Variation in Foraging Activity Areas of Worker Ants in Newly Established Red Imported Fire Ant Colonies Across Different Habitats

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Introduction
of the red imported fire ant in mainland China may continue for another 12 to 14 years, with an estimated total number of suitable administrative areas ranging from 1,400 to 1,600 (Wang et al., 2022;Lu Yongyue et al., 2015).
The foraging behavior of the red imported fire ant is a critical factor in its establishment and expansion in new ecological environments.Researchers have proposed several theories for reference while exploring worker ant foraging behavior, including Optimal Foraging Theory, Diminishing Returns Theory, Collective Cognition Theory, Individual Differences Theory, and Interaction Strategy Theory.The Optimal Foraging Theory posits that worker ants adjust foraging investment levels and colony size based on the quality and quantity of food to maximize their energy gains (Kolay, 2020).The Diminishing Returns Theory suggests that worker ants, when foraging, follow the principle of diminishing marginal returns, and the population decreases its effort to obtain food as its scarcity increases (Roeder, 2020).The Collective Cognition Theory indicates that ant colonies can achieve global coordination and decision-making through local interactions between individuals (Couzin, 2009).The Individual Differences Theory proposes that behavioral differences among workers significantly impact collective cognition and decision-making (Modlmeier et al., 2012).The Interaction Strategy Theory suggests that the foraging strategies adopted by ants in interactions with other ants, including competition, cooperation, predation, and defense, depend on their ecological niche, resource distribution, colony structure, and information transmission methods (Gordon, 2010).During the invasion of new habitats, the red imported fire ant competes with nearby ant colonies and maintains a clear territorial boundary.Researchers view the red imported fire ant nest as a superorganism and focus on how the ant colony allocates labor.Studies have found that the proportion of foraging worker ants in the ant colony decreases with an increase in colony size, indicating that ant colonies cannot expand without restraint and are regulated by the proportion of foraging worker ants (Tschinkel, 2011).
To further investigate whether there are corresponding patterns in the foraging behavior of red imported fire ant workers in different habitats and at different times, this experiment employed bait trapping to measure the foraging activity of newly established red imported fire ant colonies in various environments and time periods.The aim was to clarify the foraging strategies and adaptability of red imported fire ant workers in different environments and explore the foraging patterns of red imported fire ant workers, providing a basis for the prevention and control of red imported fire ants.

Experimental design
This study aims to investigate the impact of different crop environments on the red imported fire ant, providing insights into its control.At the South Asian Tropical Crop Research Institute (21°15′70″N, 110°27′38″E), four different crop environments -bare land, sugarcane, pineapple, and sweet potato -were selected for transplanting red imported fire ants.Bare land was chosen as the control.Each crop occupied 0.5 acres, with a spacing of 1 m between plants and an 8 m wide isolation zone separating different environments.Active and large-volume red imported fire ant nests from areas invaded by the target crop were excavated, and before transplantation, the length, width, height, and perimeter of the nests were recorded.We transplanted three nests to each type of crop field.The transplanted ant nests were arranged in groups of three from north to south, with a minimum spacing of 20 m between each nest to avoid interference during baiting.Four tubes containing 10 ml of tap water and ham sausage (Shuanghui) were placed around the periphery of each transplanted nest as a source of food and water for the ants.The recovery of the red imported fire ant nests was observed daily.The worker ant foraging activity range was investigated when the ant activity range reached 15 m after 14 days.

Ant Sampling Methods
The experiment was conducted from August to November, with temperatures ranging from 20 to 30°C during baiting.Five bait bottles containing ham sausages were placed at 1 m intervals in eight directions from the ant nest center: east, southeast, south, southwest, west, northwest, north, and northeast, extending 5 m outward.The bottle openings were placed close to the ground, facing the nest direction, and marked with red flags.Bait was released at each point every half an hour, and the number of worker ants attracted was recorded after waiting 15 minutes for ant activity to stabilize.After the final baiting session, all residual baits were cleared to ensure that a large number of worker ants were not still foraging near the nests.

Data Analysis Methods
Worker ant foraging data were processed and analyzed using the Jupiter programming software based on Python.The ant nest data were imported into the polar function to generate a polar coordinate graph, marking the maximum range for display.The program extracted the intersection point of the maximum range and the polar coordinates, calculating the total area of the worker ant activity range with the ant nest center as the origin.The theoretical area of worker ant activity range generated by the program was subjected to analysis of variance using SPSS 27.Since data were not normally distributed, a Kruskall-Wallis test was performed to determine significant differences in the effect of vegetation cover types on activity range and the number of foraging ants.

Impact of Different Environments on the Daily Average Activity Range and the Number of Foraging Red Imported Fire Ant Workers
After calculation, the results for the daily average activity range and the number of foraging red imported fire ant workers in different environments are shown in Figure 1 and Table 1.The daily average activity range of red imported fire ant workers in open land is significantly higher than in sweet potato and pineapple fields (Kruskal-Wallis test, χ 2 = 14.947, df = 3, P = 0.02).The number of foraging ants in open land is significantly higher than in the other three environments (Kruskal-Wallis test, χ 2 = 15.096,df = 3, P = 0.02), with an average activity range of 26.36 m 2 and 1,102 foraging ants.The average activity ranges for sweet potato and pineapple fields are 16.87 m 2 and 16.25 m 2 , respectively, with both environments having around 500 foraging ants.There was no significant difference in the ant activity range in sugarcane fields compared to open land.
These results indicate that vegetation cover is an essential factor influencing the foraging behavior of red imported fire ant workers.In the open land environment without vegetation cover, the foraging ant quantity and search range of red imported fire ant workers are significantly higher than in the other three environments with vegetation cover.

The daily activity variation patterns of red imported fire ant workers in different habitats
The daily average activity patterns of red imported fire ant workers vary in different environments.In sweet potato fields, the daily average activity of red imported fire ant workers shows a clear bimodal pattern, with two peaks occurring at 10:00 and 16:00.At the same time, it is lower during noon and in the evening.In open land and sugarcane fields, the daily average activity of red imported fire ant workers exhibits an unimodal pattern, reaching the highest levels at 10:00 and 16:00 and lower levels at other times.In pineapple fields, the daily average activity of red imported fire ant workers shows a fluctuating pattern, with some fluctuations at various time intervals but without distinct peaks or troughs.In Figure 2, it can be observed that the curves for the total activity range and average activity distance of red imported fire ant workers in open land, sweet potato fields, pineapple fields, and sugarcane fields are relatively stable, without significant peaks or troughs.Therefore, in these four environments, there is no apparent regularity in the changes in total activity range and average activity distance of red imported fire ant workers.

Discussion
In four different habitats, red imported fire ants exhibit two distinct foraging patterns: 1) the bare-ground habitat pattern, where the quantity and foraging range of worker ants are significantly higher in environments without vegetation cover than those with vegetation.This increase may be attributed to the absence of vegetation, hindering worker ants' effective access to food and water, thereby significantly increasing the quantity and range of foraging and reducing the pressure on the colony's survival.2) the vegetationcover pattern, where there is no significant difference in the Activity range (Kruskal-Wallis test, χ 2 = 14.947, df = 3, P = 0.02) Foraging ant number (Kruskal-Wallis test, χ 2 = 15.096,df = 3 P = 0.02) Table 1.The effects of different habitats on the daily average activity range and foraging quantity of red imported fire ant workers.foraging range and quantity of worker ants among sugarcane, pineapple, and sweet potato habitats.In these environments, red imported fire ant workers can quickly obtain food and water in the vegetation near their nests, sustaining their colony life.Both of these patterns align with the Optimal Foraging Theory, indicating that animals adjust their foraging efforts and colony size based on the availability and quality of food to maximize energy gain (Kolay, 2020).
In addition to habitat effects, the foraging behavior of red imported fire ants is influenced by factors such as temperature, humidity, light, and food resources, leading to different rhythms and dynamics in various environments (Xu et al., 2007).Studies have indicated that the foraging activity of red imported fire ants follows distinct daily and seasonal rhythms.During winter and spring, activity peaks at noon, while in summer, it peaks in the morning and evening.The most frequent activity occurs from April to June, followed by September to November, with the lowest activity from December to March (Xi et al., 2010).Generally, red imported fire ants exhibit the most frequent and vigorous foraging activity in warm, humid, dimly lit, and food-rich conditions (Zhou et al., 2011).Research has shown that the foraging activity of red imported fire ants in lawns and grasslands demonstrates clear seasonality and daily variation.In contrast, foraging activity in banana and lychee orchards is more stable and uniform (Xu et al., 2014).
The analysis of the daily average activity patterns of red imported fire ant workers in this study indicates that workers in pineapple fields exhibit a fluctuating activity pattern throughout the day, lacking distinct peaks or troughs.Workers in sweet potato fields show a bimodal activity pattern.In contrast, those in open areas and sugarcane fields display an unimodal pattern, with peaks occurring at the same times in all three habitats, namely 10:00 and 16:00.This suggests that the activity patterns of red imported fire ants are indeed influenced by environmental factors such as temperature, humidity, and light, resulting in different foraging rhythms and intensities in different habitats.
This study unveils the relationship between red imported fire ant worker foraging behavior and their habitat, providing valuable information for understanding the ecological dynamics of their invasion.However, the study has certain limitations, such as a relatively limited range of experimental subjects, few experimental locations, and relatively simple experimental methods.Future research could further explore whether there are additional patterns in the foraging behavior of red imported fire ants concerning other environmental factors.

Fig 1 .
Fig 1.Schematic diagram of the daily average activity range of red imported fire ant workers in different habitats.Note: A: bare land B: sweet potato field C: pineapple field D: sugarcane field

Fig 2 .
Fig 2. Daily activity variation patterns of red imported fire ant workers in different habitats.Note: E: bare land F: sweet potato field G: pineapple field H: sugarcane field.