Management of citrus thrips to reduce the evolution of resistance

Transcription

1 Management of citrus thrips to reduce the evolution of resistance Joseph Morse and Beth Grafton-Cardwell Editor s Note: Work on citrus thrips is now a part of the CRB s Core Program of Integrated Pest Management research with Drs. Grafton-Cardwell and Morse as lead investigators. Background The citrus thrips, Scirtothrips citri (Moulton), is one of the few pests of California citrus which is native to California. In this case, the exotic organism is the citrus tree, and the native is citrus thrips. Dudley Moulton, a USDA scientist, named this insect in 1909 (calling it the orange thrips) after damage to citrus in southern Kern County made it clear that the characteristic surface scarring of citrus fruit that had been seen for many years was not wind rubbing or cold injury as had previously been thought. Even to the present day, it can be a challenge to differentiate citrus thrips damage from wind-caused fruit scarring. We like to use the presence of a partial or complete ring scar around the button as one good criterion this is normally present with citrus thrips damage. In addition, checking fruit for damage shortly after petal fall will reveal the early stages of citrus thrips-induced fruit scarring. The article listed in Further Reading, Grafton-Cardwell et al. 2003, contains pictures that aid in differentiating citrus thrips fruit scarring from other types of fruit injury. The life cycle of citrus thrips Citrus thrips starts its life as an egg laid inside young leaves, twigs, flowers, or fruit. The most damaging stages are the first and second instar larvae (Figure 1), because they prefer to hide under the button when the fruit is small, concentrating their feeding in that area, which causes the characteristic ring scar as the fruit expands. Mature second instar larvae crawl towards the inside, dark portions of the 22 Citrograph March/April 2012 tree, looking for a place to hide, where they pass through two relatively inactive and non-feeding stages, the propupa and pupal stages. Typically, about onethird of the thrips pupate in cracks and crevices in the tree. and two-thirds drop to the soil to pupate in the upper layers of the leaf duff and soil beneath a tree. Fig. 1. It is the first and second instar (larger, above) citrus thrips that cause most fruit scarring. Photo by Jack Kelly Clark, courtesy UC Statewide IPM Program. Fig. 2. The predaceous mite Euseius tularensis will feed to some degree on citrus thrips (here feeding on a late second instar thrips). Photo by Jack Kelly Clark, courtesy UC Statewide IPM Program. The adults then emerge, mate, and produce the next generation of thrips. Adult females concentrate their feeding in one area to a lesser extent than larvae, and males don t feed all that much or live very long -- thus they are not considered as damaging as the larvae in most situations. There are eight or more generations of citrus thrips attacking the leaves and fruit of citrus over the spring, summer, and fall. The first generation in the spring feeds on the leaf flush prior to bloom, and the second generation of larvae typically appears about the time of petal fall. It is the second and third generation of citrus thrips that normally are of economic concern. Small fruit are fairly susceptible to citrus thrips scarring, and as the fruit grows it becomes less susceptible. Once fruit are larger than about 1.5 inches in diameter, citrus thrips do not typically cause economic scarring because the fruit is tough enough to make extensive feeding difficult (thrips feed best on tender leaf and fruit tissue). Coastal lemons are an exception to the above scenario because multiple fruit sets are produced over the year. Rather than being a spring pest, citrus thrips on coastal lemons typically isn t a concern until the mid-to-late summer fruit set. Natural enemies of citrus thrips We have searched for many years for ways in which to manage citrus thrips non-chemically and, in particular, with natural enemies. A number of natural enemies will feed on citrus thrips, but, unfortunately, in years when thrips levels are high they can cause substantial fruit injury in a relatively short period of time. Natural enemies often cannot keep up with the rapid growth of spring thrips populations. The only stages readily available for natural enemy attack are the first and second instar larvae. The two pupal

2 stages are typically hidden in cracks or crevices in the tree or the soil, the winged adults are difficult for most predators to capture, and the egg stage is fairly protected inside plant tissue. Thus, any predator species trying to make a living off citrus thrips has sporadic availability of the larvae and cannot respond well numerically to increased thrips levels within a particular year. Triapitsyn & Morse (1999) searched for wasp parasitoids attacking citrus thrips and, although they found low levels of two parasitoid species on laurel sumac (a common native host, see below), these insects were not found associated with citrus. The citrus thrips has adapted over time to citrus, but perhaps the parasitoids have not, or they are not present at high enough levels to be detected easily. Jones & Morse (1995) used isoelectric gel electrophoresis to study to what degree the predaceous mite Euseius tularensis (Figure 2) feeds on citrus thrips, as this predator has been proposed as one of the more common natural enemies of citrus thrips on California citrus. Only 7 of 556 (1.3%) adult female E. tularensis tested positive for citrus thrips in their gut. Given this, we wonder if E. tularensis perhaps reduces citrus thrips levels only when the pest first starts to build from low levels but not when both species are present at moderate to high levels. Euseius spp. are generalist predators that feed on pollen, mites, insects and leaf sap, so they are not specifically tracking citrus thrips populations. Grafton-Cardwell demonstrated that pruning and fertilizing trees generated higher densities of Euseius than augmentative releases by providing the environment Euseius prefers. It is generally accepted that densities of >0.5 Euseius per leaf are associated with good citrus thrips control, but it is possible that the presence of this level of Euseius is indicative of good biological control in general because a suite of natural enemies provide citrus thrips suppression rather than Euseius specifically. Monitoring for citrus thrips Strategies used by pest control advisors and growers for managing citrus thrips vary. PCAs typically monitor citrus thrips levels on young, developing fruit immediately after petal fall to decide if treatments are needed. Post-petal 24 Citrograph March/April 2012 Fig. 3. We rate navel orange scarring caused by citrus thrips on a 0-4 scale where 0 = no scarring by citrus thrips (not shown), 1 and 2 are slight scarring (not sufficient to cause fruit to be downgraded from first to second grade) and 3 and 4 are severe (economic scarring). The threshold for fruit downgrading varies from year to year but is typically set at a level of 3 scar or worse. fall treatments are not needed every year. This is because thrips levels vary from year to year, and also the timing of when the second and third generation of immature thrips appears in relation to fruit size varies. In the San Joaquin Valley, wet weather during bloom typically results in lower thrips levels after petal fall, in part due to greater mortality of the pupae in the soil. Careful monitoring can reveal orchards that have low levels of immature thrips on young fruit, and treatments can be delayed or eliminated altogether. Reducing the number of treatments will reduce the selection pressure for resistance to insecticides. Problems controlling citrus thrips in a particular grove are more likely if one or more treatments are used each year in contrast to a treatment perhaps being used only 5 years out of 10 years based on sampling for thrips severity each year. Insecticides can aggravate thrips populations Over the period , we ran citrus pesticide screening trials in Field 12 (Atwood navel oranges) at the Lindcove Research and Extension Center (LREC). Untreated control plots were always included in order to assess how much fruit scarring would result if no treatment were applied. We used a 0-4 rating scale to assess the severity of citrus thrips-caused fruit scarring (see Figure 3), and scarring levels 3 and 4 were categorized as economic scarring. We set the threshold for economic scarring as the level that would typically lead to fruit being downgraded from first to second grade. Over the 20 years (data prior to 1981 from O. L. Brawner and Dr. Bill Ewart), citrus thrips economic scarring on untreated trees ranged from 1.2% (1986) to a maximum of 69.0% (1988) on outside lower fruit with a mean of 30.2% economic scarring. In contrast, over the 12 years , economic scarring ranged from 0.1% (2000) to a maximum of 10.7% (1997) with a mean of 4.4%. Clearly, something changed dramatically between these two time periods. The maximum level of severe scarring over the latter time period was about 1/3 of the average level over the earlier period. We believe the reason for this is that citrus thrips is, to a considerable degree, a pesticide-induced pest.

3 During the first time period, , broad-spectrum organophosphate, carbamate, and pyrethroid insecticides were used in the SJV and in the test areas of this field at Lindcove for citrus thrips control. Many pest control advisors mentioned to us that spraying thrips only makes them mad. They found that if the first spray did not control them well, they came back at very high levels and were more difficult to control. We believe that what was happening was that citrus thrips populations had developed resistance in some areas and to varying degrees to organophosphates, carbamates, and/or pyrethroids. The level of resistance varied greatly depending on how often and which chemicals had been used in the past and how long thrips had NOT been exposed to that chemistry so that resistance could revert. When citrus thrips are sprayed with a broad-spectrum insecticide (as is the case for most products in these three classes of chemistry, see Table 1) these sprays reduce most natural enemies that might help hold the thrips in check. If the thrips are somewhat resistant, they are not completely killed. Hormoligosis is the term used to describe the stimulation of insects or mites when they are exposed to sublethal rates of pesticides or other toxins. As pesticide residues drop to sub-lethal rates, citrus thrips can be stimulated (depending on pesticide and dose) to lay more eggs, contributing to a resurgence of the thrips population several weeks later. Hormoligosis the stimulation of insects or mites when they are exposed to sub-lethal rates of pesticides or other toxins. What happened to contribute to lower thrips levels over ? The relatively soft insecticides Agri-Mek and Success (Entrust is the organic version) were registered for use on California citrus in 1994 and 1998, respectively, and growers largely switched to using those products, especially Success, for citrus thrips control (see Figure 4). In addition, growers switched to Esteem or Applaud for red scale control, in both cases replacing broad-spectrum organophosphate, carbamate, and pyrethroid treatments with softer insecticides that allowed more natural enemies to survive. Thus, although citrus thrips can still cause economic damage when weather conditions are conducive, in general, citrus thrips is less of a problem than it used to be. Whereas citrus thrips insecticide screening trials at LREC were quite productive prior to 2003, once the organophosphate era ended, it was difficult to consistently see differences between fruit scarring on trees treated with the standard, effective product versus levels on untreated control trees. We shifted in 2004 to screening experimental pesticides on what we believe is one of the major natural hosts of citrus thrips in California (before citrus was introduced), laurel sumac, in greenhouse trials (see Morse 1995). Treatments on non-bearing citrus Some growers and PCAs believe that treating citrus thrips on non-bear- Table 1. Pesticides that might be used in rotation for citrus thrips control. Trade name Common name Pesticide class Mode of Action a Critical as part Resistance situation with Comments and application of future ACP citrus thrips methods to improve efficacy control? Dimethoate (and Dimethoate Organophosphate 1B Yes Resistance in some areas Moderately systemic material generics) depending on the degree of past organophosphate use Carzol Formetanate Carbamate 1A No? Resistance in some areas hydrochloride depending on the degree of past carbamate use Veratran D Sabadilla alkaloids Botanical unclassified No Resistance not yet seen Adding 1-2 gallons of with citrus thrips (seen with molasses/acre assists with avocado thrips) efficacy and persistence; Critical to reduce spray tank ph to 4.5 prior to adding material; Works poorly in cold weather (active only as a stomach poison) Baythroid XL Beta-cyfluthrin Pyrethroid 3A Yes Resistance in some areas Danitol Fenpropathrin depending on the degree of Mustang Zeta-cypermethrin past pyrethroid use Agri-Mek (and Abamectin Chloride channel 6 Somewhat Possible cross-resistance Translaminar, add oil (1/4% generics) activator (adults) with class 5 or more) to aid leaf penetration and persistence Success Spinosad Spinosyn 5 Delegate Possible cross-resistance Translaminar, add oil (1/4% Entrust Spinosad (organic) Yes with class 6 or more) to aid leaf Delegate Spinetoram penetration and persistence Movento Spirotetramat Inhibitor of acetyl 23 Yes Resistance management Highly systemic; add oil to CoA carboxylase critical to protect this improve leaf penetration material s efficacy for ACP, (surface residues are NOT red scale, and citrus thrips active) control a The IRAC MoA (mode of action) for each class of chemistry (see 26 Citrograph March/April 2012

4 ing, young citrus has value in terms of enhancing tree growth and/or bringing the tree into production sooner. We suggest it is perhaps worthwhile treating for citrus thrips only in year 1, when the trees are first planted in the ground to ensure they get a good start. For older trees, we admit that the leaf scarring of young leaves by citrus thrips is unsightly (Figure 5), but does citrus thrips really slow the growth of young trees, if they are well irrigated, well watered, and otherwise healthy? We believe the answer to that is no, based on two research trials reported in Grafton-Cardwell et al. (1997) in the San Joaquin Valley. The first study was done on navel oranges at LREC using 50 single-tree replicates over a three-year period. Treatment 1 never received any pesticides, and treatments 2-6 received 2 summer treatments in year 1. In years 2 and 3, the treatments were: (2) no citrus thrips treatments; (3) 2-3 spring treatments/year; (4) 1 fall treatment/ year; (5) 2-3 spring and 1 fall treatment each year; and (6) 2-3 spring, 4 summer, and 1 fall treatment each year. Thus, trees received as many as 17 treatments over a three-year period. As an indication of tree growth, we measured trunk circumference at 4 cm above the bud union 1, 2, 3, and 4 years after the trees were planted. To summarize the results, none of the 6 treatments had a differential impact on tree growth; that is, we could detect no difference in tree size during years 1-3 whether they were untreated, treated with 17 treatments, or with an intermediate number of treatments. The only significant effect we measured was a loss in citrus thrips susceptibility to Carzol. The second study was done with commercial Valencia oranges planted in April in Fresno County over a threeyear period. A 20-acre block was divided into 18 plots of trees each, and 9 plots were randomly assigned to be treated with (1) no citrus thrips treatment over years 1-3 or (2) grower choice of treatments including a range of insecticides used for citrus thrips control (7 treatments in year 1, 7 in year 2, 5 in year 3). The entire field was treated after petal fall in the spring of year 4 to protect fruit from citrus thrips scarring. Again, we measured trunk diameter in years 1-4 and saw no difference between 0 treatments and, in this case, 19 treatments over years 1-3. In this study, we also measured fruit production in year 3. Although we saw a slight numerical trend with somewhat more fruit in the treated plots, this difference was not statistically significant. By year 4, this slight numerical trend had disappeared (an average of fruit per tree on treated trees, on untreated trees). Our conclusion for both the navel and Valencia studies is that despite leaf scarring caused by citrus thrips being unsightly, treating young citrus very much, if at all, is likely wasting money and can significantly contribute to the Acres Treated Insecticides Used for Citrus Thrips & Katydid Control in the San Joaquin Valley 450, , , , , , , ,000 50, Cygon Carzol Baythroid Baythroid XL Danitol Agri-Mek Veratran Success Delegate evolution of pesticide resistance. We strongly suggest that growers not treat citrus thrips on non-bearing citrus except perhaps the first year when trees are very small. The cost, in terms of losing the efficacy of pesticides to resistance, is too high and this is going to be even more important once treatments are needed for Asian citrus psyllid (ACP) control. The history of citrus thrips pesticide resistance Table 2 shows that over the years, citrus thrips has evolved pesticide resis- Fig. 4. Damage of young flush on citrus can be unsightly but does not warrant treatment except perhaps on very young trees just after planting. Photo by Jack Kelly Clark, courtesy UC Statewide IPM Program. Fig. 5. Insecticide acreage treated with various insecticides for citrus thrips and katydid control demonstrating the changes in grower uses during for the San Joaquin Valley. Totals for Cygon and Agri-Mek include generic formulations of the same chemical March/April 2012 Citrograph 27

5 tance to a number of different pesticides from DDT to pyrethroids in as short a time as 1 year (to Dieldrin after resistance to the related DDT had appeared) and in as many as 18 years (dimethoate). We now have evidence for resistance to Delegate in a population of citrus thrips in the San Joaquin Valley. Citrus thrips ability to rapidly develop resistance concerns us each time growers consistently rely on one or a small number of insecticides for the control of a particular pest (citrus thrips, red scale, ACP, etc.). Unfortunately, we often do not have as many effective pesticides from different classes of chemistry available for rotation as is desirable. What is meant by the word resistance? There are a number of definitions, but we like two of the more common ones: (1) if the resistant population of insect or mite develops a >10-fold increase in the LC 50 or LC 90 (the pesticide concentrations needed to kill 50 or 90% of the population, respectively) or (2) if one sees clear evidence of a lack of field control (either complete failure or reduced persistence) when the material is used properly. The first definition recognizes that there is variability in the response of various insects and mites to pesticides, and a population has not developed resistance until at least a 10-fold level has been reached. The latter definition is more of an operational term when the material stops working or works less well, resistance has occurred. In most cases, laboratory measurements and field experience correlate well. Researchers typically try to take baseline resistance data in the labora- tory before a pesticide is used widely so they can later watch for changes in responses to the insecticide and confirm those changes based on field observations. Sometimes resistance to one insecticide also confers resistance to another insecticide. This is known as cross resistance. One way that insects resist pesticides is by breaking down (metabolizing) the pesticide more quickly; this is called metabolic resistance. Citrus thrips that have evolved resistance to organophosphates have increased levels of enzymes that break down the organophosphate relatively quickly compared to susceptible strains, and this also gives them resistance to other organophosphate and carbamate insecticides (MoA category 1, Table 1). A second type of resistance is due to an altered target site for the pesticide. The rapid development of citrus thrips resistance to pyrethroids may be due to previous exposure to DDT because these two types of pesticides have similar target sites (MoA 3). Following the registration of Success (spinosad) for use on California citrus in 1998, Success and Entrust were widely used for citrus thrips control, accounting for an average of 43% of the spring thrips/katydid treatments in the San Joaquin Valley between 1999 and 2007 (Figure 4). Delegate (spinetoram) was registered in 2007, is in the same class of chemistry as Success (cross resistance is expected), and is somewhat more effective and persistent against citrus thrips than Success. The use of Delegate until recently was hampered by the lack of MRLs (maximum residue limits) by some of the foreign trading partners to which California citrus is shipped. Each year over the last six years or so, we have offered to test for citrus thrips susceptibility to either Success or Delegate based on baseline data we took before these products were widely used. Each year, we have been pleasantly surprised to see a lack of documentable resistance. However, following a report of poor citrus thrips control from a Delegate application during the spring of 2011, we measured a significant increase in the spinetoram LC 50 for citrus thrips collected at this location (9.4 to 19.8-fold higher LC 50 than baseline values determined in 2008). Actually, we should feel very fortunate that it has been 14 years before the first signs of field resistance were observed with the Success/Delegate chemistry in the San Joaquin Valley. The need to manage citrus thrips resistance We plan to continue to monitor the Delegate resistance situation to determine whether this is a fairly isolated incident and whether such resistance is developing in other areas of the SJV. In addition, we are accelerating the testing of new products and chemistries which might be used to help manage citrus thrips resistance (several are moving closer to registration). As mentioned by Morse & Grafton- Cardwell (2009), once Asian citrus psyllid enters the SJV, it will be even more important to manage pesticide resistance by rotating between products with different modes of action, as many Table 2. Partial history of citrus thrips pesticide resistance evolution in California. Pesticide common Class of chemistry Mode of Action a Year first used Year first field name commercially failure reported Reference DDT Sodium channel modulator Morse & Brawner 1986 Sabadilla + sugar Botanical bait unclassified Morse & Brawner 1986 Dieldrin GABA-gated chloride channel antagonist 2A Morse & Brawner 1986 Malathion Organophosphate 1A Morse & Brawner 1986 Dimethoate Organophosphate 1A 1962 b 1980 Morse & Brawner 1986 Carzol Carbamate 1B early 1980s 1986 Immaraju et al Baythroid Pyrethroid Morse & Grafton-Cardwell 2009 Abamectin Macrocyclic lactone Success + oil Spinosyn Delegate + oil Spinosyn Morse et al. unpublished Movento + oil Acetyl CoA carboxylase inhibitor a The IRAC MoA (mode of action) for each class of chemistry is listed (see Cross-resistance is expected between chemicals with the same mode of action. b Non-bearing (limited) use only, until Citrograph March/April 2012

6 of the materials that are effective against citrus thrips also will assist in control of ACP (see Table 1). There are currently six modes of action (1, 3, 5, 6, 23, and unclassified) for insecticides registered for citrus thrips control (Table 1). The best advice regarding resistance management for citrus thrips, or most pests for that matter, is to: (1) minimize pesticide use to the extent that is possible by sampling carefully to make sure the treatment is needed; (2) maximize the use of non-chemical control methods; (3) rotate among effective available chemistries to the maximum extent possible (clearly understand what mode of action each pesticide has and where the potential for cross resistance exists); and, (4) when a treatment is needed, apply it at the optimal time and with the best possible application method so as to avoid the need for re-treatment (Table 1). We view the first observation of citrus thrips resistance to Delegate in 2011 as an early warning. To maintain the effectiveness of Delegate and Success against citrus thrips, avoid making more than one application of a Group 5 insecticide (Delegate, Success, or Entrust) to a block each year. If additional applications are needed, other effective insecticides with different modes of action should be used. Also, try to make applications to adjacent blocks or groves at the same time or within a few days of each other to have an area-wide impact and thus slow re-infestation. It is important that we hold this situation in check as best we can until new chemistries become available for citrus thrips control. It is hoped we will have at least one new chemistry to use against citrus thrips prior to the 2013 spring field season. Acknowledgements We would like to thank the Citrus Research Board for funding to support in part the research described herein. We also thank Alan Urena, Lindsay Robinson, Pamela Watkins, and Heavenly Clegg for technical support and past graduate students/postdoctoral scientists Tim Grout, Bill Wiesenborn, Alex Rhodes, John Immaraju, Nasser Zareh, Jim Ferrari, Steven Jones, Heinrich Schweizer, Inamullah Khan, Kris Tollerup, and Dr. Serguei Triapitsyn for contributing to past citrus thrips research efforts which lead to some of the information in this article. Photographs 1, 2, and 4 were provided by Jack Kelly Clark, courtesy of the UC Statewide IPM Program and are copyrighted by the Regents of the University of California. Dr. Joseph G. Morse is a Professor of Entomology and Dr. Beth Grafton- Cardwell is an Extension Specialist and Research Entomologist. Both are members of the Department of Entomology, University of California Riverside. Further reading Grafton-Cardwell, E.E., J.G. Morse, and A. Gjerde Effect of Insecticide Treatments to Reduce Infestation by Citrus Thrips (Thysanoptera: Thripidae) on Growth of Nonbearing Citrus. Journal of Economic Entomology 91(1): Grafton-Cardwell, E.E., N.V. O Connell, C.E. Kallsen, and J.G. Morse Photographic Guide to Citrus Fruit Scarring. University of California Division of Agriculture and Natural The SOURCE for all your citrus tree needs SuperCitrus Trees B&B Trees Seedlings Starter Trees/Citrus Liners Rootstock Seed Budwood of all Types W W W. C I T R U S T R E E S O U R C E. C O M March/April 2012 Citrograph 29

7 Uniquely effective products for controlling major pests in citrus with minimal disruption to IPM programs. Citricola Scale Cottony Cushion Scale California Red Scale Citrus Red Mite Two-spotted Spider Mite Bud Mite Contact Your area Nichino America sales representative to learn more Nichino America, Inc. All rights reserved. APPLAUD and FujiMite are trademarks of Nichino America, Inc. Farm Safely. Always read and follow label directions Resources Publication 8090, Oakland, CA. 8 pp. Haviland, D.R., S.M. Rill, and J.G. Morse Southern Highbush Blueberries Are a New Host for Scirtothrips citri (Thysanoptera: Thripidae) in California. Florida Entomologist 92(1): Jones, S.A. and J.G. Morse Use of Isoelectric Focusing Electrophoresis to Evaluate Citrus Thrips (Thysanoptera: Thripidae) Predation by Euseius tularensis (Acari: Phytoseiidae). Environmental Entomology 24(5): Immaraju, J.A., J.G. Morse, and D.J. Kersten Citrus Thrips (Thysanoptera: Thripidae) Pesticide Resistance in the Coachella and San Joaquin Valleys of California. Journal of Economic Entomology 82(2): Immaraju, J.A., J.G. Morse, and R.F. Hobza Field Evaluation of Insecticide Rotation and Mixtures as Strategies for Citrus Thrips (Thysanoptera: Thripidae) Resistance Management in California. Journal of Economic Entomology 83(2): Lovatt, C.J., S.M. Streeter, T.C. Minter, N.V. O Connell, D.L. Flaherty, M.W. Freeman, and P.B. Goodell Phenology of Flowering in Citrus sinensis (L.) Osbectk, cv. Washington navel orange. Proceedings of the International Society of Citriculture 1: McMurtry, J.A. and B.A. Croft Life-styles of Phytoseiid Mites and Their Roles in Biological Control. Annual Review of Entomology 42: Morse, J.G Prospects for IPM of Citrus Thrips in California. Pp , In: Thrips Biology and Management. Proceedings, 1993 International Conference on Thysanoptera, Towards Understanding Thrips Management. Editors: B.L. Parker, M. Skinner, T. Lewis. Sept , 1993, Burlington, VT. Plenum, New York, NY. 636 pp. Morse, J.G. and O.L. Brawner Toxicity of Pesticides to Scirtothrips citri (Thysanoptera: Thripidae) and Implications to Resistance Management. Journal of Economic Entomology 79(3): Morse, J.G. and H. Schweizer Citrus Thrips Resistance A Problem Requiring Grower and PCA Restraint. Citrograph 81: Morse, J.G. and E.E. Grafton- Cardwell Bear Citrus Thrips Resistance in Mind When Deciding Whether and How to Treat in Topics in Subtropics 4: Morse, J.G. and N. Zareh Pesticide-Induced Hormoligosis of Citrus Thrips (Thysanoptera: Thripidae) Fecundity. Journal of Economic Entomology 84(4): Morse, J.G. and E.E. Grafton- Cardwell Managing Insecticide Resistance will be Key to the Future of Effective Citrus Pest Management. Topics in Subtropics 7(1): 6-8. Morse, J.G., E.E. Grafton-Cardwell, and A.A. Urena Management Options for Citrus Thrips in the San Joaquin Valley. Citrograph 86: 4-5, 12. Rhodes, A.A., J.G. Morse, and C.A. Robertson A Simple Multigeneration Phenology Model: Application to Scirtothrips citri (Thysanoptera: Thripidae) Prediction on California Oranges. Agriculture, Ecosystems and Environment 25(4): Triapitsyn, S.V. and J.G. Morse Survey of Parasitoids of Citrus Thrips, Scirtothrips citri (Moulton, 1909), in Southern California. Russian Entomology Journal 8(1): l 30 Citrograph March/April 2012