Feature

Turning pests into allies for citrus farmers

Angela Bownes, Sean Moore and Martin H. Villet

Ant bands are collars fastened around the tree trunk and smeared with sticky chemicals that prevent ants from climbing the tree to collect honeydew.

Whether an animal is a pest or not depends on your perspective. For example, ants are commonly regarded as pests in many crops because of their close association with crop-damaging insects. In the case of citrus orchards, these crop pests include mealybugs, scale insects, aphids and psyllids. Ants gather a sugary secretion called honeydew from these pests and in return they defend the pests from their natural predators and parasites (known in crop protection circles as 'biological control agents'). This breakdown in natural pest control often leads indirectly to outbreaks and epidemics of citrus pests. For this reason, farmers use financially and ecologically expensive insecticides to combat the unwanted effects of ants. The ecological expense comes from the loss of pollinators and beneficial pest-eating predators and parasites that are killed by the insecticides.

However, there is another side to ants: they can also be effective predators themselves, and there is abundant evidence that predacious ants can inflict heavy mortality on agricultural pests. This gives them the potential to be important biological control agents in agro-ecosystems. The big question is whether it is worth farmers' time and money to try to turn these 'pestilent ants' into allies, and how this can be done. This has been the focus of research done by Rhodes University and Citrus Research International in the Eastern Cape. As it turns out, it is worth while, and because the pests that ants encourage and the pests that they combat live in different parts of the orchard, it is largely a matter of confining the ants to the areas where they will be beneficial.

In southern Africa, the brown house ant (Pheidole megacephala) and the pugnacious ant (Anoplolepis custodiens) are widespread and abundant in citrus-producing areas. They are also largely responsible for outbreaks of citrus pests like red scale, and are therefore often controlled with insecticides. J.J. Steyn, an entomologist working on citrus pests noted that these pest ants prey on the pupae of soil-pupating citrus pests like bollworm (Helicoverpa armigera), false codling moth (Cryptophlebia leucotreta), fruitflies (Ceratitis rosa) and citrus thrips (Scritothrips aurantiaca). In order to work out the economic benefits of having the ants, a study to quantify the effectiveness of their predatory activity was launched by planting pupae of these other pests in orchards. If there were significant benefits to the crop, the conservation of ants would then be strongly justified as a method of pest control, as opposed to poisons.

For the study the scientists selected two orchards of each of four different ages: 2, 4, 15 and 30 years old. Within each of these eight orchards, three non-continuous subunits consisting of a block of 16 trees were demarcated, one subunit of each of which was left as an untreated control experiment in which ants could forage wherever they wished, while the other two received one of two different experimental treatments. In one of the treated subunits, the ants' nests were poisoned, and in the second the trees were protected with ant bands around their trunks. Ant bands are collars fastened around the tree trunk (see image above) and smeared with sticky chemicals that prevent ants from climbing the tree to collect honeydew. This enabled us to investigate the effect of predation in the relative absence of the two ant species (because they were poisoned) and where they were present on the ground but prevented from foraging in the trees by the ant bands.

During the study, bait pupae of three soil-pupating pests, bollworm, false codling moth and fruit fly were buried under a layer of soil in trays beneath the citrus trees. The trays were protected from other predators like birds and rodents. Bait trays were checked at the end of each trial (the duration of which depended on the duration of the pupal stage of each pest species) and the number of whole pupal cases found were counted, indicating the number of pupae that survived and reached adulthood.

The results showed that the average proportion of surviving pupae for all three pest species was significantly higher in subunits of all orchards where the ants had been poisoned than where they were allowed to forage on the ground or wherever they liked. This means that predation on the pest pupae was less efficient where populations of the brown house ant and the pugnacious ant had been suppressed. This suggests that predacious ants have the potential to substantially reduce the survival rate of soil-pupating citrus pests. No difference in pest mortality between the banded and control subunits was found, indicating that preventing ants from collecting honeydew does not alter their willingness to attack pupae in the soil.

Ants do need to be controlled because they encourage other pests in the canopies of citrus trees. However, the research showed that this problem can be solved by using ant bands to eliminate their harmful affects in citrus orchards while allowing them to play a key role in the control of pest species. Serendipitously, conserving ants also has other benefits in crop systems - they move and aerate soil and facilitate the cycling of nutrients in the soil.

Another interesting outcome of the experiment was that even in the poisoned subunits, predation on pupae still occurred, and at a fairly high rate. Apparently the predator complex as a whole, which included spiders, parasitic wasps and certain flies and beetles, inflicted heavy mortality on the pupae and thus has a significant influence on the growth of pest populations. By not spraying, this predator community can be conserved and encouraged to help in combating pests. It is therefore recommend that citrus crops should be managed so as to optimize the benefits of predacious arthropods to the crop. If chemical control is needed, safe and selective pesticides like microbial formulations should be used that will leave beneficial species unharmed. By conserving these species, more sustained pest suppression should be obtained with a decreased need for pesticides.

The take-home message for farmers is that ant bands are an alternative method for controlling ants that is ecologically appropriate, environmentally friendly, and cost effective. They can also be used for other tree crops where predacious ants are part of the agro-ecosystem. So, there are ways to turn pests into allies!

More information:

Angela Bownes and Martin Villet are with the Department of Zoology & Entomology, Rhodes University, Grahamstown, 6140 South Africa. Sean Moore is with Citrus Research International, Humewood, Port Elizabeth

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