What is pesticide resistance?

Pesticide resistance is the ability of an insect or mite (arthropod) to survive a rate of pesticide that other individuals in the population cannot survive. This characteristic is inherited and the survivors pass the gene(s) for resistance to the next generation. The more often we spray, the faster we remove the susceptible individuals and select for a population that has mostly resistant individuals.

Why should we be concerned about resistance?

Resistance costs money. When insects develop resistance the grower needs higher rates and more frequent applications of the pesticide to kill the pests and eventually needs a new pesticide to replace the old one. In addition, when insects become resistant the pesticide does not control them as well and they do more damage to the crop. Pesticides are extremely expensive to develop and register and so those replacement pesticides do not come along very quickly. We have to be careful stewards of the pesticides that we do have registered to make them last as long as possible.

How do arthropods resist the killing action of pesticides?

When we spray, we select for insects with genes for resistance. The genes can provide the insect protection many ways. The most common way is to provide the insect with more enzymes in their body to break down the pesticides into chemicals that do not kill them.

Are resistant arthropods stronger than susceptible ones?

When insecticides are sprayed the resistant arthropods will survive better. However, often there is a cost to the insect to have extra enzymes ready to resist the pesticide. And so, in the absence of the pesticide spray, susceptible individuals may reproduce faster or survive better. Thus, there is a biological trade-off for an arthropod to be resistant so that it is not stronger in all situations.

What is cross resistance?

If an arthropod develops resistance to one pesticide, it has a gene that may allow it to be resistant to another closely related pesticide (or even one not so closely related) even if the population has not been exposed to it yet. For example, insects that become resistant to one organophosphate tend to be resistant to all organosphosphates. Also insects that develop resistance to organophosphates usually have partial cross-resistance to carbamates. Insects that became resistant to DDT many years ago often have some resistance to pyrethroids when they are first used. Cross-resistance is important to understand because if you spray insecticides from these cross-resistant groups, then your are essentially spraying the same pesticide over and over, and you will select for resistance faster.

What is multiple resistance?

Multiple resistance is when an arthropod has more than one mechanism of resistance. For example, the cuticle (skin) may be thicker to reduce penetration of the pesticide into the resistant insect and the resistant insect may also have more enzymes inside its body to break down the pesticide once it gets through the cuticle.

What are the mechanisms of resistance?

1) Metabolic resistance is the most common mechanism of resistance. The insect uses various enzymes to detoxify (destroy) the pesticide before it poisons the insect. Many insects use esterase or mixed function oxidase enzymes to break down pesticides. 2) Target site resistance. The insect changes the structure of an enzyme or the function of part of its nervous system to reduce the effect of the pesticide on that site. 3) Behavioral resistance consists of changes in the habits/actions of the insect to avoid exposure to a pesticide. For example mosquitos that hover a lot don’t land on the pesticide treated surface and aren't killed. A resistant population shows this increased hovering behavior.

How do low and high rates impact the development of resistance?

It depends on the insect or mite being studied. Generally speaking, use the lowest effective rate of a pesticide and avoid repeat applications of the same chemical class. If a low rate kills an economical number of pests and allows natural enemies to survive and they help kill the rest of the pests, then it is a good rate. If it is too low to kill enough pests and the grower ends up putting on repeat applications then resistance will be selected for quickly. High rates tend to select for resistance because they remove the susceptible individuals from the population and so interbreeding can’t help to reduce resistance.

How do tank mixes impact the development of resistance?

In general, tank mixes are likely to speed up development of resistance because they select for pest individuals with multiple genes for resistance. Tank mixes should be used only in situations where multiple pests are present or when the pesticides of choice only kill certain stages of the pest. For example, Savey only kills eggs of spider mites and so it may be tank mixed with another miticide that kills adults.

What is an insecticide resistance management program and when should it be initiated?

Insecticide resistance management is a strategy of using as few applications of any pesticide class as possible, in order to delay the development of resistance in insects. The strategy includes spraying only when economic thresholds are reached, using the most selective pesticides first so that natural enemies will survive and help control the pest population, and rotating between different pesticide classes. A resistance management program needs to be initiated before the problem arises because once the insects have the genes for resistance, you can't get rid of them.

What is a bioassay?

A bioassay is a test of the ability of a living insect or mite to survive a pesticide. For example, with spider mites and aphids, a petri dish is treated with a concentration of pesticide that kills susceptible individuals. If more than a few individuals survive, then we know that population has some resistance to the pesticide.

Why should we monitor resistance?

Resistance management anticipates and slows the rate of development of resistance. We use resistance monitoring to detect a resistance problem and avoid using that pesticide. We can reduce wasted pesticide applications by knowing ahead of time if they will work. In addition, resistance is rarely an area wide problem, so if we can detect it and stop using the problem pesticide, the susceptible and resistant pests can interbreed and the resistance may decrease.

Spider mites are key arthropod pests in San Joaquin Valley cotton fields. First reported as pests after the introduction of synthetic insecticides, they have become an annual problem accounting for estimated losses as high as 4%, not counting the cost of control. Spider mites cause damage by feeding on the cotton leaf surface, thereby reducing photosynthetic activity of the plant and yield. There are three species of spider mites in cotton; strawberry spider mite (Tetranychus turkestani), two-spotted spider mite (T. urticae), and pacific spider mite (T. pacificus). One, two, or all three species may be found in a cotton field. In most years, strawberry mite is more likely to cause defoliation and is the first to appear. The other two species gradually build in numbers as the season progresses. Strawberry mite prefers field crops, pacific mite prefers trees and vines, and two-spotted spider mite has no preference. Therefore, the species present in a cotton field is influenced by the neighboring crops.

A bionomial sampling plant is used for spider mites involves inspecting leaves for the presence of immature and adult spider mites. The treatment threshold is 30% of the 5th main stem node leaves infested with mites.

In general, spider mites are an early to mid-season pest. Spider mite populations begin in cotton because overwintering adult mites emerge from the soil or move through the air into a field from neighboring crops or weeds. These populations usually develop gradually with ample time for growers to monitor and make decisions. In fields with low natural enemy populations, the spider mite population can increase more quickly. Late season problems are often associated with the use of disruptive, broad spectrum insecticides, which release spider mites from their natural enemies, thus allowing damaging populations to build up. Spider mite problems can also be due to heavy infestations blowing in from neighboring crops such as corn, alfalfa, sugarbeets, or beans that are drying out.

Miticide Resistance

Currently registered selective miticides include sulfur, KelthaneÒ, ComiteÒ, ZephyrÒ, and OvasynÒ. The broad spectrum systemic insecticides Temik^® and Thimet^® can also effectively control mites. In laboratory bioassays conducted during the 1980s to the present, strawberry mite has never been shown to be resistant to any of the currently registered miticides. Laboratory bioassays have demonstrated that Kelthane and/or Comite resistance has been detected in 25% of two-spotted spider mite and 40% of pacific mite populations. Bioassays have been conducted for Zephyr as well and as yet no resistance to Zephyr has been detected in spider mites from cotton. While Kelthane and Comite resistances can be a problem in the San Joaquin Valley, these resistances are not dominant in inheritance and so with sufficient mixing of susceptible and resistant spider mites resistance frequently declines during field seasons. Careful rotation of miticides will help keep resistance frequencies low. Often, growers think they have resistance because the pesticides are not working, but the spider mites densities stay high because mites are blowing in from neighboring crops. In addition, some broad spectrum insecticides used for the other pests actually make mites reproduce faster (most pyrethroids). None of the miticides are very effective when spider mite densities are very high even if there is no resistance, thus, avoid flaring mite populations with broad spectrum pesticides.

Miticide Resistance Management Strategy

To manage miticide resistance in spider mites, we need to limit the total number of sprays of each pesticide. The best way to do this is to practice the basic principles of IPM including:

• monitor pests
• maximize the use of biological and cultural controls
• spray only when pests reach action thresholds
• use the most selective insecticides first so that natural enemy populations can build and help out
• follow good application guidelines
• rotate among different chemistries when pesticides are needed to reduce selection for any one
• save the broad spectrum insecticides for the end of the season

Table 1. Summary of miticide guidelines

Early Season (@ plant - early squaring)	Mid - Season (early squaring - layby)	Late Season (post layby)
Kelthane+	Comite+	Comite (if not used previously)
Zephyr	Zephyr(if not used previously)	Ovasyn
Sulfur#	Ovasyn	Alert**
Temik/Thimet (@plant)	Temik (sidedress)
Savey**

Lygus bugs are pests of numerous crops in the San Joaquin Valley. They attack dry beans, seed alfalfa, fruit trees, strawberries, lettuce, and cotton. They also use many other crops for shelter such as alfalfa hay, safflower, sugar beets, and understory plants in vines and orchards. This key cotton insect can decide the profitability of a production season depending on the timing and severity of migrations. There are no selective insecticides for controlling Lygus thus, as Lygus bugs go so goes all insect pest management. Lygus can threaten a crop from earliest squaring through cutout and final boll set. The severity of Lygus in a field is dependent on many factors, including spring temperatures, rainfall patterns, surrounding crops, and proximity to large areas of host plants, such as foothills or weedy lands.

Lygus feed in the upper portion of the plant, probing small squares and feeding on anther sacs. Migrations can occur quickly, but depending on the environment, can move off or establish residency through reproduction. Using a standard insect net, fields must be checked twice weekly to ensure accurate assessment of Lygus populations. Lygus treatment decisions are based on insect densities, expected square retention, and other plant based measurements, as described in IPM for Cotton in the Western U.S. Square retention, number of fruiting branches and boll retention are important components in making sound Lygus management decisions.

Insecticide Control Factors

There are no selective insecticides for Lygus bugs and any application can cause a reduction in natural enemies and thus, de-stabilize the natural biological control system. As more chemical intervention is required, other pests such as spider mites, aphids, and worms can become more troublesome. A single broad spectrum application during July usually does not caused secondary outbreaks, but repeated applications that begin in June can cause late season mites to increase beyond the natural enemies’ ability to manage.

There is a range in the impact an insecticide can have on natural enemies. For example, ProvadoÒ is useful for managing low to moderate populations in situations where residual control is not required. While it does effect some natural enemies, it does so much less. TemikÒ side dressed at first square, is useful in maintaining the natural enemies complex. Organophosphates appear to impact natural enemies much less than pyrethroids.

Unfortunately, organophosphate insecticides are not as effective in reducing Lygus. Thus, under heavy pressure, sustained migrations, or periods of high reproduction, the knockdown and residual control of pyrethroids is required. However, in most situations of low populations, single migration episodes or limited reproduction, organophosphate and carbamate insecticides can provide adequate control without sacrificing the biological element the cotton field.

Table 2. Summary insecticide guidelines for Lygus bugs⁺

¹ UC DANR Publication 3305

² Kidd, P.W., R.R. Rummel, and H.G. Thorvilson. 1996. Effect of cyhalothrin on field populations of the cotton aphid in the Texas High Plains. Southwestern Entomologist. 21 (3):293-301.

³ Mueller, S.C. 1997. Monitoring resistance development in seed alfalfa. Plant and Soil Conference Proceedings. Pp. 48-50.

⁴ Russell, J.E., T.J. Dennehy, L. Antilla, M. Whitlow, R. Weeb, and J. Pacheco. 1997. Lygus bugs in Arizona regain susceptibility to key insecticides. Cooperative Extension, Unversity of Arizona, Extension Arthropod Resistance Management Laboratory. 10 pp.

⁵ Summers, C.G. 1976. Population fluctuations of selected arthropods in alfalfa: Influence of two harvesting practices. Environ. Ent.:5:1:103-110.

The Cotton Aphid Problem

The cotton aphid (Aphis gossypii) has become a significant pest of San Joaquin Valley cotton over the last 10 years. During the 1980s and early 1990s, high levels of cotton aphids were seen, primarily on seedling cotton (April and May) and also on late-season cotton (September and October). During the squaring and boll-filling period, aphid populations only appeared in isolated fields along the citrus belt in Tulare County. This situation changed in 1992 when very high densities of cotton aphids appeared in July and persisted into August throughout much of the San Joaquin Valley. This occurrence was repeated in most years since then. In 1995, cotton yield losses from cotton aphids were particularly severe and equaled or possibly exceeded losses from spider mites and Lygus bugs. Cotton aphid outbreaks were localized in 1996 and 1997, but were severe in several areas.

The cotton aphid is an extremely adaptable insect and, under favorable conditions, reproduces and builds high densities extremely fast. Under favorable field conditions, mid-season aphid numbers have been observed to double about every 6-8 days. However, populations have also been observed to crash quickly when high populations of beneficial insects are present during the early- and late-season. The cotton aphid exists in several forms or morphs. These are all the same insect species even though the color varies from pale yellow to almost black. The dark green to black individuals are generally found during periods of comparatively cooler weather and on cotton plants with higher levels of nitrogen. This morph has also been observed to produce more offspring and may be an important factor in aphid outbreaks. Conversely, the light morph cotton aphids (pale yellow in color) persist during periods of hot weather and do not reproduce as much. They are thought to be the "survival stage" to bridge periods of unfavorable environmental conditions. These trends generally occur in the field, but not in every case. When conditions become unfavorable for cotton aphids, such as very high densities, poor food quality, etc., winged aphids form and aphids migrate to other areas. The wings are most evident on the adult aphids, but can be seen as small buds on nymphs.

Several agronomic factors are related to high cotton aphid densities and may influence aphid reproduction and susceptibility to insecticides. Late-planted cotton, hairy-leaf varieties, high levels of nitrogen, and excessively-irrigated cotton generally have higher aphid densities. Leaf hairiness does not vary greatly among SJV-approved varieties, but is generally lowest on DP6100. Research in other states and in the SJV has shown that cotton aphid populations are stimulated by pyrethroid applications. This effect appears to result because these chemicals directly increase cotton aphid reproduction in addition to a disruption of predators and parasites. Studies in Texas were done with Karate, whereas observations from California involved Capture. Aphid population flare-ups following pyrethroid applications should be monitored.

Natural enemies are an important factor in aphid management. Lady beetles, lacewings, big-eyed bugs, and damsel bugs are important predators of aphids. Parasites such as, Lysiphlebus testaceipes, mummify and kill cotton aphids. Populations of these natural enemies should be preserved to assist in management of cotton aphids. Small grains appear to be important sources of parasites and predators that eventually move to cotton.

The available insecticides differ in their effects on natural enemies. Soil-applied and seed treatments have minimal effects on beneficials. Provado effects some natural enemies, but generally is not too detrimental to the overall system. Many natural enemies have built up some tolerance to organophosphates. Foliarly-applied carbamates have severe impacts on beneficials and pyrethroids are the most detrimental. Refer to Table 2 (Selectivity and Persistence of Key Cotton Insecticides/Miticides) in , Cotton Pest Management Guidelines, UCPMG Publication 17).

Action Thresholds and Sampling

Thresholds for cotton aphids vary with the cotton plant growth stage. Aphids feed by removing plant juices from the plant leaves. These are the same energy reserves that the plant needs to develop squares and to fill/mature bolls. The source:sink relationship (production vs. use of energy) varies throughout the development of the cotton plant and cotton aphid infestations act as competition for energy reserves.

Seedling Cotton

Research has shown seedling cotton can generally withstand and fully compensate for cotton aphid infestations when the plants reach the ~3-leaf stage before they are infested and the aphid infestation lasts about 10-14 days before natural enemies remove the aphids. In this case, the plants are able to compensate and to produce a "normal" yield. Plant growth is stunted for a few weeks, but this is quickly compensated for and by season-end plant height, maturity, yield, etc. is unaffected. In some areas such as the eastern Tulare County "Citrus Belt," cotton aphids occur earlier and infestations persist longer then in the rest of the SJV. In these cases, when the plants are infested at emergence, and the infestation persists for several weeks, cotton plants are not able to compensate for the aphid feeding. In this situation, insecticidal control is warranted.

Squaring and Boll-Filling Period

During the squaring and boll-filling period, aphid feeding competes directly with the squares/bolls for resources. Therefore, cotton aphids during this stage can substantially reduce yield and may need to be controlled. Threshold levels are not clearly defined because the damage depends not only on the aphid population, the duration of the infestation. Texas and New Mexico have recommended 50 aphids per leaf as an action threshold. Research results from California at this level have been somewhat variable. Some studies have demonstrated a yield loss only after 100 aphid per leaf was reached, however other studies demonstrated a significant yield loss occurred when aphids increased to a level between 50-75. Therefore, delaying insecticidal control of aphids until infestations exceed 50 aphids per leaf is a good guideline. This allows time to schedule and apply insecticides before this threshold is reached and for the insecticide to bring the infestation under control (usually 1-3 days) before an economic yield loss occurs. Aphid population trends, increasing or decreasing should also be considered.

Sampling should be done on the 5^th main stem node leaf. All aphids, adults and nymphs, should be counted for this determination. In the field, aphid densities are often underestimated; training of field scouts to ensure consistency is important. Dark morph aphids are thought to stress the plant more than the light morph aphids. The dark morphs are larger and probably remove more plant sap than the light morph aphids.

Boll Crack to Harvest

Cotton aphid feeding at this time can potentially contaminate open bolls and lint. As the aphids feed, a sticky honeydew is excreted. This material, once deposited on lint, leads to sticky cotton which is difficult to harvest and/or process. Thresholds during this phase are low to prevent lint contamination. Treatments should be initiated at 10-15 aphids per 5^th main stem node leaf.

Cotton Aphid Resistance Management

The cotton aphid has a long history worldwide of developing resistance to insecticides. Within California, the level of resistance varies across regions and during the growing season. Resistance to Capture is widespread as of 1994. Resistance has also been documented to Lorsban, Metasystox-R, and endosulfan (Thiodan, Phaser); however, these three products still provide excellent field control in many cases. Therefore, procedures should be undertaken to manage resistance to these materials (and to all other insecticides) and to extend their usefulness. This is a particular concern as the same material are used to control cotton/melon aphid on multiple crops. There are few new insecticides under development to manage cotton aphid.

The factors which influence the level of resistance in cotton aphids are still being studied. However, it is clear that minimizing the number of applications and making sure that when an application is made it is effective and is the best resistance management strategy is important. Since cotton aphids infest the undersides of leaves and may be on lower portions of plants, insecticide coverage is critical. Inconsistencies in insecticide performance are often due to application rather than increased resistance levels, although resistance does magnify the coverage problem.

Ground applications should be made at speeds less than 6 mph with the use of at least 3 nozzles per row. A crop oil carrier may reduce the fuming action of Lorsban, and researchers in Texas reported that crop oil carriers appear to impede the movement of the locally systemic insecticides into leaf tissue.

Practice the basic principles of IPM, including monitoring pest levels, maximizing the use of biological and cultural controls, treating only when aphids reach the action threshold, using the most selective insecticides first so that natural enemy populations can build and be maintained, and saving the most disruptive, broad-spectrum insecticides for the end of the season.

Table 3. Insecticide resistance management guidelines.

Special Considerations:

The use of foliar insecticides on seedling cotton for cotton aphid control should be avoided except for cases of chronic, long-lasting early cotton aphid infestations. If this situation is expected based on field history, the use of a seed treatment or planting time systemic may be helpful. Orthene seed treatments may enhance aphid populations occurring during the seedling stage. Gaucho seed treatment may increase spider mite level. Rotate treatments among insecticide classes; each insecticide class should be used only once per growing season. Cultural and biological controls are also extremely important for managing cotton aphids. Among the important considerations are:

• manage for earliness including timely planting and proper plant populations
• maintain beneficial insect populations by avoiding unnecessary insecticide applications
• minimize excessive nitrogen and irrigation by using soil/petiole testing and pressure bomb analyses, respectively, and by utilizing a realistic yield goal
• timely crop termination and harvest including proper irrigation cutoff and optimal defoliation timing using NACB method
• limit regrowth
• limit use of broad-spectrum insecticides to later in the growing season
• Choice of late season tank mixes depends on other pests present and costs. The wrong choice may aggravate other pests.

Silverleaf whitefly first appeared in cotton and melons in the San Joaquin Valley during 1992. Occasionally there were localized outbreaks of whitefly in the southern and eastern parts of the San Joaquin Valley during 1993-95. In 1996 we saw a major outbreak of silverleaf whitefly as a result of warm fall and winter temperatures. The increasing distribution and host range of silverleaf whitefly continued in 1997 with heavy migrations observed in August and September. In addition to the difficulty of managing high populations, there is evidence that silverleaf whitefly are developing resistance to insecticides. These factors, compounded by the diverse cropping patterns of the San Joaquin Valley, increase the complexity of silverleaf whitefly management. However, based on our experiences in 1997 a program placing a high value on cotton management, host plant sanitation, intensive monitoring and close adherence to the suggested action thresholds resulted in success at season’s end.

We have borrowed from Arizona Guidelines (IPM Series No. 2, 3, 6, and 8) and acknowledge their work. However, the San Joaquin Valley environment is very different than the southern desert and central Arizona. Whitefly dynamics in the San Joaquin Valley result in different situations often related to local cropping patterns. The first situation exists in fields adjacent to overwintering whitefly populations and sources of spring development. The insect growth regulators (IGRs) are best used when adults and nymphs are present. This situation is commonly observed after the initial period of invasion and internal buildup of low numbers of silverleaf whitefly. The size of cotton plants and potential for continued immigration should also be considered. The second situation (non-pyrethroids) occurs when there is a gradual invasion of adults into fields before the bolls open particularly when other pests may require treatment with the same insecticides. The last situation occurs at the end of the season when silverleaf whitefly is migrating heavily, the bolls are opening, lint is at risk, and quick knockdown is required. Pyrethroids in combination with the organochlorine, endosulfan, or organophosphates can best be used in this situation.

Insecticide Resistance in Silverleaf Whitefly

During 1996 & 1997, the responses of adult silverleaf whitefly to various insecticides in six locations in the San Joaquin Valley were monitored during the season using yellow sticky cards treated with insecticides. These populations of whitefly showed changing levels of resistance through the season, but during several weeks populations showed significant resistance to the organophosphate chlorpyrifos (Lorsban^®), the pyrethroids fenpropathrin (Danitol^®), and bifenthrin (Capture®). In some cases combinations of organophosphates or the organochlorine with the pyrethroids increased susceptibility of the silverleaf whitefly and allowed full control. However, mixtures have been used to control silverleaf whitefly populations in Arizona, and significant resistance to these mixtures has developed. Thus, the mixture approach to reducing resistance and controlling silverleaf whitefly is a delaying or short-term approach to managing insecticide resistance. The result is that mixtures only control silverleaf whitefly in Arizona for a limited number of applications before they become ineffective.

Insecticide Resistance Management Strategy

To manage insecticide resistance in silverleaf whitefly, we need to limit the total number of sprays of each pesticide. The best way to do this is to practice the basic principles of IPM; monitor pests, maximize the use of biological and cultural controls, spray only when pests reach economic thresholds, use the most selective insecticides first so that natural enemy populations can build and help out, and save the most toxic broad spectrum insecticides for the end of the season to protect the exposed lint.

We recommend sampling both nymphs and adult whitefly. When the threshold for a particular situation is reached (see below for details), the first group of insecticides used should be relatively soft on natural enemies and could include non-pyrethroids such as Provado® or Thiodan/Phaser or IGRs, depending on the situation. Beneficial insects are needed for both silverleaf whitefly and for other cotton pests such as spider mites and aphids. Thiodan and Provado are effective on aphids as well and if their presence coincides with significant whitefly populations then both pests could be controlled with the same pesticide application. Pyrethroid mixtures should be delayed until the end of the season (September) when the bolls are open. First, there is evidence that pyrethroids can increase populations of spider mites and aphids (hormoligosis) by causing them to reproduce faster. Second, pyrethroids are toxic to natural enemies needed for aphid, spider mite, and silverleaf whitefly control. Third, pyrethroids are most effective against adult silverleaf whitefly and non-pyrethroids are most effective against nymphs. When the cotton bolls are open, both the adults and nymphs can produce honeydew that rains down on the bolls. The pyrethroid mixed with an organophosphate or chlorinated hydrocarbon kills the silverleaf whitefly much more quickly than the IGRs and so protects the cotton from stickiness.

Situation I: Initial (internal) Buildup

INSECT GROWTH REGULATORS

This situation uses the IGRs to reduce the population over time. These insecticides require time to work and have very little knockdown effect. The decision threshold requires the presence of both adults (5/leaf) and nymphs (1/leaf disk) on the fifth main stem leaf down from the terminal. For details on sampling, see Insert.

This situation is represented by low numbers of adults invading a field followed by a buildup of nymphs on the leaves. It differs from other situations by the very low numbers of adults initially migrating into the field and the presence of both adults and nymphs.

Chemicals include Applaud® (buprofezin) and Knack® (pyriproxyfen).

Benefits:

• Good residual control
• Good nymph control
• Each IGR has a different mode of action
• Selective, minimal disruption to natural enemies in cotton

Special Concerns:

• Use only one application of each IGR per season to help prevent resistance.
• IGRs are most effective when all stages of whitefly are present and beginning to increase. Using both the nymph and adult thresholds helps to insure that the population is in this state of growth.
• IGRs often have a better fit on larger plants based upon their cost and activity (e.g. translaminar movement / vapor activity).
• IGRs do not provide quick knockdown of adult whiteflies.
• IGRs do not control Lygus bugs, or mites.
• Specific restrictions apply to the use of these products. See Section 18 for details.

Situation II: Gradual Invasion by Adults

NON-PYRETHROIDS

This situation occurs when whitefly populations established in cotton begin to migrate to other cotton fields or when whitefly populations migrate to young cotton from overwintering sites. Adults and eggs are commonly found but nymphs are rare. The economic threshold is 10 adults per leaf on the fifth main stem leaf from the terminal.

This situation differs from Situation I in that the population of adults is larger and nymphs are not present yet. Under high pressure circumstances (e.g. cotton fields near the citrus belt), young plants may require non-pyrethroids followed by IGRs after immigration from the overwintering sites has subsided. Edge treatments of non-pyrethroids may be helpful under these conditions.

Chemicals include, organochlorines, Ovasyn (formamadine) and Provado (chloronicotinyl).

Threshold (adults / leaf)	Infested Leaves*
1	14%
2	28%
3	39%
4	49%
5	57%
6	64%
7	70%
8	75%
9	79%
10	82%

*From: Arizona Guidelines (IPM Series No.2) Naranjo, Ellsworth, Diehl, Dennehy.

Nymph sampling plan^* for decision making:

Necessary for the proper use of IGRs 60 plants per 80 acre field (15 leaves from each quadrant) Sampling unit: 5th main stem leaf from the terminal Count visible nymphs (3rd and 4th instars) within the leaf disk Note yellow spots & red eyes Leaf disk = size of quarter, or sight of a large hand lens Calculate average number of nymphs / leaf Number of red eyed nymphs predict subsequent adult levels A presence / absence sampling plan is in development

* From: Arizona Guidelines (IPM Series No.6) Naranjo, Ellsworth, Diehl.

General sampling rules:

1. Sample greater than 50 feet from the field edge.
2. Zig zag through the field randomly selecting plants.
3. Avoid sampling plants being sweep sampled.
4. Sampling in the morning before adults move down in the canopy is suggested particularly in mid-summer.

The Insect Growth Regulators (IGRs) will be an integral component of whitefly management in the San Joaquin Valley. Some general properties of these materials are outlined below:

* Note: both materials act mainly on the immature stages therefore, nymphs should be present prior to treatment.

Situation I: Initial (internal) buildup

IGRs: KNACK & APPLAUD

Action Threshold: 5 Adults/leaf, 1 Nymph/leaf disk

Benefits: Good residual control, selective, minimal disruption of beneficials

Special Concerns: IGRs do not provide quick knockdown, are most effective when all stages are present and populations are beginning to increase, Use only one application of each IGR per season

Situation II: Gradual Invasion by Adults

NON-PYRETHROIDS (Examples: Endosulfan, Ovasyn, Provado)

Action Threshold: 10 Adults/leaf

Benefits: less disruptive to beneficials, some adult knockdown, some control of other pests present (lygus, aphids)

Special Concerns: Limit the use of any one pesticide class

Situation III: Heavy Migration (lint exposed)

PYRETHROIDS + NON-PYRETHROIDS

Action Threshold: 10 Adults/leaf

Benefits: Quick knockdown of adults, good to control hot spots

Special Concerns: Early use significantly impacts beneficials, early use may also increase resistance and reduce effectiveness later in the season when protection of lint is a must.

* whenever possible ground applications are recommended