Red Imported Fire Ants in Merced and Stanislaus County Almond Orchards
Kris Tollerup, Areawide IPM Advisor, University of California Cooperative Extension and Statewide IPM Program
The red imported fire ant (RIFA), Solenopsis invicta (Buren), ranks among the most serious of the 33 invasive ant species introduced into California. The species not only poses a substantial economic threat to California agriculture, but to wildlife, and human health as well.
The species is native to an area in the central part of South America that extends from northwest of Porto Velho, Brazil to Santa Fe, Argentina. Due to accidental introductions, RIFA has established in the Southeastern United States, Australia, New Zealand, and Several Asian and Caribbean countries. The introduction into the United States, occurred in the early 20th Century and to date, the geographical distribution of RIFA in the United States includes: all the southern states from Florida to Texas, west into parts of California, and as far north as Arkansas, Oklahoma, Tennessee, and South Virginia.
In 1997 red imported fire ant arrived in Kern, Fresno, and Stanislaus county almond orchards purportedly on honeybee hives from Texas. The initial San Joaquin Valley infestations were confined to single properties and believed eradicated. However, RIFA infestations were detected again in the early 2000s in Merced County and in 2006 in Stanislaus County. Very soon after its detection, Merced County aided with personnel and funds from the California Department of Food and Agriculture (CDFA) began RIFA surveys and eradication efforts. By the end of 2009, Merced County eradicated RIFA from approximately 3845 acres of irrigated pasture and almond orchards. By about 2010 or 2011, CDFA stopped supporting RIFA programs and eradication efforts within the county ended. Currently, we do not have a full understanding of the extent of RIFA infestations in the Merced and Stanislaus counties, but clearly severe infestations do occur.
Soon after RIFA’s initial introduction into California, entomologists began, and justifiably so, making dire predictions of the ant’s impact on agriculture, wildlife, and the urban environments if not eradicated. And although there are still fairly substantial infestations in Orange, Los Angeles, Riverside, Merced, and Stanislaus counties, the predictions of widespread and serious economic impacts have not come to fruition.
Biology. The main reason for the slower expansion of RIFA into California almond is the Mediterranean climate and near absence of cues required for RIFA reproduction. RIFA reproduces via nuptial flights that occur at an altitude of about 1000 ft. Red imported fire ant colonies produce a reproductive caste, males (always with wings) and winged females during spring and summer. When the proper cues are received, such as high humidity indicating a rain event, the reproductives accumulate at the top area of the nest just below the soil surface. The worker ants leave the nest in mass and expand out, surrounding the nest opening, up to several feet; this is done to protect the reproductives as they exit the nest and prepare for their nuptial flight. Anything that wanders into the protected area, including humans is immediately attacked.
Almost immediately after mating the males fall to the soil surface and die. Females descend to the ground, pull off their wings, and begin digging a nest. If they land in a suitable soil environment, they dig a nest up to 3 to 4 feet below the soil surface and begin laying eggs. If the nascent nest is successfully established, the new queen does not ever leave the nest to forage but rather rears her first brood by digesting her wing muscles. Once established nests grow rapidly and not uncommonly have upwards of 120,000 to 220,000 workers.
Damage. Agricultural ant pests cause economic damage in a couple of ways, either by directly feeding on parts of a crop plant and/or indirectly by tending honeydew-producing hemipterans such as soft scale and aphids. In almond, direct feeding damage occurs to the nut meat (Fig. 1) while the crop is on the orchard floor at harvest. Soft-shelled cultivars such a Nonpareil and Carmel often suffer a greater amount of damage than hard-shelled cultivars such as Mission and Monterey.
Only two species, the southern fire ant, Solenopsis xyloni, (a close relative of RIFA) and the pavement ant, Tetramorium caespitum, cause the lion’s share of economic damage in infested orchards throughout the San Joaquin Valley. In almond orchards infested with RIFA, damage can easily exceed 10%. Also, it is important to note that RIFA, as well as southern fire ant and pavement ant do forage in tree canopies prior to harvest, however, foragers typically don’t feed on nuts, but rather collect nectar from extrafloral nectaries.
Identification. Red imported fire ant appears similar to southern fire ant. RIFA workers are red to brown in color with a black abdomen (gaster) (Fig. 2). Workers come in several sizes ranging from 0.12 to 0.24 in (2.4–6 mm). The species has two nodes and a sting at the tip of the abdomen (Fig. 2). Nests of RIFA are large with mounds of established nests measuring up to 1 or 2 ft high (Fig. 3). However, nest mounds also can appear low and dispersed, similar to southern fire ant nests (Fig. 4). Often RIFA nests are associated with vegetation (Fig. 5). Interestingly, RIFA nests have subterranean tunnels associated with them that reach out several feet from the nest’s primary entrances in all directions.
If it is suspected that an orchard is infested with RIFA, samples should be collected using plastic vials baited with a piece of hot dog or corn chip. Place two to three vials a few feet from the suspect nests for about 15 minutes. Carefully cap the vials, and place in the freezer for several hours to euthanize the ants. Governmental agencies such as CDFA, USDA, or county agricultural commissioner offices do not have any RIFA detection or eradication programs in infested San Joaquin Valley counties. Samples should be sent to the nearest University of California Cooperative Extension office.
Management. The University of California does not have official guidelines for managing RIFA. However, since RIFA feeds primarily on lipids and protein, the available options for controlling southern fire ant and pavement ant found in the University of California Almond Pest Management Guidelines work well. However, since RIFA colonies are large it typically requires multiple bait applications. One strategy that has substantially reduced colonies consisted of baiting with abamectin in early June followed by an insect growth regulator bait about two to three weeks later, then metaflumizone applied about two weeks before harvest. Importantly, apply bait to soil dry enough soil to prevent the bait granules from becoming moist. Also, if morning dew occurs, allow it to dry before making an application. Moreover, any bait application should be made during the morning hours before temperatures exceed 90°F and foraging activity decreases. For additional information on identification and information for managing RIFA, as well as other pest-ant species, view the Almond Pest Management Guidelines website .
Figure 1. Direct feeding damage to almond caused by red imported fire ant. Photo by K. Tollerup.
Figure 2. Red imported fire ant. Photo from www.antweb.org. Accessed 7 June 2018.
Figure 3. Red imported fire ant nest in an almond orchard. Photo by K. Tollerup.