Consequences of Mound Disturbance

Results from our research provide evidence that disturbing a mound immediately before treatment does not reduce fire ant bait foraging, despite a short delay in foraging activity in the disturbed mound treatment. More than 95 percent of the applied baits are foraged after three hours, regardless of mound disturbance or not. Bait placement, relative to mound, had an impact of the rate of bait removal, in that the fastest foraging rate occurred on mound, followed by 0.18-0.3-m from mound base, and the slowest 1.08-1.2-m from mound base. The fastest foraging rate occurred in the first post-treatment hour, resulting in more than 78 percent bait removal. This trend was held consistent, again, regardless of the treatment and season. All treatments gave 100 percent control one month later without colony relocation or new colony invasion in the test plots. Bait application should be done around 9 a.m. in spring, but before 7 a.m. in summer.

RATIONALE. One approach to control fire ants is the use of bait containing a food attractant and poison. Bait is considered a requisite component in a fire ant IPM program because it is easy to use, less expensive, safer and effective.

Registration labels on commercial fire ant baits generally suggest, with no research-based data, not to disturb the fire ant mound and to place bait on the ground in a circle less than 1.5 m around the mound. The theoretically speculated reasons are: 1) fire ants may switch on colony defense when disturbed or intruded, so they may reduce bait foraging or stimulate colony relocation; 2) fire ant-foraging tunnel entrances often do not open on mound, therefore placing baits on mound surface may reduce the chance of workers locating the baits.

Despite the labels, homeowners — and some pest professionals — claim that applying baits around pre-disturbed mounds via foot-kicking or raking or shoveling provides better control.

We tested the premise that disturbing and placing bait on a mound will reduce the fire ants foraging for bait, and therefore provide less effective control. We used paired plots and disturbed one but not the second. A subplot of each mound received baits in three locations. This was repeated in winter, spring and summer.

RESULTS. What follows are the results of our research:
1. Our results provided evidence that mound disturbing did not discourage fire ant foraging for bait despite a short delay in foraging activity. Mound disturbance treatment and undisturbed control (see Figure 1 below) resulted in similar amount of bait removal.
2. Our results indicated a considerable effect of bait location on bait foraging. Contrary to general assumption, faster and more bait removal occurred in the baits placed on the mound’s surface, followed by 0.18-0.3m around the mound and the slowest 1.08-1.2m away from the mound in the winter and summer (see Figure 2 above). The possible reasons for this phenomenon are: 1) fire ants quickly emerge on the mound surface when they are disturbed and start picking up the baits conveniently placed right there on their doorstep; 2) the urgent need for defense and nest repair stimulate a great number of workers from distant foraging territory to mound site who later engaged in bait forage.
3. One month after treatment, all the treated mounds became inactive. Furthermore, no new active mounds were observed in the study plots.

CONCLUSIONS. Previous studies have reported that disturbing mounds with liquid insecticides induce colony relocation or fragments. Our study shows no association of mound relocation with disturbing mounds when baiting.

The author is extension specialist associate and professor of entomology & plant pathology, Auburn University and can be reached at xhu@giemedia.com.