**4. Response to insecticides**

One of the most important factors in leading to *Liriomyza* spp. becoming pests is their ability to evolve resistance to insecticides (Parrella & Keil, 1984). Leibee (1981) compiled a list of insecticides used against *Liriomyza* spp. in Florida and the life spans of their field efficacy in commercial use. The list of ineffective materials includes almost all classes of insecticides de‐ veloped up to that time. Some insecticides became ineffective in as little as two years. This review confirmed the widespread importance of insecticide resistance in driving the pest status of *Liriomyza* spp. Despite the rapid failures of different insecticides, there has been a general belief, at least through the middle of the 20th century, that new chemistries would become available to replace ineffective ones, and provide a few additional seasons of con‐ trol. Consequently, there was little emphasis on alternative management techniques until the advent of the worldwide leafminer crisis in the 1970s (Leibee & Capinera, 1995).

Intense insecticide use is the most common strategy used to eradicate newly discovered out‐ breaks of *Liriomyza* spp. (Bartlett & Powell, 1981). The success of this strategy is dependent on the susceptibility of invasive populations to available insecticides. Because invasive pop‐ ulations are already likely to be resistant to various insecticides (MacDonald, 1991; Parrella & Keil, 1985), eradication programs may not be successful.

Cross resistance to multiple classes of insecticides is also likely in *Liriomyza* spp. Despite a short history of pyrethroid use in Hawaii, high levels of tolerance to fenvalerate and perme‐ thrin were detected in field populations of both *L. sativae* and *L. trifolii* (Mason et al., 1987). The authors speculated that the tolerance/resistance arose as a result of cross-resistance to longer used organochlorine insecticides, which have a similar mode of action to pyrethroids. Populations of invasive *L. trifolii* obtained from greenhouses in Canada treated intensively with the organophosphate pyrazophos for less than 1 year showed high levels of resistance to that insecticide and to other types of organophosphates that had not been used previous‐ ly (Broadbent & Pree, 1989). Fortunately from a pest management perspective, reversion to susceptibility to organophosphates and pyrethroids has been shown to occur within a few generations (within 1 year) (Broadbent & Pree, 1989; Parrella & Trumble, 1989). Interesting‐ ly, these Canadian populations showed no susceptibility to carbamates. It is possible that these populations were already resistant to carbamates and that laboratory-reared flies maintained their resistance for 5 years, or that carbamates are not toxic to *L. trifolii.*

At present, two of the most effective insecticides for *Liriomyza* management are abamectin and cyromazine. Both insecticides target the larvae inside the plant foliage. Cyromazine acts as a growth regulator; whereas abamectin is a neurotoxin that acts as a GABA agonist. Both have translaminar properties, allowing them to reach the larvae within the plant. Research by Schuster and Everett (1983) documented the effectiveness of both insecticides under field conditions. Since that time, both have been commercially available. Despite this long history of use, resistance has not been a major problem in their use (Ferguson, 2004). The one re‐ corded case of resistance to cyromazine cited in that study showed that reversion to suscept‐ ibility occurred within 8 generations in a laboratory strain and that field efficacy was restored within 2 seasons of reduced exposure.

Another class of insecticide with efficacy against *Liriomyza* spp. is the spinosyn class (spino‐ sad and spinetoram). Spinosyn insecticides have been widely used since their introduction in the US in 1997. Similar to abamectin and cyromazine, spinosyns have translaminar prop‐ erties, enabling them to target leafminer larvae. Spinosyns are neurotoxins also. However, they have a different mode of action than abamectin, one that disrupts nicotinic acetylcho‐ line receptors (Salgado, 1998). Spinosyns are classified as Group 5 insecticides and abamec‐ tin is classified as a Group 6 insecticide by the Insecticide Resistance Action Committee (IRAC International MoA Working Group, 2011). There have been few reports of resistance to spinosyns to date among *Liriomyza* spp. (Ferguson, 2004). The lack of reported cases of spinosyn resistance may be considered surprising, given that spinosyn products are widely used against other key pests that co-occur with leafminers, including thrips and Lepidoptera pests (Demirozera et al., 2012; Reitz & Funderburk, 2012; Reitz et al., 1999). Incorporating the use of a penetrating surfactant improves the efficacy of spinosad against *Liriomyza* larvae (Bueno et al., 2007), allowing growers to improve management with lower rates of insecti‐ cide. This approach may also help reduce selection pressures. It is reasonable that increasing penetration of abamectin or cyromazine into plants would, likewise, increase their efficacy.

Selection of appropriate insecticides and rates for use in the field also depends upon proper identification of leafminer species. Parrella and Keil (1985) found that *L. trifolii* was much less susceptible to methamidophos than was *L. sativae* or *L. langei.* Likewise, *L. trifolii* populations in China are significantly less susceptible to abamectin and cyroma‐ zine than are populations of *L. sativae* (Gao et al., 2012) In contrast in Japan, *L. sativae* populations were less susceptible to several commonly used insecticides than were local populations of *L. trifolii* (Tokumaru et al., 2005). There is evidence that invasive popula‐ tions of *L. huidobrensis* are more tolerant to certain commonly used insecticides than are sympatric populations of *L. trifolii* (Weintraub, 2001a).
