Piperonyl butoxide (PBO) is frequently used as a synergist in combination with other pesticides, particularly pyrethrins. Recent studies by Satoh and Plapp (1992) indicate that PBO may also act as a juvenoid type of insect growth regulator (IGR). These trials were conducted to determine if PBO had an effect on the development of the red imported fire ant, Solenopsis invicta Buren. Fenoxycarb, the active ingredient in Logic® and Award® fire ant baits, acts as a juvenoid IGR. Colonies treated with fenoxycarb in soybean oil fail to produce worker ant larvae and pupae and larvae present at the time of treatment develop only into larger reproductive larvae and pupae. These effects are observable within several weeks of treatment.

The first trials were conducted to determine the most acceptable concentration of PBO in soybean oil to fire ants. Thereafter, trails were conducted to determine if PBO produced insecticidal or IGR effects in laboratory colonies of fire ants.
 

Materials and Methods

I. Preference Trials

Trial 1. Three red imported fire ant colonies were field collected, extracted from the soil, and placed in Fluon®-coated plastic boxes. On 12 January 1994 a set of capillary tubes were filled with varying concentrations of PBO in soybean oil and placed together in the ant colonies. Concentrations exposed to the ants were:

 1) Technical (100%) in soybean oil
 2) 10,000 ppm (1%) in soybean oil
 3) 100 ppm (0.01%) in soybean oil
 4) 1 ppm in soybean oil
 5) 0.01 ppm in soybean oil
 6) 0 ppm, soybean oil alone

The capillary tubes were filled with the solutions to approximately 35 mm of the tubes' length. Exact distances (corresponding to volume) were recorded for each tube. The tubes were then exposed to ant colonies until one tube in each colony was emptied by the ants' feeding activities. The entire set of tubes was then removed and frozen (to kill any ants) before the amount of PBO-soybean oil solution was measured (in mm) and recorded to determine solution consumed. The entire test was repeated (Run 2).
 
 
Trial 2. Results from the first set of trials were used to formulate a narrower range of PBO concentrations in soybean oil (PBO was acceptable to ants at concentrations between 1 and 100 percent). The procedure described above was repeated, 13 January, 1994 using the following concentrations:

 1) 50% PBO in soybean oil
 2) 25% PBO in soybean oil
 3) 10% PBO in soybean oil
 4) 1% PBO in soybean oil
 

II. Efficacy Trials

Twelve freshly collected fire ant colonies were separated into a treatment and a control group based on the approximate number of workers and quantity of brood present. The colonies of the two groups were paired according to colony size so that each group had an approximately equal number of large and small colonies.

Trial 3. On 7 February 1994, a capillary tube with a known volume of solution was placed in each of the six treatment colonies. In order to deliver as much PBO as possible to the colonies, 25 percent PBO was chosen. A capillary tube of soybean oil alone was placed in the control colonies.

Ants were not apparently attracted to the 25 percent solution of PBO within an hour. The capillary tubes were removed and replaced with ones containing a known volume of 10% PBO. These tubes were removed later in the day and the volume of oil or solution consumed was recorded for all tubes. Results indicated very erratic acceptance of the PBO solution. The procedure was repeated with a 1 percent solution of PBO with tube removal at the end of the day. Consumption was again erratic.

On 8 February 1994 the above procedure was repeated using a 0.1 percent PBO solution. This concentration appeared to be acceptable to all colonies and the procedure was repeated again that day when the tubes were emptied and again on 11 February. In an effort to introduce as much PBO as possible into the colonies, two tubes were used on 11 February and three on 13 February.

The colonies were then examined weekly. Estimated numbers of worker ants and presence of worker brood (ant larvae and pupae) was recorded for each colony through 17 March 1994.

Trial 4. Because of poor results of Trial 3, another trial was conducted to attempt to deliver more PBO to treatment colonies. Colonies from Trial 3 were cleaned of debris and moved to new boxes as necessary. The procedure used in Trial 3 was repeated except that the PBO concentration was increased to 5 percent and was delivered in a disposable 1 ml pipette plugged with cotton to slow leakage. Colonies were observed periodically through 21 April. Worker ant numbers were estimated and presence of worker brood (larvae and pupae) was noted for each colony.
 

Results and Discussion

I. Preference Trials

Trial 1. No clear correlation occurred between amount of piperonyl butoxide concentrations in soybean oil and amount of solution consumed by foraging red imported fire ant activity. The debris piled up by foraging worker ants around the end of the tubes containing technical material indicates that the material was not repellent, but not attractive, at least not compared to the other food sources available (Table 1). The 1 percent solution was as "attractive" as any other formulation or oil alone.

Trial 2. Based on the results of Trial 1, the concentrations of this test were arrayed between 1 and 100 percent. The debris piling phenomenon was observed again on several tubes making the results difficult to interpret. The technical material was clearly not attractive as was the 50% concentration (Table 2). The lower concentrations yielded highly variable results, but the 25% solution appeared to be attractive enough to use in the delivery of PBO to the Efficacy Test colonies.
 

II. Efficacy Trials

Trial 3. As indicated by the data in Tables 3 and 4, ants or brood (worker ant larvae and pupae) were not eliminated by the addition of piperonyl to soybean oil. Contrarily, most colonies were observed to increase brood production and some in worker numbers at some point during the duration of this trial. These observations indicate that either PBO is ineffective as an juvenoid insect growth regulator or not enough active ingredient was administered to colonies to cause an effect.

Trial 4. In this trial, the capillary tube method that delivered at most 6.2 mg PBO over a week was abandoned in favor of disposable pipettes containing 0.4 ml of 5 percent PBO that could deliver a maximum of 14 mg PBO within 24 hours. Though large amounts of debris were piled around the mouths of the tubes and some leaked out onto their mounting surface, all material was gone within 24 hours. In the larger colonies, ants were seen feeding on the puddles of material that leaked out.

As indicated in Table 5, the piperonyl butoxide administered to fire ant colonies in soybean oil again had no visible effect on either brood production or worker mortality as compared to the control. All colonies experienced some decline in brood and worker numbers over the course of the course of the trial.
 

Conclusion

The results obtained from these trials failed to document piperonyl butoxide formulated as a bait in soybean oil as an effective insect growth regulator for the red imported fire ant.
 

Satoh, G.T. and F.W. Plapp, Jr. 1992. Use of juvenoid insect growth regulators for management of cotton aphid and sweet potato whitefly populations. Proc. Beltwide Cotton Conf. 2:751-757.
 
 
 
Table 1.  Amount (mm) of piperonyl butoxide (PBO) in soybean oil removed from capillary tubes by red imported fire ant workers in laboratory colonies, Trial 1, 12 January 1994.
 

* Indicates that ants piled debris around the end of capillary tubes causing the oil solution to soak out of the tube. Consumption cannot be confirmed, so these values should be treated as much less than indicated.
 
 

Table 2. Amount (mm) of piperonyl butoxide (PBO) in soybean oil removed from capillary tubes by red imported fire ant workers in laboratory colonies, Trial 2, 13 January 1994.
 
 

* Indicates that ants piled debris around the end of capillary tubes causing the oil solution to soak out of the tube. Consumption cannot be confirmed, so these values should be treated as much less than indicated.
 
 

 Table 3. Amount of soybean oil or piperonyl butoxide in soybean oil consumed by untreated control (CK) or PBO treated (TRT) colonies during efficacy Trial 3, February 1994.
 

Table 4. Vigor and brood status of red imported fire ant laboratory colonies fed either soybean oil (CK) or piperonyl butoxide concentrations in soybean oil (TRT), February through March 1994.
 
 

Table 5. Vigor and brood status of red imported fire ant laboratory colonies fed either soybean oil (CK) or piperonyl butoxide concentrations in soybean oil (TRT), March through April 1994.
 
 

   * Worker numbers are estimated on the basis of 20,000 in a "full" large petri plate and 10,000 in a "full" small petri plate. Brood quantity was evaluated on the basis of the amount in an average petri plate. LB indicates a "full" large petri plate, SB a "full" small petri plate. The number indicates the number of plates. The addition of + or ++ indicates an unusually large amount of brood. #+# indicates number of petri plates of both sizes with brood.
 

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