Behavioral resistance of house flies, Musca domestica (Diptera: Muscidae) to imidacloprid.
House flies (Musca domestica L) are a significant pest associated with military operations where proper sanitation of manure and refuse is often limited. (1,2) Developmental sites for house flies include human and animal waste and food waste, all of which may be common at temporary military encampments or in the communities surrounding military encampments. In addition to the considerable nuisance caused by large populations of these flies, they have also been implicated in the transmission of a phylogenetically diverse group of human pathogens including Escherichia coli O157:H7, Salmonella enteritidis, Yersinia pseudotuberculosis, Helicobactor pylori, Vibrio cholerae, and rotavirus, (3-11) as shown in Table 1. The extent to which house flies may be involved in pathogen persistence and dispersal among troops or neighboring communities remains unclear; however, it is well known that flies may disperse a considerable distance (5 km or more) from breeding sites. (32,33) Flies dispersing from their developmental sites are commonly attracted to human habitations and food preparation sites where they can transmit pathogens. (34-36) Annual increases in human infections with enteric pathogens can demonstrate a distinct seasonality that coincides with increasing abundance of house flies. (13,37) Further, area-wide control of house fly has been associated with a concurrent reduction in human sickness due to enteric pathogens, (16,38-40) providing strong evidence that house flies can be important in the spread of these enteric diseases.
With the advent of DDT (dichlorodiphenyltrichloroethane) for control of insects, house fly control has been largely through use of chemical insecticides.
However, house flies have shown a great adaptability to these insecticides, developing resistance first to DDT, (41) and then to most of the chemicals that have later become available for their control. (42,43) The extent of house fly resistance to any particular chemical appears to be principally determined by the historical use of that chemical regionally. (44-48) However, cross-resistance where selection with one chemical may result in resistance to another has also been shown. (42) In the military environment, rapid assessment of house fly resistance to chemicals approved for management of house flies by the Armed Forces Pest Management Board is needed whenever military units are deployed to a location where sanitation is lacking and house fly populations exceed acceptable numbers.
A rapid assessment of the physiological resistance of an insect population to chemicals is typically accomplished using some form of contact assay, such as the World Health Organization bottle assay, (49) or topical application of the material to the insect dorsum. A standard assay for determining house fly resistance to chemicals has been developed using 230 ml glass jars. (50) For chemicals formulated into fly baits, a standard feeding assay has also been developed. (43) In each of these assays, tested house flies are ensured substantial contact with the chemical. However, under natural field conditions, fly contact with a chemical may be limited due to detection and avoidance of the chemical, or due to irritation following contact with the chemical resulting in movement away from the chemical. Such behavioral resistance to chemicals has been shown in some house fly populations using "choice assays" even where physiological resistance was limited. (44,51)
In this study, we examined both physiological and behavioral resistance of a wild house fly population from southern California to imidacloprid, the active ingredient found in one of the most commonly used fly baits in the United States during the last 5 years.
Materials and Methods
House flies from a southern California dairy (BS Strain) were collected by sweep net in October 2008 and colonized at the University of California at Riverside (UCR) using standard methods. (52) Field collected flies were maintained in colony through 7 generations for use in this study. Susceptibility of this fly population to imidacloprid was evaluated relative to a UCR susceptible strain population of house flies that have been in colony since their collection in 1982 from a dairy in Mira Loma, California.
Because imidacloprid is typically formulated into a fly bait material, house fly susceptibility to this chemical was assessed using either a no-choice feeding bioassay (43) or a modification of this bioassay (choice feeding assay), providing flies the opportunity to feed on a food source with or without imidacloprid. Colony -reared house flies were chilled in a freezer for approximately 4 minutes, and then 25 female house flies, 3 to 6 days old, were placed into 230 ml glass jars (VWR International (West Chester, PA) catalog No. 16195-008) containing either one 4 cm cotton dental wick (Richmond Dental Co., Charlotte, NC) soaked in 20% sugar water containing technical grade imidacloprid (Chem Service Inc., West Chester, PA) (no-choice feeding assay), or two 4 cm dental wicks, one with sugar water and imidacloprid and the second with sugar water only (choice feeding assay). Bioassay jars were then covered with mesh netting and kept at 25[degrees]C with a 12:12 L:D photoperiod. Dental wicks were hydrated at 24 and 48 hours, and mortality was assessed at 72 hours with house flies considered dead if they were unable to right themselves.
For both the no-choice and the choice feeding assays, a minimum of 5 jars were used for each imidacloprid concentration, with an additional 5 jars provided dental wicks with 20% sugar water only as a negative control. For both the BS strain and the UCR strain flies, a minimum of 5 different concentrations of imidacloprid resulting in >0% and <100% mortality were used. Bioassay data was pooled and analyzed by standard probit analysis (53) with Abbot's correction to adjust for control mortality using POLO-PC (LeOra Software, Petaluma, CA). (54) Resistance ratios (RR) were calculated by dividing the lethal dose (LD) values of the wild house fly population by the corresponding LD value of the UCR susceptible colony for each assay type. A significant difference (P<0.05) in the susceptibility of house fly strains to imidacloprid was determined by nonoverlap of the 95% confidence interval of the LD value of each fly strain.
Results and Discussion
The field-collected BS house fly strain was significantly less susceptible (P < 0.05) to imidacloprid as compared to the UCR susceptible strain using both the no-choice and the choice feeding assays, as shown in Table 2. House fly resistance to imidacloprid when measured by the no-choice assay was moderate (RR=10), but was very high when measured by the choice assay. Low field fly mortality at even the highest concentration of imidacloprid tested in the choice assay (50 x LD99 of the susceptible UCR strain) resulted in the lack of a significant regression line to estimate a 50% mortality value (data presented in the Figure). From these data, it is reasonable to infer that the BS strain of house flies would be minimally affected by the use of toxic fly baits containing imidacloprid as part of a fly management program under natural field conditions.
The L[D.sub.50] values for the UCR and BS strain of house flies tested for susceptibility to imidacloprid using the no-choice feeding assay were similar to previously published LD50 values for susceptible and resistant fly strains, respectively. (48) The UCR susceptible flies will readily feed on commercial fly bait containing 0.5% imidacloprid (A.C.G., unpublished data, 2006). The 10 x higher L[D.sub.50] value for UCR susceptible strain flies when tested with the choice assay is a result of altered test conditions providing both a treated and an untreated dental wick on which the flies may feed over the 72 hour assay period. Small changes in the test conditions can result in significant changes in calculated LD values, (44) thus a direct comparison between the L[D.sub.50] value for the no-choice and the choice feeding assays should be avoided.
Fly baits containing imidacloprid were first introduced in California in 2003. Studies conducted in 2003 showed imidacloprid baits to be quite effective at attracting and killing field fly populations in southern California. (47) From 2003 to 2008, fly baits containing imidacloprid dominated the fly management market due to serious house fly resistance to the older baits containing the toxicant methomyl (44,47) and the lack of alternative granular fly baits with quick fly kill characteristics. With nearly year-round use of this single imidacloprid bait product by most facilities or agencies engaged in fly management, it should be no surprise that house fly populations quickly became resistant to imidacloprid. At a diagnostic imidacloprid concentration of 2 x L[D.sub.99] for a susceptible laboratory colony, survival of house flies from field sites in southern California exposed to dental wicks containing imidacloprid in no-choice assays has increased from <40% in 2005 to 45%--51% in 2006, (48) and finally to 73% for BS strain flies in this study. While this physiological resistance is significant, substantial mortality of the BS strain flies could still be achieved using high concentrations of imidacloprid, as indicated in the Figure.
In contrast to the no-choice assay, the choice feeding assay indicated that 72% of the BS strain flies could not be killed by even the highest concentration of imidacloprid used (50 x L[D.sub.99] for a susceptible laboratory colony). The choice feeding assay is certainly more indicative of field conditions where flies have many available food options and are not forced to contact or consume toxic bait. Under field conditions, flies may be selected for behavioral responses to avoid or limit contact with toxic bait. The results of the choice feeding assay support field observations during 2008, indicating substantial field failures of imidacloprid baits in southern California (A.C.G., unpublished data, 2008).
The failure to achieve complete control of the BS strain flies at higher imidacloprid concentrations in the no-choice assay may also be indicative of behavioral resistance. In these assays, flies were not evaluated for sugar consumption, and behaviorally resistant flies may have limited sugar contact or consumption over the 72 hour assay period, even in the no-choice assay.
This study showed that house flies can develop resistance to toxic baits through altered behavioral response to a toxicant. These findings are in agreement with earlier studies (44,47,51,55) showing that avoidance behaviors against a chemical toxicant may be selected for in house flies. Most volatile compounds in commercial fly baits are selected for attractiveness to house flies, indicating a priori value to house flies in persisting to express appetitive response to these compounds. It seems unlikely that behaviors would be selected to avoid these attractive volatiles. More reasonable would be the selection for flies exhibiting either an avoidance response to nonattractive toxicant volatiles, or an irritancy response to the toxicant that limits fly contact with the toxicant. If true, then a change of toxicant in the fly bait product may be all that is needed to rescue a failing fly bait.
Further studies are needed to examine whether house fly resistance to toxicants is a result of altered house fly perception and response to volatiles associated with toxicants, or due to irritation and subsequent movement of flies following contact with toxicants. Also needed are studies to develop field deployable kits for rapidly evaluating behavioral resistance of flies, mosquitoes, and other pest arthropods to pesticides formulated into baits or applied as space or surface sprays.
Funding for this research was provided by the West Valley Mosquito and Vector Control District and by federal Hatch funds.
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MAJ Gerry is an Associate Medical/Veterinary Entomologist on the faculty of the University of California at Riverside, California. He is an Army Reserve Entomologist assigned to the US Army Center for Health Promotion and Preventive Medicine-South at Fort Sam Houston, Texas.
Dr Zhang is a Research Associate at the University of California at Riverside. Her interests include management of disease-transmitting insects and human parasitism by arthropods (real or delusional).
MAJ Alec C. Gerry, MS, USAR Diane Zhang, MD
Table 1. House fly-transmitted pathogens known to have the most serious affect on human health. Note: The list is not exhaustive for all pathogens carried by house flies. Bacteria Viruses Acinetobacter spp (3) Coxsackievirus (3) Campylobacter spp (12,13) enteroviruses (24) Chlamydia trachomatis (14,15) poliovirus (3) Enterobacter spp (3) rotavirus (11) Enterococcus spp (3) Parasites Escherichia coli O157:H7 (6,16-19) Cryptosporidium parvum (25) Helicobactor pylori (9) Endolimax nana (26) Klebsiella spp (3) Entamoeba coli (26) Proteus spp (3) Entamoeba histolytica (26) Psuedomonas spp (3) Giardia lamblia (5,27-29) Salmonella enteritidis (7,20) Isospora spp (26) Shigella sonnei (16,21,22) Sarcocystis spp (30) Staphylococcus aureus (22,23) Toxoplsma gondii (31) Streptococcus (22) Vibrio cholerae (10) Yersinia pseudotuberculosis (8) Table 2. Physiological and behavioral resistance to imidacloprid demonstrated by wild-type house flies captured from a dairy in southern California relative to a susceptible laboratory strain. Fly Strain (Assay) N Slope (SE) [LD.sub.50] (95% CI) ([micro]g/ml) UCR (no-choice) 1,250 4.27 (0.29) 15.1 (13.1-17.5) BS (no-choice) * 750 2.12 (0.22) 155.9 (110-194) UCR (choice) 725 2.36 (0.13) 68.2 (38.4-117.3) BS (choice) ([dagger]) 725 0.78 (0.12) NA Fly Strain (Assay) RR UCR (no-choice) -- BS (no-choice) * 10.3 UCR (choice) -- BS (choice) ([dagger]) NA * Significantly different from UCR Susceptible strain based on nonoverlap of 95% confidence intervals in no-choice assay. ([dagger]) Significantly different from UCR Susceptible strain based on failure to achieve 50% mortality at maximum dose (2,500 [micro]g/ml) of imidacloprid tested.
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|Author:||Gerry, Alec C.; Zhang, Diane|
|Publication:||U.S. Army Medical Department Journal|
|Date:||Jul 1, 2009|
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