The red imported fire ant, Solenopsis invicta Buren (Hymenoptera: Formicidae), is an invasive ant of serious public health significance. In this study, a device and a protocol were developed for collecting brood of the red imported fire ant in the field. Field trials showed that within 30 min about 700 larvae and pupae plus about one half this number of workers were collected per device. This collection procedure takes advantage of the innate brood rescue behavior of alarmed workers, which involves strong negative phototaxis. This brood collection device is not useful for collecting queens, but it provides a practical means to separate the brood from the mound soil.
The red imported fire ant, Solenopsis invicta Buren (Hymenoptera: Formicidae) is one of the most notorious invasive ant species, because of its significant impact on human health, agriculture, livestock and wildlife (Vinson 1997; Chen 2007). Techniques have been developed for the separation and maintenance of red imported fire ant colonies in laboratories (Khan et al. 1967; Markin 1968; Banks et al. 1981; Chen, 2007) because they are crucially needed for research. Solenopsis invicta colonies usually have been collected by shoveling mound soil into a container, such as a plastic bucket (Banks et al. 1981). Various laboratory studies require the separation of a colony from is mound soil. In the laboratory, the waterdrip method was developed for colony separation (Jouvenaz et al. 1977; Banks et al. 1981). However, few studies have focused on techniques for collecting brood of the fire ants in the field. In current study, devices and methods were developed for collecting brood of red imported fire ant in the field.
Materials and Methods
Device for Collecting Solenopsis invicta Brood
The device developed in this study consists of an opaque light-tight covered column and a circular base (Fig. 1), which henceforth is referred to as the cover. The cover was 5 cm high with a diam of 20 cm. Three gaps were made in the cover, and the cover was fixed on the base with 3 plugs. The purpose of the gaps was to serve as entrances through which workers could deliver rescued brood into the device. The innate brood rescue behavior of the workers results from their negative phototaxis.
Procedures for Collecting Solenopsis invicta Brood
Thirty-two colonies were scooped by shovel for brood collection in the field. The procedure to separate the brood from mound soil by the device included the following 3 steps:
Several shovel scoops of mound soil were scattered thinly over a sorting sheet as described by Porter (1993);
The inside surface of the wall of the cover of the brood collector was coated either with vaseline or Teflon®. Two or 3 brood collectors were placed on the sheet at its edge and immediately adjacent to the scattered mound soil. Care was taken to assure complete contact of the base of the brood collector with the sorting sheet to prevent workers from moving brood underneath the base;
After 30 min the workers had transferred almost all the brood into the device, then the device was turned upside down and the brood collection with brood and workers was transported to the laboratory, where the brood and the workers were counted.
The device successfully provided a shelter for brood rescue. Usually the brood was moved from the sorting sheet into only 1 or 2 of the devices on the sheet. The device(s) nearest to the brood on the sorting sheet were chosen by the workers as the shelter for the brood. An average of 745 ± 358 individuals of larva and pupa were collected per collector, depended on the quantity of brood on the sorting sheet. Accordingly, the workers were also caught in the device as 32 ± 5% of the combined catch of workers plus brood individuals; but only 1 queen was transferred into a collection device while collecting brood from 32 colonies.
The brood collection device was designed on the basis of the worker rescue behavior, and especially on their strong negative phototaxis. The brood rescue behavior of workers was also previously used for colony collection, especially for searching out the queen (Tschinkel & Howard 1978). In addition the negative phototaxis of alarmed fire ants was also recognized and used in laboratory by Porter & Tschinkel (1985). We were successful in developing a device for red imported fire ant brood collection based on the brood rescue behavior of workers.
The workers were also caught in the device in a ratio to brood individuals of about 50: 100, and therefore the brood cannot be separated from the workers by this device. Moreover, the device is not useful for collecting queens because only one queen was transferred into a collecting device in this study. Nevertheless, rescue behavior was successfully used to collect queens by Tschinkel & Howard (1978).
Brood can be separated from the soil in about 30 min by the device used in this study. By the water drip method a colony could be separated overnight (Banks et al. 1981), and within 2–3 h at the increased drip rate developed by Chen & Wei (2005). However the water drip method cannot be used to collect brood in the field. Accordingly, this study has provided a practical method of brood collection.
This work was supported by the National Natural Science Foundation of China (Grant No. 31100316) and Science and Technology Program of Guangdong province (2012B061800091), and we express deep gratitude to the Foundation.