[Annual Ant Control Issue] Is the National Zoo Haunted?

The ghost ant (order Hymenoptera, family Formicidae) is a common species of ant that is not invisible, but sometimes appears to float around because of near-transparent legs and abdomen. Ghost ants are similar to other ant species in that they play a small role as part of a larger collective, working together to benefit their colony.

Colonies tend to range in number and size with small- to medium-sized colonies whereby numbers often range from several hundred to several thousand with workers being monomorphic (i.e., having only one form). While ghost ants do not sting and do not typically bite, one interesting fact is that they are sometimes predaceous upon the medically important kissing bug (Rhodnius prolixus), a known vector of Chagas’ disease endemic to South America.
 

Distribution.

Originally thought to be from Africa and Asia, ghost ants are now commonly found worldwide with greatest activity occurring within tropical climates of North and South America and the Caribbean. Ghost ants are common pests of greenhouses and zoos and are one of several species commonly referred to as “tramp ants.” Colonies can have multiple queens and propagate via budding whereby various queens and select nestmates will migrate to new nesting sites to establish new satellite colonies. While mating flights are common for many ant species, they are not common within ghost ants.
 

Identification.

Ghost ants are similar in appearance to their relative, the odorous house ant (Tapinoma sessile). Both produce a distinct odor (sometimes described as rancid coconut) when crushed; this technique is useful for quick field identification when magnification is not readily available.

Adults are small, measuring about 1.5 mm in length (1/16th inch). The head and thorax are typically black while their abdomens, legs, and antennae are translucent. In some regions/cultures, ghost ants are called black-headed ants because their head and thorax colorations are so much more pronounced. Like all ants, they have a pinched wasp-like waist where one flattened node (pedicel) or segment is found.
 

Damage.

Adults tend to be the most problematic as they are the ones commonly encountered as they forage for food. Ghost ants are omnivorous, with food preferences ranging from sweet substances such as sugars and honeydew to proteins depending upon colony nutritional requirements. Ghost ant adults will sometimes “farm” or protect aphids and mealybugs that produce honeydew.

Ghost ants are commonly found within leaf litter, underneath tree bark, underground and within vegetation. Indoors, they can be found within almost any available hollow cavity such as those found within wall, floor, and ceiling spaces, behind and between cabinetry and within interior plantings.

While they cause no structural damage, ghost ants are nuisance pests that are often not well tolerated within most buildings. Within the Smithsonian National Zoological Park, building environments with higher humidity levels tend to be favored by ghost ants; these ants are commonly found within potted plants, planter beds, wall voids (concrete masonry unit/hollow blocks), as well as within boxes located near and within greenhouse and similar tropical exhibits.
 

Scouting Procedures.

Visual inspections should be made for adults within atriums, kitchens, bathrooms and other areas with high humidity. While trails can be difficult to find, adults often travel along intersecting edges (e.g., walls/floors, countertops). Using baits to attract them and then tracing trails back toward nesting areas can be helpful in identifying entry points as well as colonies.

Economic thresholds have not been established, but will depend upon environmental conditions; more sensitive accounts will require swift action while less sensitive ones will allow more time for more thorough inspection and resolution.
 

Management Methods.

Ant identification is helpful in managing this species. Sanitation to remove food residues will help as well as minimizing available moisture/humidity to as great an extent as possible. Baiting can be helpful and is recommended, but is often slow (see “Ghost Ant Bait Trial” later in this article), especially when dealing with larger ant colonies. Identifying entry points and tracing those back to nest sites to allow direct treatment provides the most effective means of control.

Other ghost ant management efforts include the need for exterior inspections to identify colonies that may be occurring outside. For outdoor colonies and greenhouse environments, removing or addressing aphid and mealybug populations will help to manage ghost ants. Exclusion includes using sealants such as caulking and mortaring compounds to close gaps and cracks through which ants may enter and should be performed to as great an extent as possible. Areas to be sealed include wire penetrations, pipe penetrations and expansion joints.
 

Ghost Ant Bait Trial.

A bait trial was initiated to implement an in situ evaluation of ant baits in the presence of readily available alternative food resources. Initially, two ant bait formulations (one thick liquid, one gel) with two different active ingredients (5.0 percent borax and 0.05 percent indoxacarb, respectively) were used to evaluate efficacy and test bait acceptance. Four tubes of bait (two of each bait type) were installed along ghost ant foraging trails located external to an animal holding habitat. Ants were observed trailing out of the exhibit along the wall and floor edge.

Both bait types were injected into plant water tube picks and tubes were placed along ant trails. Baited tubes were weighed for eight weeks to determine bait consumption and evaluate efficacy. The original intent was to bait for only two weeks so as to reflect a real-world baiting scenario whereby treatment time is often limited and successful results are needed quickly. After the initial two weeks, bait was left in place to measure long-term effects on activity and bait consumption.

Traits of a good bait include one that is accepted in lieu of alternative food options. Challenges include competing food resources and baits that become unpalatable as they age. Some bait formulations are hygroscopic (i.e., absorb moisture) while others desiccate. Regardless, if alternate food options are available and, more importantly, selected, baits become irrelevant since baits generally must be consumed to be effective. Such a small trial serves as an excellent indicator of whether or not to continue baiting and if so, with which bait to proceed with treatment regimens. Baits that are accepted regardless of the presence of other food options serve to indicate effective bait formulations.

Results and Discussion.

Ghost ants displayed an affinity for one bait (indoxacarb gel matrix) over the other (borax thick liquid matrix) (Figure 1). Average amount of bait placed was 29.45 grams for borax and 26.55 grams for indoxacarb. Average consumption was quite different, with 0.20 grams consumed on average per day for the borax-based bait compared to 1.65 grams for indoxacarb.

The greatest difference in bait consumption, however, occurred during the first week of baiting, with 0.60 grams borax bait consumed vs. 10.20 grams of the indoxacarb bait; that means 17 times more indoxacarb-based bait was consumed during the first week compared to borax-based ant bait. This difference in preference is likely a function of the bait matrix. Results are further supported by ant recruitment to the two different bait formulations; low to no recruitment was observed in response to the borax-based thick liquid bait while medium to high numbers (i.e., dozens of ants) were recruited to the indoxacarb gel bait. There was a measurable decline in bait consumption during weeks two through seven, with an average of 0.13 grams of borax liquid bait consumed per week versus 0.80 grams indoxacarb gel bait.

Results do not suggest that one bait is necessarily superior to the other, but that it is merely preferred given certain parameters, those being the pest species being treated, time of year, and current colony nutritional needs. Ant colony nutrition is dictated by brood production and energy needs. Like most animals, ants too utilize carbohydrates, lipids and proteins; those nutritional requirements are determined by colony activities.

In summary, baiting trial and error will dictate which baits should work best for different situations. Results here suggest that if bait is accepted within the first week, it should remain available and be replenished while alternate baits should be tried after two weeks once initial acceptance is followed by declining consumption as long as the number of foragers remains high.

Gregory A. Ose, B.C.E, is an associate entomologist in the Center for Animal Care Sciences at the National Zoo in Washington, D.C. Email him at oseg@si.edu.

References

Chin, D. 2008. Factsheet ENT2: Ants in the Household and Garden. http://www.nt.gov.au/d/Content/File/p/Plant_Pest/ENT2%20Ants%20in%20the%20household%20and%20garden%20Factsheet.pdf Accessed 25 March 2013.

Mallis, A. 1997. Ghost Ant. Handbook of Pest Control, Eighth Edition. pp. 543 - 544.

Nickerson, J. C., C. L. Bloomcamp, and R. M. Peirera. 2003. Ghost Ant, Tapinoma melanocephalum (Fabricius). EENY-310. UF/IFAS Extension. http://edis.ifas.ufl.edu/pdffiles/IN/IN53200.pdf Accessed 25 March 2013.