Article Super Absorbent Polymers Buried within Dry Soil Attract Formosan Subterranean (: )

Qinxi Xie 1, Zhengya Jin 1, Wei Lin 2, Kena Xue 2, Xuemei Chen 2, Kai Zhao 3, Xiujun Wen 1 and Cai Wang 1,*

1 Guangdong Key Laboratory for Innovation Development and Utilization of Forest Plant Germplasm, College of Forestry and Landscape Architecture, South China Agricultural University, Guangzhou 510642, China 2 Foshan Institute of Forestry, Foshan 528222, China 3 College of Agriculture, South China Agricultural University, Guangzhou 510642, China * Correspondence: [email protected]; Tel.: +86-20-85280256

 Received: 26 June 2019; Accepted: 15 July 2019; Published: 16 July 2019 

Abstract: Baiting is one of the main methods to control subterranean termites. Many previous studies showed that subterranean termites avoid making tunnels within dry soil and feeding on dry wood, which may decrease bait infestation and consumption in drought areas. Super absorbent polymers are a group of materials that can retain large amounts of water and improve the moisture content of soil and bait matrices, and therefore may attract termites. In the present study, choice tests were conducted in the laboratory to investigate the aggregation and feeding behaviors of Formosan subterranean termites, formosanus Shiraki, in response to the three super absorbent polymers—sodium polyacrylate (Na-PAM), potassium polyacrylate (K-PAM), and poly(acrylamide-co-acrylic acid) potassium salt (P(AM/AA))—that were either placed within soil or filled in the void volume of baiting containers. Under dry-soil (30%-moisture) conditions, termites consumed significantly more wood in the chambers where super absorbent polymers were buried than in the control chambers (super absorbent polymer was not placed within soil). In addition, Na-PAM placed within dry soil significantly increased aggregation compared with the control chambers. However, no aggregation or feeding preference was detected when super absorbent polymers were placed within wet soil (60%-moisture). Also, filling super absorbent polymers into the void volume of baiting containers did not attract termites, whether the soil was dry or wet. Our study showed that placing super absorbent polymers within soil around bait stations may increase bait consumption by subterranean termites in drought locations.

Keywords: aggregation; bait; Coptotermes formosanus; drought; feeding preference; super absorbent polymers

1. Introduction The Formosan subterranean termite, Coptotermes formosanus Shiraki, is an economically important pest with a wide distribution in subtropical and temperate areas [1,2]. This pest attacks buildings and wooden structures in urban areas, and caused huge economic losses worldwide [3]. Suszkiw [4] estimated that the annual repair and control cost for C. formosanus was >1 billion dollars (USD) in 11 US states. In China, the economic losses caused by C. formosanus was ~0.1 billion dollars each year [5]. Unlike many other subterranean termites, C. formosanus can also attack live trees. A recent study conducted by Evans et al. [6] showed that large proportions of forests in South Carolina and Louisiana have been infested by C. formosanus, which may have resulted in the death of the trees. Since

Forests 2019, 10, 591; doi:10.3390/f10070591 www.mdpi.com/journal/forests Forests 2019, 10, 591 2 of 14

C. formosanus is more likely to attack some tree species (e.g., bitternut hickory, bald cypress, blackgum and sweetgum in Louisiana) than others, it may change the community structure in forests [6]. Baiting is one of the main strategies to control subterranean termites. This method applies a small amount of active ingredients (e.g., hexaflumuron, noviflumuron, etc.) in bait matrices, which can be searched for and consumed by termites, and eventually causes the elimination of the whole colony. Although the success of baiting against termites has been shown over the past 20 years [7], the effectiveness of baits may be reduced under certain environmental conditions. For example, previous studies showed that termites had limited ability to make tunnels in dry soil/sand [8]. Also, low moisture content of wood significantly decreased the feeding activities of termites in the laboratory and field [9,10]. Few studies tried to increase the moisture conditions of bait matrices or soil around bait stations and therefore improve bait attractiveness to termites. For example, Xiong et al. [11] reported that filling the void volume of bait stations with sodium bentonite, a clay material with the ability to absorb and retain large amounts of water, can significantly increase wood moisture and bait consumption by termites under dry conditions. However, filling the bait stations with another water-retaining material, polyacrylamide/attapulgite composite, showed no promise to increase wood consumption by termites [12]. Some patents on termite bait stations claim the use of super absorbent polymers that may improve the moisture of bait matrices [13], but the effectiveness of such potential applications have not yet been investigated. In the present study, we hypothesized that: (1) placing super absorbent polymers within soil around bait stations would attract foraging termites and increase bait consumption; and (2) filling the void volume of bait stations with super absorbent polymers would create a favorable micro-environment for termites and therefore increase their aggregation and feeding activities. To test these hypothesizes, we conducted different choice tests to investigate the preferences of termites in response to super absorbent polymers (either placed within soil or filled in the bait containers) under dry or wet conditions.

2. Materials and Methods

2.1. Termites Three colony groups of C. formosanus (>500 m apart from each other) were collected in the campus and arboretum of South China Agricultural University (SCAU), Guangzhou, China, using underground monitoring stations containing pine wood sticks [11]. The monitoring stations infested by C. formosanus were brought to the laboratory and maintained in 60 L plastic containers for <1 month. Just before the experiment, the wood sticks were gently knocked with each other to extract termites.

2.2. Super Absorbent Polymers and Soil (Substrate) Three super absorbent polymers—sodium polyacrylate (Na-PAM), potassium polyacrylate (K-PAM), and poly(acrylamide-co-acrylic acid) potassium salt (P(AM/AA))—were purchased from online sellers. We chose these particular polymers because they have been widely used to increase the water-holding capacity of agricultural and forest soil [14,15]. The basic information (i.e., manufacturer and water absorption ability) of these super absorbent polymers is shown in Table1. Top soil ( <5 cm) was collected from the arboretum of SCAU and used as the substrate in the present study. A sample of soil was sent to the Laboratory of Forestry and Soil Ecology (College of Forestry and Landscape Architecture, SCAU), and classified as sandy clay loam (54% sand, 23% silt, and 23% clay). The soil was dried at 50 ◦C for ~2 weeks and sterilized at 80 ◦C for 3 days. Wooden pestles and mortars were used to ground the dried soil, which was then sifted through a 2-mm sieve to remove the coarse particles. The moisture level of soil was determined using the formula provided by Chen and Shelton [16] as follows: moisture level (%) = [weight of distilled water/(weight of saturated soil weight of dried soil)] − 100%. The water saturation level of super absorbent polymers was determined using the formula × provided by Xie et al. [12] as follows: water saturation (%) = [weight of distilled water added/(weight Forests 2019, 10, 591 3 of 14 of saturated super absorbent polymers weight of dried super absorbent polymers)] 100%. One day − × before the experiments, the required amount of deionized water was added to prepare the super absorbent polymers at the 30% and 60% water saturation, as well as soil (substrate) at the 30% and 60% moisture levels.

Table 1. Basic information of each super absorbent polymer (SAP) used in the present study.

Water Absorption at Each Super Absorbent Abbreviation Saturation Level Brand Manufacturer Polymer 100% a 60% a 30% a Sodium polyacrylate Sigma Aldrich, St Na-PAM 795.1 b 477.1 b 238.5 b Sigma (cross-linked) Louis, MO, USA Zibo Jadreh Polymer Potassium polyacrylate K-PAM 355.2 213.1 106.6 Jadreh Technology Co, Ltd., (cross-linked) Zibo, China Poly(acrylamide-co-acrylic Sigma Aldrich, St acid) potassium salt P(AM/AA) 183.9 110.3 55.2 Sigma Louis, MO, USA (cross-linked) a Water saturation level. b Amount of deionized water (g) can be absorbed by 1 g dry powder of super absorbent polymer to reach the 100%, 60%, or 30% water saturation level at room temperature (25 ◦C).

2.3. Preference of Termites to Super Absorbent Polymers Placed within Soil Two-choice tests were conducted to investigate whether placing super absorbent polymers (Na-PAM, K-PAM, or P(AM/AA)) within dry or wet soil surrounding the baiting containers would increase aggregation and wood consumption of termites. A total of six multiple-choice tests (3 types of super absorbent polymers 2 moisture conditions) were conducted. Each test was repeated 12 times × (4 replicates for each colony group 3 colony groups). × The bioassay arenas were plastic boxes (153 72 49 mm [L W H]) with three chambers × × × × (69 50 40 mm [L W H]). There was a small hole (~5 mm in diameter) at the bottom center × × × × of each of the two inner walls of the chambers. A baiting container was prepared by drilling 10 holes (diameter = 5 mm, lined up in 2 rows and staggered distributed) on the wall of a plastic box (diameter of upper side = 41 mm, diameter of bottom side = 31 mm, height = 33 mm). A wood block (pine wood, 20 20 20 mm) was oven dried (80 C for 5 days) and weighed using a 0.1 mg electronic × × ◦ balance, and then placed inside the baiting container. Baiting containers were placed in the side chambers of the bioassay arenas (Figure1A). Under dry-soil conditions, the baiting container in one side chamber (randomly assigned) was buried using 30% moisture soil, but the top of the baiting container was unburied (Figure1B). On the other side, soil was first added to partially bury the baiting container in the depth of 1.5 cm (Figure1C), and 8 g of 30% water-saturated super absorbent polymers (Na-PAM, K-PAM, or P(AM/AA)) was evenly placed on the surface of soil (Figure1D,E). Additional soil was then added to bury the baiting container (Figure1F). Similar procedures were conducted to prepare the bioassays under wet-soil conditions, but 60% moisture soil was used as the substrate, and 60% water-saturated super absorbent polymers were placed within soil. Forests 2019, 10, 591 4 of 14 Forests 2019, 10, x FOR PEER REVIEW 4 of 14

FiguFigurere 1. 1. ExperimentExperiment settings settings to to investigate investigate the the preference preference of of termites termites to to super super absorbent absorbent polymers polymers placedplaced within within soil. soil. Baiting Baiting containers containers were were placed placed in the in side the sidechambers chambers of bioassay of bioassay arenas arenas (A). The (A). baitingThe baiting container container in one in side one chamber side chamber (randomly (randomly assigned) assigned) was buried was buried using using soil, but soil, the but top the of top the of baitingthe baiting container container was unburied was unburied (B). O (nB the). On other the side, other soil side, was soil first was added first to added partially to partially bury the bury baiting the containerbaiting container in the depth in the of depth 1.5 cm of ( 1.5C), cm and (C 8), g and super 8 g superabsorbent absorbent polymers polymers were wereevenly evenly placed placed on the on surfacethe surface of so ofil soil(D [side (D [side view] view] and and E [topE [top-down-down view]). view]). Additional Additional soil soil was was then then added added to to bury bury the the baitingbaiting container container (F (F).).

OneOne hundred hundred termites termites (95 (95 workers workers and and 5 5 soldiers) soldiers) were were placed placed into into the the center center chamber chamber of of each each bioassaybioassay arena arena and and sealed. sealed. The The bioassays bioassays were were placed placed in in the the environmental environmental chambers chambers setting setting at at 25 25 °C◦C underunder total total darkness. darkness. On On day day 21, 21, the the bioassay bioassay arenas arenas were were opened opened and andcotton cotton balls ballswere wereused usedto seal to theseal holes the holes on the on bottom the bottom center center of inner of inner walls, walls, and and the the number number of of termites termites in in each each chamber chamber was was counted.counted. The The wood wood blocks blocks in in each each baiting baiting container container were were weighed weighed using using the the 0.1 0.1 mg mg electronic electronic balance balance asas mentioned mentioned earlier.earlier. TheseThesewood wood blocks blocks were were then then oven oven dried dried (80 ◦(80C for°C fivefor days)five days) and the and dry the weight dry weightwas measured. was measured The wood. The consumptionwood consumption was determined was determined by calculating by calculating the dry the weight dry weight change change before beforeand after and the after experiment. the experiment. The moisture The moisture of wood bockof wood was bockdetermined was determined using the formula using the provided formula by providedXie et al. [by12 ]Xie as follows:et al. [12] wood as follows: moisture wood (%) =moisture[(wet weight (%) = after [(wet the weight experiment after thedry experi weightment after − dry the − weightexperiment) after the/dry experiment weight after)/dry the weight experiment] after the100%. experiment] × 100%. × 2.4.2.4. Preference Preference of of Termites Termites to to Super Super Absorbent Absorbent Polymers Polymers Filled Filled in in Baiting Baiting Containers Containers MultipleMultiple-choice-choice tests tests were were conducted conducted to to investigate investigate the the aggregation aggregation and and wood wood consumption consumption of of termitestermites in in response toto baitingbaiting containers containers filled filled with with each each super super absorbent absorbent polymers polymers (Na-PAM, (Na-PAM, K-PAM, K- PAM,or P(AM or P(AM/AA)),/AA)), or soil, or or soil, unfilled or unfilled (controls). (controls). The tests The were tests conducted were conducted either undereither dry-under or dry wet-soil- or conditions. In total, there were six multiple-choice tests (3 types of super absorbent polymers 2 moisture wet-soil conditions. In total, there were six multiple-choice tests (3 types of super absorbent× polymers conditions), and each test was repeated 12 times (4 replicates for each colony group 3 colony groups). × 2 moisture conditions), and each test was repeated 12 times (4 replicates for each× colony group × 3 colonyThe groups). bioassay arenas were 750 mL plastic boxes (diameter of upper side = 14.1 cm, diameter of bottomThe bioassay side = 11.3arenas cm, were height 750= m6.5L plastic cm). Baiting boxes (diameter containers of were upper prepared side = 14.1 as mentioned cm, diameter earlier. of bottomUnder dry-soilside = 11.3 conditions, cm, height four = 6.5 baiting cm). Baiting containers, containers which were prepared filled with as 30% mentioned or 60% water-saturated earlier. Under dry-soil conditions, four baiting containers, which were filled with 30% or 60% water-saturated super absorbent polymers, or 30% moisture soil, or unfilled, were placed on the bottom of the bioassay

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Forests 2019, 10, x FOR PEER REVIEW 5 of 14 super absorbent polymers, or 30% moisture soil, or unfilled, were placed on the bottom of the bioassay arena.arena. Each Each baiting baiting container container was was equidistant equidistant from from the the adjacent adjacent ones, ones, and and their their order order and and cardinal cardinal directiondirection were were randomly randomly assigned assigned (Figure (Figure 2A).2A). Dry Dry soil soil (30% (30% moisture) moisture) was was used used as as the the substrate substrate to to burybury the the ba baitingiting containers containers (the (the tops tops of of the the baiting baiting containers containers were were unburied) unburied) (Figure (Figure 2B).2B). Similar proceduresprocedures were were conducted toto prepareprepare thethe bioassays bioassays under under wet-soil wet-soil conditions, conditions, but but 60% 60% moisture moisture soil soilwas was used used as theas the substrate substrate to bury to bury and and fill thefill the baiting baiting containers. containers.

FigureFigure 2. 2. ExperimentExperiment setting settingss to to investigate investigate the the preference preference of of termites termites to to super super absorbent absorbent polymers polymers filledfilled in in baiting containers.containers. FourFourbaiting baiting containers containers (either (either filled filled with with 30% 30% or 60% or 60% water-saturated water-saturated super superabsorbent absorbent polymers, polymers, or 30%-moisture or 30%-moisture soil, or unfilled) soil, or wereunfilled) placed were on the placed bottom on of the the bottom bioassay of arena the bioassay(A), and arena soil was (A), used and assoil the was substrate used as to the bury substrate the baiting to bury containers the baiting (B). containers (B).

TwoTwo hundred hundred and and fifty fifty termites termites (225 (225 workers workers and and 25 25 soldiers) soldiers) were were released released into into each each bioassay bioassay arenaarena and and sealed. sealed. The The bioass bioassaysays were were placed placed in in the the environmental environmental chambers chambers setting setting at at 25 25 °C◦C under under totaltotal darkness. darkness. On On day day 21, 21,the number the number of termites of termites aggregated aggregated in each in baiting each baiting container container or still within or still thewithin substrate the substrate was counted. was The counted. wood consumption The wood consumption and wood moisture and wood were moisture measured, were as mentioned measured, earlier.as mentioned earlier.

2.5.2.5. Data Data Analysis Analysis ForFor each each choice choice test, test, the the percentage of termites aggregatedaggregated inin each each location location was was calculated. calculated. Due Due to tothe the sum sum constraint constraint of of the the percentage percentage data,data, wewe conductedconducted the logratio transformation to to make make the the percentagespercentages data data indep independentendent [1 [717]. ].The The transformed transformed data data were were analyzed analyzed using using two two-way-way analysis analysis of varianceof variance (ANOVA, (ANOVA, Proc Proc Mixed, Mixed, SAS SAS 9.4, 9.4, SAS SAS Institute, Institute, Cary, Cary, NC, NC, USA) USA) with with colony colony group group as as the the randomrandom factor factor and and location location as as the the fixed fixed effect. effect. Wood Wood consumption consumption and and wood wood moisture moisture were were also also compared using two-way ANOVA with colony group as the random factor and chambers or baiting

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Forests 2019, 10, x FOR PEER REVIEW 6 of 14 compared using two-way ANOVA with colony group as the random factor and chambers or baiting containers as the fixed fixed effect. effect. Tukey’s Tukey’s honest significant significant difference difference (HSD) tests were used for multiple comparisons after each ANOVA. InIn allall tests,tests, thethe significancesignificance levelslevels werewere determineddetermined atatα α= = 0.05.0.05.

3. Results

3.1. Preference of Termites to Super Absorbent PolymersPolymers PlacedPlaced withinWithin Soil Soil The mean survivorship of termites was >>75%75% in all two two-choice-choice tests. Under Under dry dry-soil-soil conditions, significantlysignificantly more termites aggr aggregatedegated in the side chambers where Na Na-PAM-PAM was placed compared with the control side (Na-PAM(Na-PAM waswas notnot placed within the soil) (Figure3 3A).A). Also,Also, woodwood consumptionconsumption and wood moisture wwereere significantly significantly higher when Na Na-PAM-PAM was placed within within soil soil (Figure (Figure 3 3B).B). Similarly, placing K-PAMK-PAM oror P(AMP(AM/AA)/AA) withinwithin soilsoil significantlysignificantly increased wood consumption and wood moisture compared with the control side, but no eeffectsffects on termitetermite aggregation was detected (Figures4 4 and and5 ).5). Under Under wet-soil wet-soil conditions, conditions, the the three three super super absorbent absorbent polymers polymers had had no no significant significant eeffectffect on termite aggregation and wood consumption (Figures (Figures 33––55),), thoughthough Na-PAMNa-PAM andand P(AMP(AM/AA)/AA) significantlysignificantly increased thethe moisturemoisture contentcontent ofof woodwood blocksblocks (Figures(Figures3 3 and and5 ).5).

Figure 3. 3. ResultsResults of of the the two-choice two-choice tests tests (under (under dry- or dry wet-soil- or wet conditions)-soil conditions) to investigate to investigate the preference the ofpreference termites toof sodiumtermites polyacrylate to sodium polyacrylate (Na-PAM) placed (Na-PAM) within placed soil. Percentage within soil. of Percentage termites in eachof termites chamber in (eachA), and chamber wood consumption(A), and wood (B) and consumption wood moisture (B) and (C) in wood the side moisture chambers (C) are in presented the side chambers as mean areSE. ± Dipresentedfferent letters as mean indicate ± SE. significant Different di lettersfferences indicate (p < 0.05). significant WS represents differences water (p < saturation 0.05). WS level.represents water saturation level.

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Figure 4. Results of the two choice tests (under dry- or wet-soil conditions) to investigate the Figure 4. Results of the two choice tests (under dry- or wet-soil conditions) to investigate the Figureprefer 4. Resultsence of of termites the two to choice potassium tests (underpolyacrylate dry- or(K wet-soil-PAM) placed conditions) within tosoil. investigate Percentage the of preferencetermites of preference of termites to potassium polyacrylate (K-PAM) placed within soil. Percentage of termites termitesin each to potassium chamber ( polyacrylateA), and wood (K-PAM) consumption placed (B) within and wood soil. moisture Percentage (C) ofin termitesthe side chambers in each chamber are (Ain), each andpresented woodchamber consumption as mean(A), and ± SE. (woodB Different) and consumption wood letters moisture indicate (B) (C and significant) in thewood side differencesmoisture chambers ( C ( arep)

FigureFigure 5. Results 5. Results of the two-choiceof the two- testschoice (under tests dry- (under or wet-soil dry- or conditions)wet-soil conditions) to investigate to investigate the preference the of termitespreference to poly(acrylamide-co-acrylic of termites to poly(acrylamide acid) potassium-co-acrylic saltacid) (P(AM potassium/AA)) salt placed (P(AM/AA)) within soil. placed Percentage within of termitessoil. in Percentage each chamber of termites (A), and in woodeach chamber consumption (A), and (B )wood and woodconsumption moisture (B) ( Cand) in wood the side moisture chambers (C) are in the side chambers are presented as mean ± SE. Different letters indicate significant differences (p < presentedFigure 5. as Results mean ofSE. the Different two-choice letters tests indicate (under significant dry- or differences wet-soil ( conditions)p < 0.05). WS to represents investigate water the 0.05). WS represents± water saturation level. saturation level. preference of termites to poly(acrylamide-co-acrylic acid) potassium salt (P(AM/AA)) placed within soil. Percentage of termites in each chamber (A), and wood consumption (B) and wood moisture (C) in the side chambers are presented as mean ± SE. Different letters indicate significant differences (p < 0.05). WS represents water saturation level.

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3.2. Preference of Termites to Super Absorbent Polymers Filled in Baiting Containers 3.2. Preference of Termites to Super Absorbent Polymers Filled in Baiting Containers The mean survivorship of termites was ≥80% in all multiple-choice tests. Under dry-soil The mean survivorship of termites was 80% in all multiple-choice tests. Under dry-soil conditions, conditions, filling the baiting containers ≥ with the three super absorbent polymers significantly fillingincrease thed the baiting moisture containers content with of wood the three blocks super (Figures absorbent 6–8). However, polymers filling significantly the baiting increased containers the moisturewith Na-PAM content (30% of woodor 60% blocks water (Figures saturation)6–8). had However, no significant filling the effect baiting on the containers aggregation with and Na-PAM wood (30%consumption or 60% water of termites saturation) compared had no with significant unfilled e ff orect soil on-filled the aggregation ones (Figure and 6). wood Filling consumption the baiting ofcontainers termites with compared K-PAM with significantly unfilled or decreased soil-filled wood ones (Figure consumption6). Filling compared the baiting to the containers unfilled withones, K-PAMand the significantlyleast termites decreased were found wood in consumption the baiting containers compared tofilled the unfilledwith 60% ones, water and-saturated the least termitesK-PAM were(Figure found 7). in In the addition, baiting 30%containers water filled-saturated with 60% P(AM/AA) water-saturated in the baiting K-PAM containers (Figure7). significantlyIn addition, 30%decreased water-saturated termite aggregation P(AM/AA) inand the wood baiting consumption containers significantly compared with decreased the soil termite-filled aggregation containers and(Figure wood 8). consumption compared with the soil-filled containers (Figure8).

FigureFigure 6.6. Results of the multiple-choicemultiple-choice tests (under dry-dry- oror wet-soilwet-soil conditions)conditions) toto investigateinvestigate thethe preferencepreference of of termites termites to sodium to sodium polyacrylate polyacrylate (Na-PAM) (Na-PAM) filled in filled the void in the volume void of volume baiting containers. of baiting Percentagecontainers. of Percentage termites in of each termites location in ( A each), and location wood consumption (A), and wood (B) and consu woodmption moisture (B) (C)and in wood each baitingmoisture containers (C) in eachare presented baiting ascontainers mean SE. are Di presentedfferent letters as indicate mean ± significantSE. Different differences letters ( p indicate< 0.05). ± WSsignificant represents differences water saturation (p < 0.05). level. WS represents water saturation level.

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FigureFigure 7.7. Results of the multiple-choicemultiple-choice tests (under dry-dry- oror wet-soilwet-soil conditions)conditions) toto investigateinvestigate thethe preferencepreference of of termites termites to potassium to potassium polyacrylate polyacrylate (K-PAM) (K-PAM) filled in filled the void in the volume void of volume baiting containers. of baiting Percentagecontainers. of Percentage termites in of each termites location in ( A each), and location wood consumption (A), and wood (B) and consumption wood moisture (B) (andC) in wood each baitingmoisture containers (C) in eachare presented baiting as containers mean SE. are Di presentedfferent letters as indicate mean ± significantSE. Different differences letters (p indicate< 0.05). ± WSsignificant represents differences water saturation (p < 0.05). level. WS represents water saturation level.

Under wet-soil conditions, three super absorbent polymers significantly increased the moisture content of the wood blocks (Figures6–8). However, significantly more termites were found in the baiting containers filled with soil than those that filled with super absorbent polymers (Figures6–8). In addition, termites consumed significantly more wood in the baiting containers filled with soil than those that filled with 30% water-saturated Na-PAM (Figure6), and 30% or 60% water-saturated K-PAM and P(AM/AA) (Figures7 and8).

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Figure 8.8. Results of the multiple-choicemultiple-choice tests (under dry-dry- or wet-soilwet-soil conditions)conditions) to investigate the preference ofof termitestermites to to poly(acrylamide-co-acrylic poly(acrylamide-co-acrylic acid) acid) potassium potassium salt salt (P(AM (P(AM/AA))/AA)) filled filled in the in void the volumevoid volume of baiting of baiting containers. containers. Percentage Percentage of termites of termites in each in each location location (A), ( andA), and wood wood consumption consumption (B) and wood moisture (C) in each baiting containers are presented as mean SE. Different letters indicate (B) and wood moisture (C) in each baiting containers are presented as± mean ± SE. Different letters significantindicate significant differences differences (p < 0.05). (p WS < 0.05). represents WS represents water saturation water saturation level. level. 4. Discussion Under wet-soil conditions, three super absorbent polymers significantly increased the moisture contentMany of previousthe wood studies blocks have (Figures focused 6–8). on However, the termite significantly attractants that more may termites increase were bait consumptionfound in the (asbaiting reviewed containers by Lax filled et with al. [ 18soil] andthan Juddthose [that19]). filled For with example, super Chenabsorbent and polymers Henderson (Figures [20] found 6–8). thatIn addition,C. formosanus termitesconsumed consumedsignificantly significantly more wood filter paperin the baiting treated containers with the fill solutioned with of soil certain than aminothose that acid filled additives with (i.e.,30%d water-aspartic-saturated acid, l -glutamicNa-PAM acid,(Figure and 6),l -asparticand 30% acid)or 60% than water the control-saturated paper. K- CastilloPAM and et al.P(AM/AA) [21] reported (Figures that Coptotermes 7 and 8). gestroi (Wasmann) consumed significantly more bait matrices composed of cellulose, 3% xylose, 3% casein and cassava powder compared with the control matrices (only4. Discussion containing cellulose), whereas Coptotermes curvignathus (Holmgren) consumed more bait matrices (cellulose) when 3% glucose and cassava was added. Suran and Rust [22] reported that 3% xylose Many previous studies have focused on the termite attractants that may increase bait in the food (paper discs) significantly increased the intake and horizontal transfer of hexaflumuron consumption (as reviewed by Lax et al. [18] and Judd [19]). For example, Chen and Henderson [20] among individuals of Reticulitermes hesperus Banks, and caused significantly higher termite mortality found that C. formosanus consumed significantly more filter paper treated with the solution of certain than that of hexaflumuron alone. Other reported termite attractants include wood-rotting and soil amino acid additives (i.e., D-aspartic acid, L-glutamic acid, and L-aspartic acid) than the control fungi [23–27], decayed wood extract [28], carbon dioxide [29,30], clay materials [11,31,32], and even paper. Castillo et al. [21] reported that Coptotermes gestroi (Wasmann) consumed significantly more some sport drinks [33]. bait matrices composed of cellulose, 3% xylose, 3% casein and cassava powder compared with the controlOur matrices study showed (only containing that super cellulose), absorbent whereas polymers Coptotermes can act as novelcurvignathus termite (Holmgren) attractants. consumed It is worth noting that the three super absorbent polymers tested in our study only increased wood consumption more bait matrices (cellulose) when 3% glucose and cassava was added. Suran and Rust [22] reported that 3% xylose in the food (paper discs) significantly increased the intake and horizontal transfer of hexaflumuron among individuals of Reticulitermes hesperus Banks, and caused significantly higher

Forests 2019, 10, 591 11 of 14 when: (1) they were placed within soil around baiting containers; and (2) when the substrate (soil) was dry. Evens [34] reported that the subterranean termites Coptotermes frenchi Hill made tunnels slowly in dry sand, but they accelerated tunneling by 5 times after a patch of wet sand was discovered. The super absorbent polymers in dry soil may increase the moisture content of adjacent soil so that termites can easily make exploratory tunnels and rapidly locate the food. Also, in our study the presence of super absorbent polymers significantly increased the moisture of wood blocks in baiting containers (Figures3–5), probably because super absorbent polymers created a micro-environment with high humidity so that wood can absorb more moisture. Termites may also directly transfer water from super absorbent polymers to the wood by evacuating their water sacs, as shown by Gautam and Henderson [35]. Although moisture condition is one of the most significant factors that influence the foraging behaviors of subterranean termites, few efforts have been made to improve baiting systems by increasing the moisture content of soil/bait matrices. Our study showed that placing super absorbent polymers within soil around bait stations may be a simple way to increase bait consumption by termites in dry locations, and it would be valuable to conduct field studies to verify this under natural conditions. Surprisingly, though the super absorbent polymers’ significant increase moisture content of wood blocks in baiting containers, no positive effect on the aggregation and feeding behaviors of termites was found, even when the substrate was dry. A choice test conducted by Gautam and Henderson [36] showed that C. formosanus can adapt a wide range of moisture levels (4%–24% moisture) of sand, but significantly fewer termites were found in the chambers containing water-saturated sand (28% moisture). The super absorbent polymers can absorb a large amount of water, and therefore may have created a water-saturated environment inside the baiting containers that inhibited the aggregation and feeding behaviors of termites. Also, we observed that termites cannot easily make tunnels within gel-like super absorbent polymers, which may actually have acted as a barrier for termites to access to the food. Under wet conditions, significantly more termites were found in the baiting containers filled with soil and consumed more wood compared with baiting containers filled with super absorbent polymers or which remained unfilled. In the field, termites usually transport soil or clay into the hollow spaces of trees and bait stations [11,37]. Li and Su [38] claimed that such behaviors help termites to transform the void spaces to “tunnel space during soil displacement”. Our results showed that soil filled in void spaces may not only be a byproduct of tunneling excavation and space transformation, but may also enhance the aggregation and feeding activities of termites, and we suggest filling the void volume of bait stations with soil in wet areas to increase bait consumption by termites. Super absorbent polymers have been widely used in agricultural and forest soil to increase moisture conditions and promote the growth of crops and trees [14,15,39–41]. Our study showed that C. formosanus may be attracted by super absorbent polymers placed around plant roots. We suggest paying more attention to termite monitoring and management where super absorbent polymers have been applied, especially under dry-soil conditions.

5. Conclusions This study showed that placing super absorbent polymers within dry soil can significantly increase wood consumption by termites, probably because super absorbent polymers improve the moisture of soil and wood and therefore facilitate the tunneling and feeding behaviors of termites. However, super absorbent polymers placed within wet soil had no detectable effect on termite aggregation and feeding. In addition, filling the void volume of baiting containers with super absorbent polymers cannot attract termites, probably because super absorbent polymers created a microenvironment in the baiting containers that was too wet, inhibiting the foraging behaviors of termites. However, filling the baiting containers with soil significantly increased termite aggregation and consumption under wet conditions. Based on these results, we suggest placing super absorbent polymers within soil surrounding bait stations in drought locations. Also, filling the bait stations with soil may improve bait infestation and consumption in wet locations. Forests 2019, 10, 591 12 of 14

Author Contributions: Methodology, Q.X., C.W.; Formal Analysis, Q.X., Z.J., C.W.; Investigation, Q.X., Z.J., K.Z., C.W.; Resources, Q.X., Z.J., X.W., C.W.; Writing-Original Draft Preparation, C.W.; Writing-Review & Editing, Q.X., Z.J., W.L., K.X., X.C., K.Z., X.W., C.W.; Visualization, Q.X., Z.J., W.L., K.X., X.C.; Supervision, X.W., C.W.; Project Administration, W.L., K.X., X.C., C.W.; Funding Acquisition, W.L., K.X., C.W. Funding: This research was funded by National Natural Science Foundation of China (31500530), Science and Technology Program of Foshan (2018AB003861), and Foshan Agricultural Technology Extension Project. The APC was funded by National Natural Science Foundation of China (31500530). Acknowledgments: We sincerely thank Shiping Liang and Hongpeng Xiong (College of Forestry and Landscape Architecture, South China Agricultural University) for valuable helps in termite collection and maintaining. Conflicts of Interest: The authors declare no conflict of interest.

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