In California, we have a long history of citrus thrips and California red scale developing resistance to insecticides. When insecticides from the same class of chemistry are used repeatedly or for long periods of time, insecticide resistance appears, and the first sign of resistance is a reduction in how long an insecticide controls a population.

For example, when Cygon (dimethoate) was first used for citrus thrips control in 1962, thrips levels typically remained at very low levels for six to eight weeks after treatment. In the early 1980s, resistant citrus thrips populations were seen in some parts of the San Joaquin Valley and growers began to see moderate levels of live thrips three to four weeks after treatment and, in some cases, as early as one to two weeks following a spray (Morse & Brawner 1986).

Similarly, in the early 1990s, a number of populations of California red scale began to show resistance to broad-spectrum organophosphate (Lorsban and Supracide) and carbamate (Sevin) insecticides in the San Joaquin Valley (Grafton-Cardwell and Vehrs 1995, Grafton-Cardwell et al. 2001). Instead of these insecticides providing season-long control of California red scale, treatments only lasted one generation. As resistance increased, growers needed to treat for California red scale two to three times per season.

Citrus thrips resistance to dimethoate was followed by heavy use of Carzol and resistance to this insecticide appeared in the late 1980s (Immaraju et al. 1989), which resulted in a Section 18 being obtained allowing the use of Baythroid for six years 1991-1996. Resistance of citrus thrips to pyrethroids such as Baythroid was seen as early as 1996 (Khan et al. 1998). Success was registered in 1998, and by the turn of the century, this insecticide was the main product used for citrus thrips control (Morse et al. 2001).

In 1998-1999, two insect growth regulators, Esteem and Applaud, were given emergency registration for control of California red scale (Grafton-Cardwell 1999) because of organophosphate and carbamate resistance. Esteem acts as a juvenile hormone mimic that inhibits egg hatch and metamorphosis into the adult stage (Ishaaya et al. 1994) and Applaud is a chitinase inhibitor that inhibits molting. Although both insecticides were later registered, Esteem was shown to be more effective in controlling California red scale. Thus, Esteem rapidly became the most commonly used insecticide for control of California red scale (Grafton-Cardwell et al. 2006).

In the last several years, there have been reports of lessened control with Esteem and laboratory studies have confirmed that the early stages of resistance are starting to appear in California red scale (Grafton-Cardwell, unpublished data). Treatments of Esteem initially reduced California red scale for three years and growers are now reporting that Esteem reduces scale for only one year in some locations.

• Restraint – the best resistance management tool

For both citrus thrips and California red scale, the No. 1 tool in managing insecticide resistance is restraint. That is, do not overuse any one class of insecticide during a given period of time. As a basic resistance management practice, pest populations should be monitored carefully and treatments applied only when they are warranted. For example, if citrus thrips populations are low one week after petal fall, carefully consider whether a treatment is needed this year or consider using a different class of chemistry. Use a different class even though it doesn’t have as high efficacy (e.g., Veratran D) as other options (such as the spinosyns) that thrips have been exposed to more frequently. Low populations of California red scale might be maintained at low levels using Aphytis melinus releases, rather than using an insecticide, to avoid selecting for resistance.

When treatments are scheduled, they should be applied carefully, i.e. with proper timing of the treatment and optimal spray coverage (see Citrus Pest Management Guidelines at http://www.ipm.ucdavis. edu/ PMG/selectnewpest. citrus.html) so as to extend the time before another treatment might be needed. For example, with careful spray timing and coverage, California red scale can be treated every second or third year. Growers should maximize biological control to the degree that is possible to reduce the number of insecticide treatments applied per season. This is accomplished by minimizing the use of broad-spectrum pesticides and/or by carefully timing such treatments.

For example, spring is a critical period for biological control of many citrus pests and so broad-spectrum pyrethroids, organophosphates, carbamates, and neonicotinoids should be avoided during that period. A combination of soft pesticides and natural enemies provides longer term control than broad-spectrum insecticides.

When an insect develops resistance to one insecticide within a chemical class, it usually has resistance to all of the insecticides within that class (cross-resistance), whether they are exactly the same insecticide or a closely related insecticide. A large number of generic brands of abamectin and imidacloprid have become available in the last few years.

• Delegate, a new spinosyn

Delegate was registered for use on California citrus in 2007. Although it appears to be more effective against citrus thrips than Success (or Entrust, the organically approved formulation), it is in the same class of chemistry as Success. Cross-resistance is expected between Success and Delegate and possibly with Agri-Mek and other avermectins. Considering that many growers have been using Success since 1998, there is concern that resistance to this class of chemistry (the spinosyns) may appear in citrus thrips populations in the future. Thus, growers and pest control advisors should try not to use more than a single application of Delegate per year and if possible, should rotate to other insecticides to the degree that is possible so that Delegate is used only as much as every other year.

Possible rotation insecticides include Veratran D, Agri-Mek (and generic avermectins), Movento (but see the discussion below), and possibly dimethoate, Carzol, Baythroid, or Danitol if resistance to one or more of these insecticides is not present or they haven’t been used in a particular grove for three to five years (i.e. try them some period of time after their last use to see if resistance has reverted; it is hard to generalize how well they will perform based on our limited experience to date — generally, one might expect to obtain fair control with a use once every 3-5 years if resistance has not reached high levels).

• Movento, a new class of chemistry

Movento was registered in California in 2008 and is effective against California red scale and citrus thrips and suppressive against a number of other pests including citrus red mite, citrus leafminer, and aphids. It will be a very important product for future control of Asian citrus psyllid (as will Delegate), should that pest spread outside the current eradication area in southern California into commercial citrus. Thus, it is critical that growers and pest control advisors be conservative in using Movento during the coming years so that resistance does not develop in one or more of these pests.

We suggest that growers use Movento primarily for California red scale control (unless they have a citrus thrips-Delegate resistance concern) and strongly suggest that Movento not be used more than once a year. This is critical because of Movento’s persistence and will be even more important should Asian citrus psyllid be present. With the beginnings of California red scale resistance to Esteem showing, growers should carefully consider the variety of options that are available for control of this insect including minimizing broad-spectrum pesticide use coupled with releases of Aphytis melinus, and use of chemicals such as Movento, Esteem, Applaud and oils on a rotational basis.

When Asian citrus psyllid moves into California citrus, we will have to be very careful to not overuse any one class of chemistry such as Movento, spinosyns, avermectins, pyrethroids, and neonicotinoids. Unfortunately, relatively few new classes of chemistry are coming to the marketplace and invasive pests such as Asian citrus psyllid may require increased pesticide use, making resistance management an important consideration in optimizing a pest management program.