Ants: Monitoring and your Management Options

Kris Tollerup, University of California Cooperative Extension Area-wide IPM Advisor

California has approximately 270 described ant species, although very few of the species pose an economic threat to agricultural crops.  Pest ants can cause direct damage, such as feeding on the nuts, or indirect damage by protecting honeydew-producing hemipterans such as aphids and soft scale.  In almond, the primary concern is the direct damage caused by the southern fire ant, Solenopsis xyloni, thief ant, Solenopsis molesta, and pavement ant, Tetramorium caespitum.  Other species that often occur in almond orchards but pose no economic threat because they do not feed on the nuts, include the native gray ant, Formica aerate, pyramid ant; Monomorium bicolor, and California harvester ant, Pogonomyrmex californicus.  The native gray and pyramid ant feed mostly on other small insects and arthropods, and when available, honeydew, while the harvester ant feeds exclusively on small seeds.   

Since both pest and non-pest ant species so commonly exist in the same orchard, monitoring plays an important role in efficient ant management.  The University of California Pest Management Guidelines (PMGs), recommend that the mean number of ant nest openings be determined in five survey areas of ~1,000 ft2.  The UC PMGs provides a form and table to aid in recording sampling results and management decisions. (http://ipm.ucanr.edu/PMG/C003/almond-antcolony.pdf)

In agricultural cropping environments, coarse morphological differences, such as number of nodes on the petiole, presence of a sting (Fig. 1, A), and color and size (Fig. 1, B) provide enough information to properly identify pest and non-pest ants.  Examining ants using a 10x hand lens works well to observed the characters but requires that they be collected and euthanized (an overnight stay in a freezer works well).  An effective method of capturing ants consists of placing bait such as putting whole Frito corn chips or a piece of bologna in a small plastic vial and placing 10 vials of each bait type in various locations in the orchard for about four hours during the morning.          

A drawback to the UC PMGs for ants in almond is that more information is not given to help make a distinction between pests and non-pest ants in the orchard.  Fortunately, foraging activity and behavior, nest location, and manner in which ants deposit tailings around the entrance hole and/or holes provides useful information.  For instance, during hot daytime temperatures the foraging activity of southern fire ant diminishes considerably, while that of the native gray remains high.  Also, during foraging, the native gray moves in a more herky-jerkey fashion than the southern fire ant.  Both the southern fire ant and native gray ant construct flat disperse nests in shaded areas and with multiple entrances (Fig. 2, A and B).  However, tailings deposited around entrance holes of southern fire ant tend to be finer grain size and are generally hillier (Fig. 2, B).     

Kris Tollerup, Lori Berger, and Pete Goodell developed an ant identification field guide available at: The guide was developed as part of a project between the University of California, Agricultural and Natural Resources and the California Department of Pesticide Regulation to identify and manage critical uses of chlorpyrifos against key pests.  Although, the publication focuses on ant species found in citrus, most of the species that infest citrus in the San Joaquin Valley also infest almond.  The field guide is unique in that it provides descriptions of key characteristics such as dispersion of nest tailings, foraging and trailing behavior rather than solely on the description of ant morphological characteristics.   We are currently working on a similar field guide for almond.  Employing the field guide into monitoring will enable more efficient differentiation of pest and non-pest ant species in almond.

Four, very effective bait management options exist for almond, insect growth regulator (IGR) baits, Esteem (pyrproxyfen), and Extinguish (methoprene), and stomach poison baits, clinch (abamectin) and Altrevin (metaflumizone).  Each of these baits use soybean oil as the attractant / toxicant carrier and preferentially attract lipid-feeding (protein-feeding) ants.  When deciding on a management strategy, make sure to consider the lag time between bait application and population reduction.  IGR baits work slower, taking up to eight weeks before affecting a significant reduction in foraging activity.  And, in contrast, the stomach toxicants, abamectin and metaflumizone work much faster, requiring just three and one week respectively before significantly reducing foraging activity.     

Insecticide use should occur only if monitoring indicates the presence of ant pests.  Apply bait insecticides only after considering the potential risks of the compound to water quality.  For more information on this and other topics please consult the UC IPM Pest Management Guidelines for Almonds at http://ucipm.ucanr.edu > Agricultural pests > Almond

Fig. 1. (A) Diagram of an indicating five key coarse morphological characteristics.

(B) Southern fire ant, Solenopsis xyloni. Picture by April Nobile. From www.AntWeb.

Fig. 2A Nest of native gray ant.

Fig. 2B Nest of southern fire ant. Both nests have multiple entrance holes with tailings dispersed over a large area. Tailings of southern fire ant tend to be smaller grain and hilly compared to native gray ant.