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Ants foraging in vineyards in the Western Cape Province

Pia Addison, ARC Infruitec-Nietvoorbij, Stellenbosch
Biological control of the vine mealybug (Planococcus ficus) by predatory beetles and parasitic wasps is significantly reduced in the presence of ants (Kriegler & Whitehead, 1962; Myburgh et al 1973 and Urban & Mynhardt, 1983). Ants feed on the sweet honeydew excreted by mealybugs and thereby disturb the natural enemies as they attempt to feed on the mealybugs. In this way the ants gain an easily accessible food source and, in turn, the mealybugs gain protection from predators and are able to reach very high numbers.
Some ants nest in the vines, while others nest on the ground. Direct chemical stem barriers are not effective against vine-nesting ants such as the cocktail ant Crematogaster peringueyi. All ants are beneficial if they remain on the ground, as they are predacious and feed on other pests such as the pupae of fruit flies and false codling moth. Stem banding is a very effective method for controlling ground-nesting ants, as they are still left to prey on other pests, but are not permitted access into the vine canopy.
It is therefore necessary to know which ants are present in vineyards before control methods can be developed. A survey was conducted in six major vine-producing areas in the Western Cape in order to establish which ants predominate in each area and which ants are pests (associated with mealybug).
Material and Methods
During 1998 and 1999, 22 vineyards were surveyed in the Stellenbosch/ Paarl, Klein Karoo, Worcester, Swartland, Olifants River and Hex River Valley regions. All vineyards were trellised and were of varying age and size, and all were infested with mealybug. Each vineyard was sampled intensively for two consecutive weeks shortly before harvest, the time of year when mealybug populations reach their highest levels and ants are most active in the vines. Sampling was done using 42 pitfall traps per vineyard. These traps consist of polystyrene test tubes (18 x 150 mm) and were filled with 4 ml of seven parts 70% ethyl alcohol and three parts concentrated glycerol. The traps were sunk into small holes in the ground (Fig 1). In addition to this trapping method, tuna flakes were used to sample vine-foraging and vine-nesting ants. The flakes were placed into the crutch of the vines and left for 30 minutes, after which all ants feeding on the tuna were collected and placed into 70% ethyl alcohol. Ants were counted and identified in the laboratory. A dominance index for each species was calculated according to the Berger-Parker dominance index [total number of the most abundant ant species divided by the total number of ants (Southwood, 1978) and averaged.

Figure 1: Pitfall trap inserted into the ground to monitor ants.
Results and Discussion
Forty-two species of ants were recorded during the survey (Table 1). Six ground-nesting species were dominant in the 22 vineyards sampled. The economically most important ant species were as follows:

Figure 2: The common pugnacious ant (Anoplolepis custodiens), 5 - 10 mm, media and minor worker. (Afr: gewone malmier).
The common pugnacious ant (Anoplolepis custodiens) (Fig 2).
This was the most abundant pest ant in the Klein Karoo, Hex River Valley and Worcester areas. This ant was observed to tend mealybug and was often found in severely mealybug-infested vineyards, always dominating other ant species where it occurred. Its dominance in both the vine canopy (tuna bait traps) and on the ground (pitfall bait traps) was high (Table 1), indicating that it is a severe threat to the biological control of mealybug.

Figure 3: The black pugnacious ant (Anoplolepis steingroeveri), 5 - 10 mm, major, media and minor worker. (Afr: swart malmier) .
The black pugnacious ant (Anoplolepis steingroeveri) (Fig 3).
This ant was widely distributed, but did not always dominate over other ant species. It occurred in the Worcester, Swartland and Olifants River areas and seems to prefer dry environments. It was often seen nesting on dirt roads and fields with natural vegetation adjacent to vineyards, and was frequently caught in pitfall traps outside or on the edge of vineyards rather than in traps inside of vineyards. Its dominance in both the vine canopy and on the ground was relatively high (Table 1), indicating that this ant forages readily in vines and on the ground and is a serious pest.

Figure 4: The Argentine ant (Linepithema humile), 3 mm, all workers are the same size. (Afr: Argentynse mier).
The Argentine ant (Linepithema humile) (Fig 4).
This ant was recorded in traps in all areas except the Olifants River region. The reason for this could be that this ant prefers moist environments and is prone to desiccation (Witt & Giliomee, 1999). It is therefore possible that the Argentine ant has not been able to establish itself in the outlying, drier northern regions of the South Western Cape. Its dominance in both the vine canopy and on the ground was high (Table 1), indicating that it is a serious pest. This ant can also nest in vines.

Figure 5: The Cocktail ant (Crematogaster peringueyi), 3 - 6 mm. (Afr: wipstert mier).
The cocktail ant (Crematogaster peringueyi) (Fig 5).
This ant was recorded in traps in the Klein Karoo, Worcester and Swartland areas. A possible reason why it is not widespread in other areas, particularly the Hex River Valley, is due to the relatively intensive spraying of table grape vineyards with chemicals such as chlorpyrifos. Its dominance in the vine canopy was high (Table 1), but it did not dominate on the ground. This ant was never trapped in vineyards where the Argentine ant was trapped and was seen tending mealybug colonies. Two other species of Crematogaster with high dominance in vines were trapped in the Olifants River region, Klein Karoo and Swartland areas, and it is therefore possible that they are also associated with mealybug. It appears to be less competitive than other dominant ant species, and was never trapped in the same vineyard as the Argentine ant. It is relatively widely distributed and considered a pest due to its association with mealybug. Its habit of stinging workers harvesting grapes and infesting irrigation pipes and tunnels in dead or damaged vines, however, also adds to its pest status.

Figure 6: The little ubiquitous white-footed ant (Technomyrmex albipes), 2 - 3 mm.
The little ubiquitous white-footed ant (Technomyrmex albipes) (Fig 6).
This ant only dominated in one vineyard in the Hex River Valley where the mealybug infestation was noticeably low. It was recorded in traps in all six regions. This ant was found to be an occasional pest in citrus (Samways et al, 1982), and could be important in contributing to mealybug infestations (Urban et al, 1980). Its dominance, however, was the lowest of all the dominant ants caught (Table 1) and since it is not widely distributed as a dominant ant, it is not considered a pest at this stage.

Figure 7: The big-headed ant (Pheidole sp 1), 3 - 7 mm, major and minor worker.
Big-headed ants (Pheidole spp) (Fig 7).
Three species of Pheidole were found foraging in vineyards, of which two species dominated in the Worcester and Swartland areas. One species was seen tending mealybug colonies, but dominance was low (Table 1). This species was also found in the Klein Karoo and Stellenbosch areas. It is not economically significant at this stage, as it is mostly out-competed by more aggressive ant species.

Figure 8: Messor capensis, 5 - 10 mm, major and media worker.

Figure 9: Tetramorium quadrispinosum.

Figure 10: Camponotus fulvopilosus.
All other ant species that were trapped are beneficial ants that are predators or seed harvesters, but are not known to be associated with mealybug infestations. It is therefore important to establish, if possible, which ants are found in the vineyard before control measures are applied, or to establish if they forage only on the ground or also in the vines. Two species that are very common and can be confused with pest ants are the seed harvesters, Messor capensis (Fig 8) and Tetramorium quadrispinosum (Fig 9). Camponotus fulvopilosus (Fig 10) is sometimes seen in the vine canopy, but is not considered a pest in vineyards.
Conclusions and Recommendations
According to this survey, the most important pest ants are the common and black pugnacious ants, the Argentine ant and the cocktail ant. These ants are all associated with mealybug and dominance in vines was high. They are therefore a threat to the biological control of mealybug and need to be controlled.
Recommended control methods are chemical stem sprays with alphacypermethrin SC at 10 ml/l for Argentine ants and 20 ml/l for pugnacious ants. If applied as a 10 cm wide band (50 ml per vine) above the irrigation pipe with a ring spray attached to a knapsack spray pump, vines could be kept free of ants for up to 110 days. It is recommended that vines are treated as soon as ants are seen foraging in the vine canopy. It is important to treat all vines in the infested vineyard, as ants can easily move across wires or irrigation pipes. In cases where ant infestations are severe, as is often seen with the pugnacious ants in the Breede River Valley, it may be necessary to apply a second treatment towards the end of the season. This is the only method of control that is currently acceptable for the integrated production of wine grapes and is registered for the control of ants in vineyards. In order to control the cocktail ant, an application of chlorpyrifos EC can be applied at 4 ml/l during the dormant period (Nel et al, 1999).
Future research needs to focus on establishing the effect of cover crops to deter ants from nesting in vineyards as a management tool that is sustainable, environmentally friendly, cost-effective and beneficial to natural enemies. The use of poisonous baits that are specific to ants also needs to be researched, and can be a useful control option in situations where ant infestations are not that severe.
For further information, contact Pia Addison, telephone (021) 809 3145 or fax (021) 809 3584; e-mail: pia_a@infruit.agric.za.
References
Kriegler, P J & Whitehead, V B, 1962. Notes on the biology and control of Crematogaster peringueyi var. angustior Arnold on grape vines. Journal of the Entomological Society of Southern Africa 25: 287 - 290.
Myburgh, A C, Whitehead, V B & Daiber, C C, 1973. Pests of deciduous fruit, grapes and miscellaneous other horticultural crops in South Africa. Entomology Memoirs, Department of Agriculture, Technical Services, Pretoria 27: 1 - 38.
Myers, N J, 1957. Studies on the biology of ants associated with citrus trees. MSc. Thesis, Rhodes University, Grahamstown.
Nel, A, Krause, M, Ramautar, N, & van Zyl, K, 1999. A guide for the control of plant pests. 38th edition, National Department of Agriculture, Republic of South Africa.
Samways, M J, Nel, M & Prins, A J, 1982. Ants (Hymenoptera: Formicidae) foraging in citrus trees and attending honeydew-producing Homoptera. Phytophylactica 14: 155 - 157.
Southwood, T R E, 1978. Ecological Methods with special reference to the study of insect populations. 2nd Edition. Chapman and Hall, London.
Urban, A J & Mynhardt, M E, 1983. The integrated control of the vine mealybug, Planococcus ficus (Signoret), on vines. Progress report on project PB 2 16 1/23/2/1. Plant Protection Research Institute, Stellenbosch.
Urban, A J, Stander, A J & Bradley, M E, 1980. The integrated control of the vine mealybug, Planococcus ficus (Signoret), on vines. Progress report on project (A) I-Pr. 135. Plant Protection Research Institute, Stellenbosch.
Witt, A B R & Giliomee, J H, 1999. Soil surface temperatures at which six species of ants (Hymenoptera: Formicidae) are active. African Entomology 7: 161 - 164.
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