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Pest TechnologiesControl TREATMENTS TOOLS RESEARCH Why delay when you bait and spray? UCR researchers test delayed toxicity on Argentine ants

By Dr. John Klotz, Dr. Michael Rust and Andrew Soeprono Contributors

elayed toxicity is such an obvi- (see figure 1, next ous and fundamental require- page). The area of the D ment of ant baits that we for- shaded rectangle in get that it was only 40 years ago that the graph represents the concept was introduced by Stringer the range of bait con- et al. (1964). As originally conceived, it centrations that have applied to formulations of bait toxi- delayed toxicity and cants for red imported fire ants. are readily consumed In their search for effective bait toxi- by the ants. The rec- cants to replace , the United tangle’s height is spec-

States Department of Agriculture eval- ified by LT50s that uated more than 7,200 chemicals, of range from one to which only nine became or will four days, and its become commercially available width represents the (Williams et al. 2001). This monu- optimal concentra- mental task paid off: Today’s baits for- tion range of the bait. mulated with corn grit and soybean oil Both dimensions are some of the most effective ant baits are critical: If the con- on the market for red imported fire centration of active and harvester ants. ingredient is too low, Argentine ants present a new set of the bait will be readily Argentine ants forage across a -treated panel 15 minutes challenges in bait development. What after the discovery of a (center). consumed, but it will soybean oil is to fire ants as a food not be lethal. If the attractant, sucrose water is to Argentine a delicate balance between its toxicity concentration is too high, it will kill ants. Formulating baits with sucrose and attractiveness to the ants. the ants too fast and only a small water, however, is difficult because amount of bait will be consumed. most toxicants are oil-soluble. At UC CONCENTRATION COUNTS Additionally, when ants share the bait Riverside, we have been experimenting The relationship between a bait’s toxic- with one another, the toxicant will be with toxicants that have some degree of ity (measured as an LT50) and attrac- diluted. If its optimal range of activity solubility in sucrose water. We have tiveness (measured by the amount con- is too narrow, the bait will lose its found that the optimal concentration sumed) can be graphed as a function of effectiveness. of active ingredient in sucrose water is the concentration of active ingredient continued on next page www.PestControlMag.com APRIL 2004 33 Ants from previous page

600 Mortality Talstar Mortality 500 Termidor

Intake High Tempo 400

300 Take 200

100

Low 0 Low High 0 20406080100 Rate (%) Time (min)

Figure 1. The relationship between bait toxicity and consumption. Figure 2. Number of ants crossing the treated barriers. Barriers were treated at 50% the label recommended rate.

Graphs of Argentine ant responses to form like baits (Vail et al. 2003). To ment system to fully engage for several three active ingredients dissolved in validate this, we provided outdoor hours (see figure 2). 25% sucrose water reveals important nests of Argentine ants with a cock- The beauty of these delayed-action differences in the optimal range of activ- roach placed in the center of a sand- bait toxicants and residual is ity. For example, spans covered panel that was treated with that they exploit the recruitment process a15-fold range of concentrations from 2 . If the spray is nonrepellent of ants. The active ingredient in baits is x 10-5 to 3 x 10-4%, and a and has sufficiently delayed toxicity, brought into the colony and distributed 13-fold range from 7.1 x 10-4 to 9.2 x the ants will recruit to the cockroach to other workers, brood and potentially 10-3%. Compared to , which and maintain an active trail over the even the queens. Greater numbers of spans only a 7-fold range of concentra- treated surface for several hours. workers contact barriers that have tions from 0.5 to 3.7%, thiamethoxam Just like tracking powder for delayed toxicity, increasing their efficacy. and imidacloprid are effective at ultra- rodents, the ants unwittingly trail Both baits and the nonrepellent sprays low doses over a much wider concentra- across the treated surface picking up a with delayed-action insecticides used as tion range. Thus, the “margin of error” lethal dose of insecticide. Not only standalone or combination treatments for baits formulated with thiamethoxam does the individual forager that crosses will provide powerful new tools for and imidacloprid is much greater than the treated surface eventually die, but Argentine ant control. PC those with boric acid. some ants in the nest that come into Given that an Argentine ant typical- contact with the tainted forager die as References ly feeds four to 12 other ants (Markin well. The mechanism of this horizontal Markin, G.P. 1970. Food distribution within labora- 1970), a liquid bait could eventually be transfer of insecticide to other ants is tory colonies of the Argentine ant, Iridomyrmex diluted to a point where it is no longer unknown. humilis (Mayr). Ins. Soc. 27: 127-158. effective. At the other extreme, evapo- The behavioral response of Argen- Rust, M.K., D.A. Reierson, and J. H. Klotz. 2004. ration from a bait delivery system tine ants to different active ingredients Delayed toxicity as a critical factor in the efficacy could increase the concentration of varies. When the panel is treated with of aqueous baits for controlling Argentine ants (Hymenoptera: Formicidae). J. Econ. Entomol. toxicant to a level that is also outside or , for example, (in press) the effective range. This can be particu- the ants cannot form recruitment trails. Stringer, C.E., C.S. Lofgren, and F.J. Bartlett. 1964. larly critical for baits formulated with These have fast knock- Imported fire ant toxic bait studies: evaluation of tox- boric acid, because of its narrow range downs (LT90s = 4.7 and 8.2 min, re- icants. J. Econ. Entomol. 57: 941-945. of activity. spectively), so scouts cannot make it Vail, K.M., D. Bailey, and M. McGinnis. 2003. back to the nest. At lower concentra- Perimeter spray and bait combo. Pest Control DELAYED TOXICITY tions, the lethal times increase suffi- Technology 31(7): 96-100. Delayed toxicity can also be a benefi- ciently to allow some recruitment and Williams, D.F., H.L. Collins, and D.H. Oi. 2001. cial attribute of residual insecticides. limited trailing across the panels. The The (Hymenoptera: Formicidae): an historical perspective of treatment Spray formulations of nonrepellent, delayed action of fipronil (LT90 = 114 programs and the development of chemical baits for delayed-action insecticides might per- min) is such that it allows the recruit- control. Amer. Entomol. 47: 146-159.

34 PEST CONTROL MARCH 2004 www.PestControlMag.com