Detection and monitoring

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Fire ants can be detected and monitored using 2 methods of visual surveillance: pitfall traps and baited traps. The choice of a monitoring method depends on the situation.

Visual surveillance

Visual surveillance is used in high-risk RIFA areas. Generally, high-risk areas include riverbanks, old pits, newly planted nurseries and parks, fallow or barren land, paddy fields, traffic mediums, and roadsides. Ant mounds can also be formed on cement floors and electric boxes. The mounds can be observed in open areas with direct sunshine.

In newly established nests, a clear mound is not produced during the first few months. Mounds are formed after the ant nest has matured. Immature nests can be detected by digging in the soil, probing with an iron bar, or applying water. After that, one can observe whether or not any ants run out of the nest. Any nest with a mound higher than 10 cm , deposition of large amounts of sand around the exterior, aggressive movements by large numbers of ants when the nest is disturbed, and a honeycomb structure in the nest interior is a mature mound.

The advantage of visual surveillance is that by using this method a large area can be surveyed within a short  period of time; however this method does not provide exact information. Visual surveillance can be done by either walking or driving, depending upon the situation.

i. Visual surveillance by walking

With this technique, each kilometer is randomly surveyed 3 times. Surveillance is conducted on 100-m² units. High-risk areas for fire ants are investigated using a iron bar 70~ 100 cm long and 0.4 cm in diameter. If any suspected nests are observed, the nest is disturbed using the iron bar to check whether or not ants run out of the nest. After that, the mound is evaluated, the data should be recorded according to the one of following methods.

i. Density of ants in a mound: Fire ants actively running around a disturbed mound within 60 seconds indicates that it is an active nest. Results from observations can be classified into three levels according to the density of ants. Different colored flags are inserted according to the classification of distinguishing mounds:

First level (red flag): 101~1000 ants emerge from the nest;

Second level (yellow flag): 11~100 ants emerge from the nest; and

Third level (green flag): 1~10 ants emerge from the nest.

Density of active mounds per unit area: The infested area is classified according to the number of active mounds per unit area. Different colored inserted flags can be used to distinguish the areas from each other:

First level (red flag): a seriously infested area (more than 20 mounds/ 100 m² );

Second level (yellow flag): a semi-infested area (6~20 mounds/ 100 m² ); and

Third level (green flag): a low infested area (1~5 mounds/ 100 m² ).

2.     Visual surveillance by driving

Three observations can be made while driving each 5 kilometers with 500 m² as a observation unit. The driving speed should be less than 20 km /h . When a mound is observed, that becomes an observation point, and the mound should be disturbed to observe and estimate the number of ants and record the information described above.

3.     Pitfall trap

First, sampling points are set up around the surveillance areas and the number of sampling points per square meter is fixed. The distance between 2 sampling points should be more than 10 m . Each sampling point contains 4 pitfall traps. One pitfall trap is fixed in a 1-m² area. A pitfall trap is a plastic tube 3 cm in diameter and 12 cm deep. A plastic tube half-filled with 75% ethanol is inserted into the ground. The top of the tube should be level with the surface of the soil. The pitfall trap is removed, and the data are recorded 24 h after being set up. The number of ants is counted, classified, and grouped into 5 levels according to the number of ants in each trap:

First level: a heavily infested area (more than 200 ants/pitfall trap);

Second level: a medium infested area (50~200 ants/pitfall trap);

Third level: a low infested area (5~50 ants/pitfall trap);

Fourth level: an initially infested area (0~5 ants/pitfall trap); and

Fifth level: a non-infested area (no ants/pitfall trap).

4.Bait sampling method

Generally, artificial lures (high oil-containing foods) such as potato chips, peanut butter, hot dogs, syrup, and canned tuna fish are placed in a 10-cm-diameter plastic box with holes. After that, the bait is set on the ground, and a flag is used to mark the location. One bait trap is set per 100 m² . Generally, the trap is fixed during 08:00 to 17:00 because ants are known to be active during this time. The traps are removed and recorded after 1~3 hours. According to the number of ants captured in each trap, the observed area is classified according to following 5 levels:

First level: a heavily infested area (more than 200 ants/trap);

Second level: a medium infested area (50~200 ants/trap);

Third level: a low infested area (5~50 ants/trap);

Fourth level: an initially infested area (0~5 ants/trap); and

Fifth level: a non-infested area (no ants/trap).

Estimation of the control effect

Before bait application, the density of the fire ant mounds and number of fire ants per unit area is determined by visual surveillance using pitfall or bait traps. The same methods are applied to estimate the density of mounds and the number of fire ants with 6 replications after 1, 2, 4, 6, and 8 weeks. The number of replications is adjustable according to the situation in the field. Based upon the results from monitoring, the efficacy of bait application is estimated by using the following formula:

Control rate (%) = (1 - ) ×100%, or  

Control rate (%) = (1 - ) × 100%.