Rodent-Proof Wall: An Efficient Physical Method for Controlling Rodents and its Efficiency Statistics

Abstract

Rodent damage poses a significant threat to crops, human life, and health. Compared to chemical rodent control, such as placing poisonous baits, it is more economical and environmentally friendly to use physical methods, such as building a rodent-proof wall. This study introduces a method of physically controlling harmful rodents and four methods of calculating the effect of rodents control. To understand the controlling effect of the rodent-proof wall, an investigation was conducted on the Dongting Lake beach and corresponding farmland in the embankment in April and July 2012. Our findings illustrated that the density of the reed vole Microtus fortis in the farmland with rodent-proof walls was 0.52%, significantly lower than that in the farmland without rodent-proof walls (1.76%) after artificial trapping and drug extermination (χ² = 3.900, P = 0.048). The density of M. fortis that had migrated into the farmland in dikes with a rodent-proof wall decreased by 98.53%, significantly higher than the decrease of density in dikes without a rodent-proof wall (86.61%) (χ² = 11.060, P = 0.01). The results demonstrated the effectiveness of rodent-proof wall control. Therefore, building a rodent-proof wall should be advocated and vigorously promoted to prevent the migration of rodents into the Dongting Lake area and similar environments, as they cause harm.

Introduction

Rodent damage is an important biological disaster that causes extensive damage to all aspects of human production and life^1,2. In agriculture, rodent infestation of farmland damages crops³; in forestry, rodents eat tree seedlings, roots, bark, and plant seeds, leading to delayed forest regeneration and tree death, which in turn affects forest greening and sand fixation⁴; and in grasslands, rodents eat roots and seeds, leading to degradation of grassland vegetation and increased sanding, which affects the development of the grassland livestock industry⁵. In addition, rodents are hosts for many viruses, bacteria, and parasites that can seriously endanger human health⁶.

Dongting Lake, located in the northeastern region of Hunan Province, is an important water storage and flood-regulating lake in China⁷. It has many ecological functions, such as flood regulation and detention, biodiversity protection, and water resource supply^8,9. In recent decades, there have been many rodent outbreaks in the Dongting Lake area, especially an outbreak of the reed vole Microtus fortis in 2007, which caused enormous economic losses¹⁰. During the dry season, M. fortis grows and breeds on the lake beach in the Dongting Lake area. As the water level in Dongting Lake rises during the summer flood season, the habitat of M. fortis shrinks, forcing it to migrate into the embankment by swimming, crossing the flood control embankment, and reaching nearby farmlands, causing great harm to agricultural production^11,12. Chen et al. proposed a control measure for building a rodent-proof wall to block the migration pathway of M. fortis, based on the wave-retaining wall method jointly created by Jinpen Farm and Nanda Town in Yuanjiang City, both of which are located in Yiyang City, Hunan Province^13,14. In dikes without rodent-proof walls, large numbers of M. fortis can cause devastating damage to crops during and after migration. Dikes without rodent-proof walls are typically manually trapped and drugged to exterminate rodents during and after they enter the farmland. In the case of dikes with rodent-proof walls, many rodents remain outside the dikes during M. fortis outbreaks. Thus, many trapping and elimination operations are carried out outside dikes; generally, farmlands do not need to implement chemical drugs or artificial trapping. This approach can significantly reduce the density of vermin without causing serious harm to farmland crops. Conventional rodent drug prevention methods cannot achieve the expected effects, and there are hidden dangers associated with environmental pollution and human and animal safety¹⁵. Considering the characteristics that allow rodents to easily break out and cause disaster, the following protocol introduces three physical rodent control methods, including a permanent rodent-proof wall and two kinds of temporary walls, and presents four statistical methods to measure rodent control effect, providing a scientific basis for rodent control. Instead of the traditional poisonous bait, the rodent-proof wall effectively protects the environment and safeguards the health of humans and animals; thus, it is a more effective and environmentally friendly control method that should be advocated and vigorously promoted.

Protocol

All animal experiments were approved by the Ethics Committee of the Institute of Subtropical Agriculture, Chinese Academy of Sciences.

1. Construction of rodent-proof wall

1. Construction of wave-retaining wall
   1. Build the wave-retaining wall 0.5 m higher than the embankment surface. Smooth the wall surface on the side of the lake with cement, and add a flat plate slightly wider than the wall to the top such that it extends 8 cm, similar to a tongue (Figure 1). This ensures that the rodents cannot climb the wall.
      NOTE: The height of the wave-retaining wall and the width of the flat plate can be built according to different rodents.
2. Digging a rodent-proof trench
   1. When some wave-retaining walls have gaps leading to the beach, dig a rodent-proof trench at the gap. The ditch is 0.5 m deep and slightly wider than the gap of the wave-retaining wall.
   2. Fill soil or cover a thin cement board in the trench to facilitate the passage of pedestrians and vehicles. During the flood season, remove the thin cement board and clear the soil in the trench, thereby completely blocking the rodent's migration path (Figure 2).
3. Buried pots barrier method
   1. First, erect fences along the dike.
   2. Compose the baffle plate of fiberboard, plastic film, and wood board and support it with timber piles.
   3. Bury the plate fence 5-10 cm into the soil at a height of 0.5 m. Bury the deep pots between the fixed fences at 50 m intervals.
   4. Ensure that the pots are 80 cm deep and 60 cm in diameter and buried in the soil immediately next to the baffle plate, with the mouth of the pots set flush to the ground (Figure 3).
   5. Rodents approach the dike, walk along the fences, and are channeled into the pots. Finally, dredge the pots and clean out the rodents.

2. Methods of efficiency statistics

1. Snap-trap method
   1. Use snap traps throughout the survey period using raw sunflower seeds as bait. Sample three to four plots of approximately 6-10 ha each along a line transect. Ensure that the distance between plots is >150 m, and install between 80-100 traps in each plot, with one trap placed every 5 m.
   2. After the water rises in late spring and early summer annually, conduct a survey on the farmland of the corresponding survey lake.
   3. Lay traps along the ridge of the field at 5 m intervals, with >200 traps laid in each area. Place traps in the afternoon and collect them the following morning. Then, count the captured rodents according to species.
   4. Calculate relative abundance, which is an indication of trap success, using the following equation:
      
      Where A is the number of captured rodents, B is the effective number of traps, and C is the relative abundance of rodents.
2. Crop damage statistics
   1. Use crop damage statistics to control for the effects of rodents should crops be located on the opposite side of the lake beach dike¹⁶.
      1. Sample the rice fields in the survey area randomly at the same time as the rodent relative abundance survey.
      2. Sample the selected fields according to a single five-point diagonal sampling method, with five sample squares collected from each field. Collect the rice seedlings (10 × 10 plants) in each square sample.
   2. Record the total number of seedlings and the number of seedlings broken by the rodents to determine the damage rate. Determine the effectiveness of rodent control by calculating the cost incurred by the rodent due to damage before and after rodent control.
3. Bait consumption method
   1. Place bait of the same size in the survey area, and after a certain period, calculate the bait consumption rate as an indicator of rodent density¹⁷.
      1. If the bait used was rice, calculate bait consumption as weight (g). Simultaneously, control the natural decrease in water from the bait to correct bait consumption. If the bait is a large grain or block, such as sweet potatoes, observe and record the number of grains or blocks consumed.
4. Excavation hole method
   1. Block the original holes to reduce misjudgment of abandoned holes.
   2. Determine the effectiveness of rodent prevention with or without a rodent-proof wall by comparing the number of holes dug by rodents in farmlands.
      NOTE: Plugging the hole should be carried out in a strict manner, preferably before the peak of rat activity, such as for house mice in the evening or ground squirrels in the early morning.

3. Statistical analysis

1. Use appropriate data analysis software to analyze the data and determine the significance of each capture rate using the chi-square test (χ²), with statistical significance set at P <0.05.
   NOTE: SPSS was used to analyze the data. Other statistical software can also be used for statistical analysis, such as DPS.

Representative Results

To determine the control effect of the rodent-proof wall, surveys were conducted before and after the water rose in April and July 2012, respectively, on the lake beach and corresponding diked farmland¹⁵. The survey sites were located in the Dongting Lake area of Hunan Province, namely, the lake beach outside Matangyuan in Yueyang County (29°14.5′ N; 113°03.2′ E), Beizhouzi Town in Datong Lake District (29°10.1′ N; 112°47.7′ E), Shuangfeng Dike in Nanda Town in Yuanjiang City (29°1.3′ N; 112°45.2′ E), and Muping Lake outside Munan Village Dike in Chuangyeyuan Nanzui Town in Yuanjiang City (28°59.6′ N; 112°15.1′ E). The polder in Beizhouzi and Nanda towns built rodent-proof walls, whereas those in Matangyuan and Chuangyeyuan did not.

In April 2012, a total of 815 traps were placed in the lake beach habitats of the four survey sites, and 258 animals were captured, of which 248 could be identified to the species level: 197 M. fortis, 41 Apodemus agrarius, 7 Rattus norvegicus and 3 Suncus murinus (Table 1)¹⁵. From the remaining 10 animals, only parts of their bodies, such as the tail, feet, hair, and blood, were captured; thus, the species could not be identified. In July 2012, 1141 tongs were placed at 4 survey sites in farmland habitats, and 59 animals were captured, of which 54 were identified: 38 A. agrarius, 13 M. fortis, and 5 R. norvegicus. For the remaining five animals, only parts of their bodies, such as the tail, feet, hair, and blood, were captured; thus, the species could not be identified. The farmland was dominated by A. agrarius and M. fortis, which accounted for 70.37% and 24.07% of the species composition, respectively.

The capture results for the dikes with and without rodent-proof walls are listed in Table 2. Owing to the low population of M. fortis on the lake beach of Muping Lake, the density of M. fortis on the outer lake beach without a rodent-proof wall was relatively low. However, the density of M. fortis in the corresponding farmland in July was higher at 1.76%, whereas that of M. fortis with a rodent-proof wall was only 0.52%. In terms of the absolute values, there was also a significant difference (χ² = 3.900, P = 0.048); the density of M. fortis in farmland with a rodent-proof wall was significantly lower.

Considering the density of M. fortis at the lake beach as the base, the decrease in M. fortis density in the farmland of the corresponding dike was calculated (Table 3)¹⁵. The density of M. fortis in the dike with the rodent-proof wall was further reduced. The average decrease in the density of M. fortis in the dike without a rodent-proof wall was 86.61%, whereas that in the dike with the wall was significantly higher at 98.53% (χ² = 11.060, P = 0.01). In the Nanda Town Shuangfeng polder, no M. fortis was captured in the farmland, despite the density of the lake beach reaching 29.61% (Table 1)¹⁵. This suggests that the rodent-proof wall protects against and controls rodents.

Figure 1: Rodent-proof wall. Build the wave-retaining wall 0.5 m higher than the embankment surface. Smooth the wall surface on the lakeside with cement, and add a flat plate slightly wider than the wall to the top such that it extends 8 cm, similar to a tongue. Please click here to view a larger version of this figure.

Figure 2: Rodent-proof trench. The ditch is 0.5 m deep, slightly wider than the gap of the wave-retaining wall. Usually, the trench is filled with soil or covered with a thin cement board to facilitate the passage of pedestrians and vehicles. During the flood season, the thin cement board is removed, and the soil in the trench is cleared, thereby completely blocking the rodents' migration pathway. Please click here to view a larger version of this figure.

Figure 3: Buried pots barrier method. Fences are erected along a dike; the baffle plate can be composed of fiberboard, plastic film, or wood board and is supported by timber piles. The plate fence is buried 5-10 cm into the soil to a height of 0.5 m. The deep pots are buried between fixed fences at 50 m intervals. The pots are 80 cm deep and 60 cm in diameter and buried in the soil immediately next to the baffle plate, with the mouth of the pots flushing with the ground. Rodents then approach the dike, walk along the fences, and are channeled into the pots. Finally, the pots are dredged to remove the rodents. Please click here to view a larger version of this figure.

Site	Habits	Number of traps	Number of animals	Total capture rate (%)	Capture rate for each species (%)
					Rattus norvegicus	Apodemus agrarius	Microtus fortis	Suncus murinus
Matang polder	Beach	178	61+1	34.83	0	3.93(7)	29.78(53)	0.56(1)
	Farmland	270	15	5.56	0.37(1)	2.22(6)	2.96(8)	0
Chuangye polder	Beach	233	10	4.29	0	3.86(9)	0.43(1)	0
	Farmland	297	12+2	4.71	0.34(1)	3.03(9)	0.67(2)	0
Beizhouzi	Beach	198	101+1	51.51	1.01(2)	7.58(15)	41.41(82)	1.01(2)
	Farmland	290	27+2	10	0.34(1)	7.93(23)	1.03(3)	0
Nanda shuangfeng	Beach	206	76+8	40.78	2.43(5)	4.85(10)	29.61(61)	0
	Farmland	284	0+1	0.35	0	0	0	0

Table 1: Capture of rodents at each investigation site in April and July 2012. Values after the plus sign indicate the number of captures for species that could not be identified. Values inside the plus sign indicate the number of captures for each species.

Rodent-proof wall	Habits	Number of traps	Number of animals	Total capture rate (%)	Capture rate for each species (%)
					Rattus norvegicus	Apodemus agrarius	Microtus fortis	Suncus murinus
Without rodent-proof wall	Beach	411	71+1	17.52	0	3.89(16)	13.14(54)	0.24(1)
	Farmland	567	27+1	4.94	0.35(2)	2.65(15)	1.76(10)	0
With rodent-proof wall	Beach	404	177+9	46.04	1.73(7)	6.19(25)	35.40(143)	0.50(2)
	Farmland	574	27+3	5.23	0.17(1)	4.01(23)	0.52(3)	0

Table 2: Capture of rodents on the dike with and without rodent-proof wall. Values after the plus sign indicate the number of captures for species that could not be identified. Values inside the plus sign indicated the number of captures for each species.

Rodent-proof wall	Site	Capture rate of Microtus fortis (%)		Decrease rate of population density (%)
		Beach	Farmland
Without rodent-proof wall	Matang polder	29.78	2.96	90.06
	Chuangye polder	0.43	0.67	0.00*
With rodent-proof wall	Beizhouzi	41.41	1.03	97.52
	Nandashuangfeng	29.61	0	100

Table 3: Decrease population density rate in farmland and adjacent beaches after Microtus fortis immigrated into farmland. The symbol * indicates that the capture rate in farmland was higher than that on the beach, which may have resulted from the low base; therefore, the decrease rate was calculated as 0.00%.

Discussion

There are several critical steps in the protocol. The width of the tongue-shaped flat plate of the rodent-proof wall should be sufficiently wide, and the height of the wall should be set such that rodents in the area cannot cross it. When wave-retaining walls have gaps leading to the beach, the depth of the rodent-proof trench should be set to a depth that rodents cannot climb, and the width of the ditch should be set to a distance that rodents cannot cross. The height of the baffle plate and pots in the buried-pot barrier method should be set to a height that rodents cannot cross. When an outbreak of rodents occurs in an area, the rodents in the pots should be cleaned out regularly to keep the pots clean and prevent the rodents from jumping out of them.

Assessing the effectiveness of rodent control is indispensable and important in evaluating the performance of such controls and summarizing the experience¹⁷. When investigating the effects of rodent control, the conditions before and after- rodent control must be as similar as possible. Methods are needed to set up a control area (areas without rodent control) to further calculate the corrected rodent control rate and evaluate the rodent control effect more objectively. The snap-trap method is suitable for small rodents, which mostly exhibit nocturnal activity¹⁸. The snap-trap method was used before and after rat extermination using the same size of the rat trap, with the same size of food bait, and the same method as the cloth clip. The snap-trap method is simple, has a wide range of applications, is not subject to seasonal restrictions, and can be used as a test material. However, several factors affect the capture rate, for example, the abundance of outside food and whether other animals, such as insects and ants, ingest the bait. In addition, a mouse trap can catch only one mouse; therefore, the capture rate is relatively low¹⁷.

The bait consumption method is mainly based on rodents stealing the bait to observe the degree of reduction in their population¹⁹. The water content of the bait decreases in the environment. To calculate bait consumption more accurately, it was necessary to create a control group for the natural reduction of bait water to correct bait consumption. The bait used should be different from the poisonous bait to avoid affecting the bait consumption rate in the rodent extermination effect survey. Poisonous baits are mostly prepared using grains, with sweet potato pieces often being used. Generally, they are coarse or fine depending on their specific practices for different purposes. In the former, the number of grains was fixed for each bait pile, and the piles of bait that were stolen and dragged for food (without counting the number of stolen grains or those dragged for food) were listed as the number of piles eaten. From the total number of piles laid and the total number of piles eaten, the theft rate was calculated as an indicator of rodent density; the latter, rather than comparing the lure of two baits to rodents, is a more refined approach. Briefly, each bait pile was quantified, and the consumption rate was calculated after a certain period; however, the bait can also be moved and eaten by cockroaches or other insects. We prepared larger bait pieces and focused on the timely recovery of the bait to prevent its consumption by poultry. In addition, the weather conditions before and after baiting should be the same.

The excavation hole method can be used in any area where a rodent hole is found. The excavation hole method saves labor. However, some rodents (e.g., weasels) can dig more than one hole. In the field, a detailed check on the number of rodent holes in a certain area was conducted to reduce the impact of abandoned holes. Soil, dry dung, dry grass, or twigs were then used to tightly block all the holes. This has two purposes: to prevent beetles, lizards, and snails from moving in and out of the open hole and to assist in determining whether the hole was dug by a rodent inside or outside the hole.

The use of physical methods to control the migration of rodents has improved the economic and ecological benefits over chemical rodent control. The rodent-proof wall replaces traditional poisonous bait to prevent rodents, effectively protecting the environment and ensuring human and animal safety¹⁵. In addition, the rodent-proof wall is a permanent barrier to prevent rodents from entering the farmland, based on the original "wave-retaining wall" on the dike around Dongting Lake, and according to the characteristics of the migratory hazards of the rodents and their weak climbing ability. The rodent-proof wall is suitable for rodents with migratory habits. The rodent-proof wall is suitable for areas where rodents migrate from one habitat to another, and after migration, it will cause harm to an area. This method of constructing rodent-proof walls could also be applied in other areas to control rodent infestations, for example, where an outbreak of rodent infestation occurs in a single area, or an area of conservation value needs to be protected from invasion by pest rodents. If frequent outbreaks of rodent infestation occur in an area, permanent rodent-proof walls made of cement can be constructed. It requires some time to build a rodent-proof wall; however, once built, the wall provides long-term and stable protection. The cost of building a rodent-proof wall in conjunction with existing works (e.g., wave-retaining walls) is low. Finally, for temporary rodent infestation outbreaks, it is possible to build temporary rodent-proof walls. Temporary rodent-proof walls can be constructed of plastic, fiber, or materials other than cement to reduce their building costs.

Materials

Name	Company	Catalog Number	Comments
Snap traps	Guixi Mousing Tool Factory, Jiangxi, China	large-sized	150 mm × 80 mm
SPSS	IBM	version 16.0