are caused by resistance.
• In a laboratory study exposure of adult female KCT to sublethal doses of chlorpyrifos had no effect on their fecundity.
• Based on this finding it seems unlikely that hormoligosis (the phenomenon whereby exposure of an insect to sublethal doses of a pesticide causes an increase in its fecundity) is a cause of the 1990’s emergence of KCT as a serious new pest.  
• Insecticide trials have demonstrated the efficacy of foliar spraying of Actara™ and Success™ (and the benefit of mixing Success™ with the oil products such as Brella™) for KCT control.  The results have:
  stimulated the manufacturers of these two insecticides to develop them for KCT control in Australian citrus, and
  been provided to the manufacturers to assist with APVMA registration.
• Studies with the main red scale parasitoid Aphytis melinus have revealed a similar degree of residual contact toxicity from exposure to weathered residues of chlorpyrifos (Lorsban™) and spinosad (Success™), but a significantly greater persistence of the toxic impact of thiamethoxam (Actara™).
• That is, frequent use of either of these new candidate insecticides for KCT control would be expected to disrupt the citrus IPM system.  
• A low rate of bifenthrin applied to the ground in the drip-line area significantly reduced the emergence of adult KCT in a Riverland field trail.
• The strategic application of bifenthrin to the soil of lemon orchards (important KCT breeding source) may be a cost effective, less disruptive (at the district-wide level) option for KCT control. 
• The search for biocontrol agents has identified soil-dwelling mite populations that are predacious on KCT in Riverland-Sunraysia citrus orchards. 
• Negative correlations between the abundance of these soil predators and the survival of soil-dwelling KCT suggest an important causal link between predatory mites and low thrips numbers.  
• Several factors have been identified that appear to influence the abundance of these soil predators  (soil organic carbon and run-off from chlorpyrifos sprays, and possibly the prevalence of grasses amongst the ground-cover). 
• This provides for the first time the basis for an effective biological alternative to the insecticidal control of KCT. 
• The fungal insect pathogen, Metarhizium anisopliae var. anisopliae was investigated as an alternative KCT control strategy, but proved to be ineffective in field tests.
• The results of a suction trapping study in the Riverland suggest that there is no large-scale regional movement of KCT in this region during the spring.  Instead, KCT are moving between citrus orchards at the local (settlement) level.  It is likely that the same applies in the Sunraysia region.
• Based on this evidence, and the CT97007 observation that there are few non-citrus hosts of KCT in the Riverland-Sunraysia region, it seems that KCT in this region are largely a ‘self-contained’ population cycling within citrus.
• These findings have important implications for IRM (insecticide resistance management) and the landscape management of KCT.
• They suggest that the only source of susceptible KCT available to dilute resistance will be citrus orchards that are left unsprayed. 
• A study of the role of colour in attracting adult KCT to host flowers suggests that odour is a more significant cue than colour. 
• If the chemical components of these attractant odours could be identified, their use in a trap may significantly improve its effectiveness. 
• Significant resources were directed into the delivery of field day displays, Cittgroup presentations and print media information throughout the project.
• Growers’ understanding and practice of correct thrips identification, crop monitoring and spray control practices.  Grower and industry awareness of the threat of insecticide resistance and the “chemical treadmill”, and the importance of embracing an IPM system centred on the effective boosting of KCT natural enemies, has been significantly raised.

Industry recommendations

1. The citrus industry should encourage Syngenta and Dow Agrosciences to swiftly advance the registration applications for the use of thiamethoxam and spinosad respectively for control of KCT in citrus.
2. An IRM rotation strategy, similar to the cotton and Brassica vegetable industry strategies and based on an understanding of the cross-resistance relationships between the KCT insecticides, must be devised and implemented as soon as new chemistry is registered for KCT control.  This is essential to ensure long-term, effective insecticidal control of KCT.  The assistance of SARDI Entomology should be sought to assist the interested parties (the citrus growers and their representative bodies, the insecticide manufacturers and the insecticide distributors) with concept development (refer to pages 53-54 of this report) and the planning negotiations.
3. Given that the singular reliance on insecticidal control of KCT will be unsustainable, even if an IRM strategy is adopted, the citrus industry must fund further research to enhance the contribution of biocontrol agents to KCT control. 

Research recommendations

Further research is required in four key areas

1.    Insecticide resistance studies

Important questions requiring answer are (i) what are the cross-resistance patterns of the insecticides used for KCT control? (ii) what is the stability of the resistance to chlorpyrifos and methidathion in KCT populations? and (iii) how much KCT population gene flow is occurring between neighbouring citrus orchards?  (Answers to (i) and (ii) are required for the design of a sound IRM rotation strategy, and if the answer to (iii) indicates that sufficient amounts of population exchange are occurring, then a ‘refuge’ strategy, whereby insecticides that are used for KCT control are not used in blocks that are low risk from KCT damage (eg. mandarins and juicing Valencias), would slow the development of resistance in nearby high-risk blocks (eg. navel oranges, lemons, grapefruit).)

(Please note that post CT00015, SARDI Entomology are conducting an experiment with an OP-resistant strain of KCT to assess the likelihood of cross-resistance occurring between chlorpyrifos, methidathion and methomyl, and in turn to assess the likelihood of OP resistance conferring cross-resistance to the two new candidate insecticides for KCT control.)

2.    Insecticide field trials

Different rates and spray volumes of spinosad+BrellaÔ should be trialled to determine the most cost-effective treatment for KCT control, and higher rates of the soil-applied contact insecticides bifenthrin and fipronil should be tested.

3.    Enhancement of KCT biological control

Further research is needed to answer several key questions, namely:

• Can the boosting of organic carbon (OC) levels in Riverland-Sunraysia citrus soils cost-effectively promote KCT predator activity and biocontrol, and thereby cost-effectively reduce KCT packout losses?  If so, what is the minimum OC level required?  (SARDI Entomology and the University of Adelaide, in conjunction with Jeffries Landscape Suppliers, have initiated a pilot study post-CT00015 at two Riverland citrus orchards to determine the benefit to KCT biocontrol that results from increasing OC levels with the application of organic compost.)
• Can increasing the grass-pollen production of Riverland-Sunraysia citrus ground covers promote KCT predator activity and biocontrol?  If so, which grasses are most beneficial?
• What impacts do insecticides (chlorpyrifos, methidathion, methomyl, aldicarb, spinosad and thiamethoxam) and commonly-used herbicides have on KCT predator activity and biocontrol in Riverland-Sunraysia citrus?

4.    Irrigation modification

Further research is recommended to determine whether irrigation restriction in spring can be managed to reduce KCT pupal survival (and subsequent crop attack) without adversely affecting tree health.