Control of ants using baits of low toxicity cannot be effective without knowledge of bait distribution patterns and bait station densities, which are determined by ants' foraging activities. Furthermore, the success of toxic baits also depends upon attractiveness of bait carriers. Here, we assessed ground and vine foraging activity and food preferences for the three ant species (Linepithema humile (Mayr) (Hymenoptera: Formicidae), Anoplolepis custodiens (F. Smith) and Crematogaster peringueyi Emery) under field conditions. We found that L. humile's vineyard foraging activity was high and that movement of ant bait by C. peringueyi and A. custodiens in the vineyard was relatively low. Consequently, more bait stations need to be dispensed for more effective control of C. peringueyi and A. custodiens than for L. humile. Different bait densities are discussed for the various ant species. Food preference trials indicated that vineyard foraging ants preferred wet bait attractants over dry ones, making liquids the most ideal carriers for baiting these ants. Linepithema humile was attracted to 25% sugar water, while C. peringueyi was attracted to both 25% sugar water and honey. Anoplolepis custodiens was attracted to tuna but was also attracted to 25% sugar water. Thus, future bait formulations should be tailor made to suit these specific food requirements if baits are to be successful in ant pest management.
In vineyards, the mutualism between ants and the vine mealybug, Planococcus ficus (Signoret) (Hemiptera: Pseudococcidae), results in population explosions of both insects, thereby causing pest injury and economic losses (Kriegler and Whitehead 1962; Way 1963; Myburgh et al. 1973). The Argentine ant, Linepithema humile (Mayr) (Hymenoptera: Formicidae), the cocktail ant, Crematogaster peringueyi Emery, and the common pugnacious ant, Anoplolepis custodiens (F. Smith), are the major pest ants associated with the vine mealybug in South African vineyards (Addison and Samways 2000). Linepithema humile is a cosmopolitan, invasive pest in agricultural and urban areas (Suarez et al. 2001) that nests predominantly in the soil, while C. peringueyi and A. custodiens are of southern African origin, the former nesting in the canopy and the latter in the soil (Addison and Samways 2000). Current recommendations for control include the use of chemical stem barriers, which target only ground nesting species by preventing access into the canopy (Addison 2002). This control method is designed to break the mutualism between the ants and mealybugs so that natural enemies, primarily encyrtid wasps, can effectively control P. ficus without ant interference (Samways 1990).
There is no documented control method for vine-nesting species in South Africa, but lowtoxicity baits could be a viable option, as the recruitment to food sources and subsequent food-sharing behavior of ants can be used to distribute toxicant through the colony no matter where the colony is located (Hooper-Bui and Rust 2000; Rust et al. 2004). Effective control of South African ants using baits of low toxicity is limited by inadequate knowledge on ant foraging activity and optimum bait station density. Optimal density of bait stations per unit area may vary depending on the ant species and the level of ant infestation. If ants have long foraging distances, then bait density could be lower than if ants have short foraging trails. Daane et al. (2006) dispensed liquid baits at 85–620 baits/ha against L. humile in Californian vineyards. At high densities, these baits resulted in fewer L. humile, fewer mealybugs, and less damage to grape clusters. Ant foraging ranges and behavior are affected by a number of factors, such as temperature, circadian rhythm, competition, food availability, food particle size, and photoperiod (Oster and Wilson 1978; Hölldobler and Wilson 1990). Markin (1968), for example, found that L. humile workers foraged up to 45 m away from the nest. Similarly, Ripa et al. (1999) found up to 21% of marked L. humile 54 m from the feeding station. Vega and Rust (2001, 2003) found marked ants up to 61 m from feeding stations. However, ant foraging activity in urban settings may differ from a vineyard because of varying abundance of food resources. Level of infestation by honeydew-producing mealybugs would presumably also impact foraging activity. Although L. humile can forage for long distances, these ants generally nest in close proximity to food sources and relocate nests when nearby food sources become exhausted (Suarez et al. 2001; Tsutsui et al. 2003; Heller and Gordon 2006). In South African citrus orchards, A. custodiens has been observed to forage 111 m away from their nest during peak honeydew production periods and up to 50 m in April, when the mealybugs are abundant (Steyn 1954). No research has been carried out to ascertain C. peringueyi foraging activity.
Different ant species have different dietary requirements, and consequently this must be considered during ant bait formulation. Effective proportions of fat, carbohydrates, and proteins in the bait may differ between species and with the colonies' nutritional needs (Rust et al. 2000). Physical state of the bait, liquid versus solid, and particle size also affect the rate of collection of the bait (Hooper-bui and Rust 2000). Most of the research on ant food preference has been done on L. humile (Baker et al. 1985). To date, no food preference studies have been done in South African vineyards to determine ideal food requirements for L. humile, A. custodiens, or C. peringueyi.
When dissolved in 25% sugar water, preliminary investigations showed that low toxicity baits were non-repellent to foragers (Nyamukondiwa and Addison 2011), palatable, and exhibited sufficient delayed toxicity to allow trophallaxis (Stringer et al. 1964). The present research was aimed at determining (i) the foraging activity (that is the distance each ant travels from its nest to the source of food) of vineyard ants (L. humile, C. peringueyi, and A. custodiens) using liquid baits labeled with a marker and (i