Various Novel Insecticides Are Less Toxic to Humans, More Specific to Key Pests
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REVIEW ARTICLE t Various novel insecticides are less toxic to humans, more specific to key pests Elizabeth E. Grafton-Cardwell Larry D. Godfrey William E. Chaney Walter J. Bentley t A number of novel insecticides have recently been registered for insect control in agriculture. A major advan- tage of these new products is that they act on insect biological processes that humans do not experience, such as molting. Many also have greater selectivity to target specific species, so they are less likely to harm natural enemies when compared with the While often effective at controlling specific pests, less-toxic new insecticides can also have unintended impacts. When pyriproxyfen was broader spectrum organophosphate, sprayed to control red scale in citrus, it also caused gross abnormalities in carbamate, neonicotinoid and pyre- vedalia beetle pupae, left (normal) and right (abnormal). Vedalia beetles, inset (adult stage), are predators needed to control cottony cushion scale; throid insecticides. Such novel insec- as a result, secondary outbreaks of the scale occurred. ticides currently in use include four targeting lepidopteran pests, three targeting sucking insects, one specific selectivity and so are less likely to the novel insecticides have fairly short to dipteran leafminers and one insect harm natural enemies than the broad- residual activity or affect only immature spectrum organophosphate (OP), car- stages of insects, so the treatment timing growth regulator that controls a wide bamate, neonicotinoid and pyrethroid is less flexible compared with broad- range of insects. One negative aspect insecticides. As such, they are less likely spectrum insecticides. Finally, the cost of these insecticides is that because to cause outbreaks of secondary pests of the new insecticides is usually signifi- of their narrower range of activity — that are well controlled by natural en- cantly higher than the older products. controlling only a limited number of emies, and may be used as “clean-up” It is fortuitous that in recent years pests — growers may need to apply sprays to manage outbreaks of pests insecticides from different chemical additional pesticides for secondary caused by broad-spectrum insecticides. classes have been registered to control pest groups that have poor biological The registration of these insecticides has lepidopteran (primarily moths) and ho- helped to greatly reduce OP and carba- mopteran (primarily scales and white- control, increasing the total number mate insecticide use in California. This flies) pests, because many insects in of treatments per acre and total pest- has had an especially significant impact these groups have developed resistance control costs. in cotton, citrus and stone fruits, where to the older pesticides. The simultane- OP and carbamate use has been reduced ous registration of insecticides with number of novel insecticides with by as much as 70% since the late 1990s. unique modes of action allows growers unique modes of action were reg- The new insecticides also have some to alternate the insecticides used, reduc- isteredA during the late 1990s and early disadvantages. Because of their nar- ing the rate at which resistance devel- 2000s for insect control in agriculture. rower range of activity, each insecticide ops. Insecticide resistance in key pests These new insecticides have several generally controls only one pest group will continue to be a major impetus for advantages over older classes of insec- within a crop. The grower may need adopting novel insecticides. ticides. First, most of the products in to apply additional insecticides for Insecticides for Lepidoptera this group act on insect processes that secondary pests that have inadequate humans do not experience, such as natural control, increasing the total Four insecticides that have activ- molting. Low mammalian toxicity al- number of treatments per acre and total ity primarily affecting lepidopteran lows for short re-entry and preharvest pest-control costs. In addition, many of pests — indoxacarb (Avaunt, Steward), intervals, allowing the insecticides to Insecticide resistance in key pests will continue to be easily incorporated into pest con- trol programs. Many also have greater be a major impetus for adopting novel insecticides. http://CaliforniaAgriculture.ucop.edu • JANUARY-MARCH 2005 29 tebufenozide (Confirm), methoxyfeno- to each other, as well as spinosad and emphasizes the need for the rotation of zide (Intrepid) and emamectin benzoate Bt, helping to reduce the rate that insec- indoxacarb with emamectin benzoate, (Denim, Proclaim) — are registered for ticide resistance develops. the dibenzoylhydrazine insect growth a number of crops in California. The Indoxacarb. Indoxacarb is an oxadia- regulators (IGRs) and other insecticides greatest uses of these insecticides are zine insecticide that blocks the sodium to maintain the efficacy of all groups of in cotton, cole crops, lettuce, nuts, and channels in insect nerve cells, causing insecticides. stone and pome fruits (table 1). lepidopteran larvae to stop feeding Tebufenozide. Tebufenozide is a In stone fruit, the use of these insec- within 4 hours, become paralyzed and dibenzoylhydrazine stomach poison ticides — in combination with Bacillus die within 2 to 5 days (McCann et al. that acts as an IGR specifically for Lepi- thuringiensis (Bt) products, spinosad 2001). It is more effective as a stomach doptera. It mimics a molting hormone and mating disruption during the poison than as a contact poison. Indoxa- and blocks the completion of the nor- growing season — has greatly reduced carb is fairly selective, having activity mal molting process (Retnakaran et al. the need for dormant sprays of OP, car- primarily against lepidopteran larvae 2001). The insect stops feeding within a bamate and pyrethroid insecticides for and certain species of sucking insects few hours and undergoes a premature peach twig borer (Anarsia lineatella). This such as Lygus bugs. However, the activ- lethal molt within 3 to 7 days, becom- has benefited the stone fruit industry by ity of indoxacarb against the sucking ing trapped within the shedding head reducing pesticide residues in surface wa- insects is weaker than for Lepidoptera capsule. Tebufenozide must be ingested ter, by preserving natural enemies needed because of its slower bioactivation, to take effect and is thus slow-acting, for other pests such as San Jose scale lower sensitivity and a less favorable with a residual activity of 14 to 21 days. (Diaspidiotus perniciosus), and by reducing method of oral uptake in the sucking in- Application timing is critical, because secondary outbreaks of spider mite pests sects. Indoxacarb allows most predators it is more active on early larval stages caused by broad-spectrum-insecticide dis- and immature wasp parasites to survive (Waldstein and Reissig 2001). It is non- ruption of their natural enemies. (Hewa-Kapuge et al. 2003; Studebaker toxic to honeybees and is selective, not The Central Coast Vegetable Inte- and Kring 2003). However, the wet resi- affecting most natural enemies (Dhad- grated Pest Management Program for dues of indoxacarb are toxic to bees and ialla et al. 1998). pest management in lettuce provides adult wasp parasites. The crops for which tebufenozide is another example of the significant Indoxacarb controls important currently registered include cole crops, role that these narrow-spectrum in- pests in alfalfa, apples, cole crops, cot- cotton, grapes, lettuce, tomatoes and secticides play in Lepidoptera control. ton, lettuce and pears. Populations of some nuts, pome and stone fruits. Low Lettuce is highly susceptible to pest obliquebanded leafroller (Choristoneura levels of resistance to tebufenozide damage at the seedling stage and rosaceana) in Michigan (Ahmad et al. have been found in codling moth (Cydia during head formation. Many of the 2002) have exhibited resistance to in- pomonella), beet armyworm and oblique- seedling pests — such as crickets, flea doxacarb in regions where it has not banded leafroller populations that were beetles, aphids and whiteflies — are been used, suggesting cross-resistance not exposed to this insecticide (Moulton controlled with broad-spectrum OP, to older groups of insecticides. This et al. 2002; Ahmad et al. 2002), suggest- carbamate, pyrethroid or neonicotinoid insecticides. These insecticides reduce or eliminate the natural enemies that attack the lepidopteran pests, some- Clark Kelly Jack times causing outbreaks; selective in- secticides help to bring the Lepidoptera back under control without creating additional problems. In addition, there are a number of lepidopteran pests that attack both head and leaf lettuce, including cabbage looper (Trichoplusia ni), beet armyworm (Spodoptera exigua), corn earworm (He- licoverpa zea) and tobacco budworm (Heliothis virescens). Lepidopteran pests can destroy seedlings, bore holes and leave frass or insect body contaminants throughout the growth cycle of the let- tuce, necessitating multiple treatments. Because lettuce is highly susceptible to insect damage at the seedling stage, many Indoxacarb and tebufenozide provide growers spray broad-spectrum insecticides. Some newer, more selective insecticides unique, selective chemistries for these can bring lepidopteran pests under control without hurting their natural enemies and pests and act as rotational alternatives causing secondary pest outbreaks. 30 CALIFORNIA AGRICULTURE, VOLUME 59, NUMBER 1 Omnivorous leafroller moth Peach twig borer larva attacking