Bulletin of Entomologil Reserh (29) 99, 53 512 Ó 29 Cmridge University Press irst pulished online 1 erury 29 doi:1.117/s7485386524 Printed in the United Kingdom Signifine of the tropil fire nt Solenopsis gemint (hymenopter: formiide) s prt of the nturl enemy omplex responsile for suessful iologil ontrol of mny tropil irrigted rie pests M.J. Wy 1 *nd K.L. Heong 2 1 eprtment of Biology, Imperil College London, Silwood Prk, Asot, Berkshire, SL5 7PY, UK: 2 Interntionl Rie Reserh Institute, APO Box 7777, Metro Mnil, Philippines Astrt The tropil fire nt Solenopsis gemint (riius) often nests very undntly in the erthen nks (unds) round irrigted rie fields in the tropis. Where some frmers hitully drin fields to the mud for out 3 4 dys, the nts n quikly spred up to out 2 m into the fields where they ollet food, inluding pest prey suh s the eggs nd young of the pple snil Pome niult (Lmrk) nd insets suh s lepidopterous lrve nd hoppers, notly Nilprvt lugens (Stäl) the rown plnthopper (Bph) nd green lefhoppers Nephotettix spp. Even in drined fields, the tivity of S. gemint is restrited y rinfll in the wet seson. The reltively few nt workers tht forge hrteristilly into drined fields nd on to the trnsplnted lumps of rie plnts (hills) kill the normlly few immigrnt Bph dults ut re initilly slower ting thn other speies of the nturl enemy omplex. However, lrger popultions of Bph re fierely ttked nd effetively ontrolled y rpidly reruited nt workers; wheres, in the sene of the nt, the other nturl enemies re indequte. In norml irumstnes, there is no nt reruitment in response to initilly smll popultions of immigrnt Bph nd no evidene of inomptiility etween nt forgers nd other nturl enemies suh s spiders. However, when mny nts re quikly nd ggressively reruited to ttk lrge popultions of Bph, they temporrily disple some spiders from infested hills. It is onluded tht, in suitle wether onditions nd even when insetiides kill nturl enemies within the rie field, periodi dringe tht enles S. gemint to join the predtor omplex is vlule for nt-sed ontrol of pests suh s snils nd Lepidopter, nd espeilly ginst reltively lrge popultions of Bph. ringe prties to enefit nts re fully omptile with reent reserh, whih shows tht periodi dringe omts prolems of yield deline in intensively irrigted tropil rie nd is lso needed in South Est Asi to mke etter use of seriously delining wter supplies for irrigtion. *Author for orrespondene x: +27 594 245 E-mil: 2wys@tlktlk.net
54 M.J. Wy nd K.L. Heong Keywords: tropil irrigted rie, irrigtion prties, iologil ontrol, Solenopsis gemint, nt predtion, spider predtion, Nilprvt lugens (Aepted 12 Septemer 28) Introdution Solenopsis gemint (riius) nd some other predtory nt speies n nest undntly within fields of uplnd (drylnd) tropil rie (Wy et l., 22). The nts my lso estlish nests within the fields during drylnd phses etween irrigted rie rops, prtiulrly during the long fllow seson of single ropped flooded rie. uring flooding, the nts re lrgely limited to the erthen nks (unds) used to onfine wter in eh field. Here, the numers of workers in S. gemint nests my exeed 5, per 1 m of und (Wy et l., 1998). In ontinuously flooded rie fields, S. gemint n forge ross losed lef nopy one this hs overlpped the und (Wy et l., 1998), ut most only reh the peripherl 2 3 m of the rop. However, s disussed lter, some frmers periodilly drin ropped fields to mud level for severl dys, during whih predtory S. gemint quikly spred ross wet mud nd forge on the rie plnts. This pper desries experimentl studies on the ehviour nd impt of S. gemint ompred with tht of other nturl enemies, notly spiders, mostly using Nilprvt lugens (Stäl) the rown plnthopper (Bph) s trget pest speies. Intertions etween S. gemint nd spiders re lso exmined in order to ssess possile impirment of iologil ontrol y the diverse nturl enemy ommunity, whih is potentilly very importnt in the ontrol of mny rop pests (Sunderlnd et l., 1997) nd reognized s sometimes ruil in the suppression of t lest one key pest of tropil irrigted rie, notly Bph (Kenmore et l., 1984; Wy & Heong, 1994). Mterils nd methods Some studies on forging ehviour of S. gemint were done on privte frm within 1 km of IRRI, where no insetiides hd een used for t lest ten yers. Otherwise, most field experiments were t the IRRI Experimentl rm in pestiide-free res, prt from single pre-emergene non-insetiidl heriide s used lso on the privte frm. At IRRI, trnsplnted rie seedlings were estlished using unhes (hills) of 3 5 rie seedlings rered in the nursery, whih were trnsplnted in flooded fields.2 m prt. The rie ultivr ws the Bph resistnt IR72. This is only prtly resistnt euse dpted strins of Bph n olonize nd multiply though more slowly thn on suseptile ultivrs. The dpted Bph were rered in glsshouse nd then estlished s trget pests for experimentl studies in the field. Experiments on S. gemint predtion nd its impt ompred with tht of other nturl enemies, notly spiders, were done using.7-m dimeter nd.6-m-high trnsprent Mylr ring ges with two.2 m 2 netting windows, eh usully put round five rie hills rtifiilly infested with Bph dults or 3rd 4th instr nymphs. The ges were designed to ompre the impt of ll predtors nd prsitoids inluding nts with tht of nts only nd with non-nt nturl enemies, s well s with omplete nturl enemy exlusion. All predtors nd prsitoids were exluded y Mylr rings pressed into the mud round the rie hills nd surmounted y sleeves of 18 m nylon netting. All nturl enemies were given ess through unsleeved open top ges with the ses lifted out.1 m ove the soil surfe. Suh ge frmes did not impir herivore or predtor ess (see tle 1). The tretment for sole ess of S. gemint involved sleeved ges with out six.1-m-wide nd 5-mm-high slots ut into the sunken ses of the Mylr rings out 1 mm ove the level of the mud. Suh slots were quikly found y S. gemint. In the tretment with nts exluded, the non-nt predtors nd prsitoids were given ess through the unsleeved tops of Mylr ges set into the mud. Bsed on totl spider numers, these seletive exlusion proedures proved stisftory. or exmple in one experiment (tle 2), the numers of spiders in the ll predtor nd non-nt predtor tretments in the first five dys were, respetively, 8.6 nd 7.4 per two ges, whih did not differ signifintly (P >.5); ut in the first five dys their speies omposition ws different, with 18.2% per hill Lyos pseudonnult (Böesenerg & Strnd) of totl spiders in the nt plus other predtor tretment ompred with 5.8% Tle 1. Chnges in numers of the nturl popultion nd of 4 introdued Bph per hill on five unged hills nd on five ged hills, ll given ess to oth S. gemint nd spiders during four dys when field of IR72 rie ws kept drined in riny wether. Bph numers Cging istne from und(m) Nturl popultion Highest individul hill ounts during the four-dy period S. gemint Spiders No. of Bph y 1 y 4 Not 2 3 4 2 2 ged 12 12 6 3 Cged 2 5 9 2 1 12 12 3 2 4 introdued Not 2 15 7 36 1 ged 12 16 35 26 Cged 2 5 16 37 38 12 18 4 29
Tle 2. Chnges in Bph numers eginning with () 4 initilly introdued 3rd 4th instr Bph nymphs or () ten grvid femles nd of nturlly ourring Glh on groups of five ged hills given ess either to S. gemint nd other predtors, or to other predtors only, or to no predtors on wet seson rop of Bph resistnt IR72 rie. () Predtor exposure Speies Men nos per hill for eh five-dy period ys 1 5 6 1 11 15 16 2 21 25 Ants nd other predtors Signifine of the tropil fire nt Solenopsis gemint 55 Bph 22+58 76+22 367+53 56+16 186+82 Glh 3.8+1.3 8.+2.9 9.+3.5 4.8+1.6 4.3+1.5 Ants 11.+3.9 12.+3.1 2.3+.6 8.5+1.8 5.3+1. Cyrtorhinus 4.+1.9 15+5.5 8.5+3.2 14+5.1 14+4.7 lividipennis Spiders 8.6+3.2 27+6.6 18+5.4 3+7. 27+5.6 Other predtors only Bph 41+31 168+35 1481+23 7844+299 5583 Glh 2.+1.7 4.8+1.9 9.5+2.4 2.+.5 1.7+.4 Cyrtorhinus 4.8+1.5 16+5.1 22+6.6 82+9.1 26+41 lividipennis Spiders 7.4+2.8 18+6.3 38+4.8 42+12 2+6.8 No predtors Bph 288+37 184+3 216+291 9143+216 3868 * * * Some plnts dying hopperurn. () Predtor exposure Speies Men nos/hill for eh five-dy period ys 1 5 6 1 11 15 16 2 21 25 Ants nd other predtors Bph 3.4 42 24.. Glh 4.6 15 11 2. 1.7 Ants.6 4.5 3.5.5 2. Cyrtorhinus 5.4 16 12.8 6.6 lividipennis Spiders 1 25 19 22 17 Other predtors only Bph 5.2 52 5..4.4 Glh 6.8 16 7.4 5. 1. Cyrtorhinus 3. 24 3.4 1. 1.9 lividipennis Spiders 5.4 18 18 15 14 No predtors Bph 6.9 138 194 189 21 in the tretment where predtors only hd ess only from the ge top. Lter oservtions showed tht some L. pseudonnult entered the nt only exposed tretment t ground level through holes mde for nt ess. Other spider speies, notly Atypen formosn (Oi), mde up for the fewer L. pseudonnult, ut this speies preys mostly on hopper nymphs wheres the ltter lso preys on dults (Sheprd et l., 1987). After ten dys, the proportions were restored, 14.3% nd 13.8%, respetively (P <.5). Whilst this prolem ws mostly solved in experiments with uplnd rie using n mended tehnique (Wy et l., 22), it ws unfortunte tht two well-replited experiments on wet seson irrigted rie rops using the etter tehnique were ruined y lekge from n djoining field, nd y typhoon. The numers of different speies were ounted visully on individul hills; however, when there were more thn out 1 per hill, Bph numers were estimted (Wy et l., 1998). Replited experiments were nlyzed y ANOVA nd multiple omprison of mens using unn s multiple rnge test. In generl, numers were no dout greter thn those reorded, prtly euse ll individuls re diffiult to find, prtiulrly on the older lrger hills, nd prtly euse some spiders nd Bph jump or drop from the plnts when the plnts re distured. Nevertheless, the diret ounting method is notly more urte thn, for exmple, sution smpling (Islm et l., 2) nd euse destrutive methods suh s sution smpling re useless for experiments involving sequentil ounts on prtiulr plnts. The min ging experiment to ompre effets of S. gemint with nd without other predtors omprised five replites of five hills in wet seson rop. Chnges in undne of Bph, nts, spiders nd some other predtors were reorded throughout the seson in one dry seson rop. In this experiment, 36 hills were exmined individully
56 M.J. Wy nd K.L. Heong Tle 3. Estimted totl numers of S. gemint workers returning to nests during 14 h period (6:3 2:3) long six trils diretly fter prtil dringe of dry seson irrigted rie field. Tril numers 1 2 3 4 5 6 Totl workers 18,43 16,29 869 399 35 282 Numers rrying 413 351 159 83 51 48 food items % rrying food items 22.4 21.6 18.3 2.8 16.7 17. on lok ten hills wide nd extending 36 hills long into the field from und. The field ws periodilly drined for up to four dys nd then re-flooded. Results S. gemint ehviour in periodilly drined rie fields ormtion of forging olumns A dry seson rop t the tillering stge ws llowed to drin during dry sunny wether. After one dy, there ws still some stnding wter, ut S. gemint workers were lredy forming forging trils ross the mud. Six trils from und length of out ten metres were mrked, nd the totl numers of workers nd those rrying food items rossing line t the inoming end of eh tril were ounted for three minutes in every 3 min period from 6.45 to 2.3 h. The totl numers of nts, sed on the three-minute ounts, were then estimted for the 14 h durtion of the study. Those returning vried from totls of 282 to 18,43 in different trils (tle 3) though only 17 22% were rrying food items, inluding ll or prts of hopper dults nd nymphs, leffolder lrve, dipterous detritivore lrve, lrvl Coleopter nd eggs nd young of the golden pple snil, Pome nliult (Lmrk). As n exmple long one tril, 87 hoppers, inluding ody prts nd 226 snil eggs, were mong the lulted 413 food items rried in their mndiles y 18,4 returning workers during the 14 h period. Ant predtion on rtifiil infesttions of Bph uring dry wether in the wet seson, two seprte pirs of rie hills three metres from und were eh infested diretly fter prtil dringe with out 9 nymphl nd dult Bph. One hour lter, there ws n S. gemint tril to eh pir of rie hills from two seprte olonies on the djoining und. Returning workers were reorded dy nd night s efore lthough there were gps in the reords during the night of dys 2 3. Ants rrying Bph or Bph prts nd those rrying other items were ounted, nd men numers per hour were then estimted. Bph nd S. gemint on eh pir of hills were lso ounted or estimted seprtely for suessive 5 7-h periods. In oth pirs of hills nd their ssoited S. gemint trils, there were initil peks of Bph olletion y out 1% of the returning workers (figs 1 nd 2). In one tril (fig. 1), Bph olletion ontinued overnight nd into the seond dy when S. gemint numers on the plnts nd in the tril gin peked t round middy. On dy 4, no Bph were left on the plnts (fig. 1). In the seond tril (fig. 2), the nts remined tive during the first night ut were not rrying food items. Moreover, nt numers peked round middy on the 3rd dy though there were few remining Bph on the plnts. A midnight reord on the seond night showed tht oth trils remined tive, ut neither ws rrying food items (figs 1 nd 2) There ws one nomly, nmely the estimted numers of Bph prey tken long the seond tril t round 9 h on dy 3 (fig. 2); this seemed notly greter thn the derese in numers on the orresponding hills (fig. 2). Perhps, nts olleted Bph during the night ut did not return with them until the next morning. Estimtes of totl Bph nd their ody prt numers rried long trils to eh olony during the experimentl period were 16 nd 113 ompred with initil infesttions of 9 Bph on the hills from whih Bph were eing tken. orging distne S. gemint were seen forging up to out 2 m into two-dy drined field. Tht the ehviour of different ompeting olonies vried ws demonstrted strikingly in one experiment where six hills.2 m prt in row opposite S. gemint olony A (fig. 3) were eh infested t 8 h with out 2 Bph nymphs. The originl ojetive ws to determine the speed t whih suessive infested hills were found nd the Bph removed y olony A workers. However, no forgers left olony A. Insted, t 1 h, tril of S. gemint to the Bph ws tred > 1 m k to olony B (fig. 3). At first, olony B workers removed ll Bph from hills 5 nd 6, ut y 17 h they hd rehed hills 2 to 4. On the following morning, lthough no tril ws seen to hill 1, the numers of Bph hd fllen to seven. Seemingly, workers from one of the nt olonies hd removed them. Otherwise, workers from olony B, despite the distne, monopolized hills 2 6, hving inhiited forging from olony A. Use of seletive exlusion tehniques Two experiments were done to ompre the roles of nts nd other nturl enemies, notly spiders, ginst Bph. Initilly, in the dry seson, rie field t the tillering stge ws prtly drined. The soil ws still very muddy when it ws re-flooded four dys lter. Two sets of five hills, eh infested with 2 3rd 4th instr Bph, were ged three metres from und in three onditions: to permit ess of nts nd other nturl enemies, to exlude nts ut not others, nd to exlude ll nturl enemies. These were set up t 7 h one dy fter dringe egn when the ground ws still very muddy with pools of wter. Yet, S. gemint workers were lredy mking trils in the field, nd within out four hours were removing Bph from ged plnts to whih they hd ess, leving only 43 fter eight hours nd three efore the field ws re-flooded on dy 4 (tle 4). Where only other nturl enemies hd ess, the numers of Bph deresed notly more slowly. Initilly the predtors were minly spiders, notly Lyos pseudonnult nd Atypen formosn, nd ltterly lso the heteroptern ug Cyrtorhinus lividipennis Reuter nd predtory Coleopter. In these irumstnes, 27 Bph eme dults whih reprodued using surge of newly hthed nymphs fter 18 dys when 25 were reorded on the ten plnts, ontrsting with none where S. gemint hd ess during field
Signifine of the tropil fire nt Solenopsis gemint 57 1 Men no. of S. gemint returning per hour (log sle) 1 1 1 12: y1 y2 Time y3 15: 7: 1 4 Men no. of Bph on two hills 8 6 4 2 3 2 1 Men no. S. gemint on two hills 12: 15: 7: y 1 y 2 y 3 ig. 1. Predtion y S. gemint from one olony on 9 Bph nymphs on two rie hills in drined rie field. () lulted numers of S. gemint workers returning per hour to the nt olony with nd without trnsported items (K, totls;, numers rrying other food items; &, numers rrying Bph). () hnges in numers of Bph on the infested plnts ( &, Bph; - -^- -,S. geminte). dringe (tle 4). However, where ll nturl enemies were exluded, the estimte of hthed nymphs rehed > 21, efore the experiment ws terminted y typhoon. ull wether nd some rin kept the field muddy during the period of dringe, ut this did not stop S. gemint devstting the Bph. In wet seson experiment, the sme ging regimes were used with two densities of Bph, one with five sets of
58 M.J. Wy nd K.L. Heong 1 Men no. of S. gemint returning per hour (log sle) 1 1 12: 15: 7: y 1 y 2 y 3 1 4 Men no. of Bph on two hills 8 6 4 2 3 2 1 Men no. S. gemint on two hills 12: 15: 7: y 1 y 2 y 3 ig. 2. As in fig. 1 ut using seprte S. gemint olony in the sme rie field. five ged hills initilly infested with 4 3rd instr Bph nymphs nd the other with ten dult grvid femles, greter thn the urrent level of nturl oloniztion of immigrnt dult Bph. Where the 4 nymphs per ge were given ess to nts nd other nturl enemies, the numers of Bph initilly deresed ut susequently inresed to up to 5 per five hills during the 25-dy experiment (tle 2). In ontrst, there ws muh egg-lying in the no-nt other nturl enemy tretment nd in the no-nturl enemy ontrols, reting surge of newly hthed Bph nymphs. This ws followed y ollpse when plnts egn to die of hopper urn (tle 2), despite n inrese to 26 C. lividipennis per hill. However, with n initil totl of ten dult femle Bph, there were no differenes etween the two nturl enemy-exposed tretments (tle 2). In these irumstnes, nts were not reruited to the few Bph prey, whih,
Signifine of the tropil fire nt Solenopsis gemint 59 B BUN 6 5 4 3 2 1 2 m ig. 3. Approximte line of n S. gemint tril ross mud to nest of olony B from row of rie hills eh rtifiilly infested with 2 Bph. Note sene of tril from nest of olony A (, pprox. line of nt tril;, S. geminte nest). however, were suppressed suessfully y the other predtors in the sene of nts, s were nturlly ourring immigrnts of the green lefhopper (Glh) Nephotettix viresens (istnt) (tle 2). This ws despite periodi rinfll, whih no dout impired the rte of predtion y S. gemint on the lrge popultions of Bph (tle 2). The next wet seson experiment ws hosen to show the effet of wether onditions unfvourle to nts when muh rinfll used nturl flooding suh tht rtifiil reflooding ws only needed one during the first 15 dys of the experiment. S. gemint nd few Prtrehin spp. spordilly entered the rop, yet they were seemingly still suffiient to keep the Bph popultions very low in irumstnes where there were ttrtively lrge hopper popultions. However, the indequte sole effet of other B U N A predtors on the lrge initil Bph popultions (tle 2) must e qulified y the evidene (see Methods) tht some of one importnt spider predtor speies, L. pseudonnult, were initilly exluded from the non-nt open-top ge tretment lthough this speies ws repled y more of the other ommon spiders, notly A. formosn nd Tetrgnth spp. Nevertheless, it is reveling tht suh non-nt predtors still effetively suppressed smll dult Bph numers nd lso immigrnt N. viresens equivlent to nturl numers of inoming olonists (tle 2). In third wet seson experiment in July, the ojetive ws to ompre the predtory ontriution of S. gemint ginst Bph on hills two metres from the und nd ner the middle of field out 12 m from the und. Unreplited groups of five hills were kept either in open ges whih llowed ess of ll predtors from the ground s well s from ove, or they were entirely unged. These were set up one dy fter field dringe, five inluding the nturl immigrnt Bph popultion nd five infested with 4 3rd 4th instr Bph nymphs per hill. One dy nd four dys fter dringe (tle 1), no S. gemint were reorded on hills 12 m from the und, no dout euse frequent rinfll limited nt forging exept lose to the unds. However, t 12 m, the other predtors solely prevented inrese of the smll nturl Bph popultion. S. gemint were still reruited to the rtifiilly infested hills 2 m from the und where, together with other predtors, they gretly deresed the Bph popultions in oth ged nd unged onditions (tle 1). However, t 12 m distne, with nts sent, the other predtors lone hd omprtively little effet on the lrge introdued Bph popultions, whether ged or unged. Aundne of S. gemint, spiders nd other insets on hills throughout the rop seson In the 36-hill dry seson experiment, S. gemint moved immeditely into the drined field nd onto some hills on 1 Mrh. When the field ws re-flooded four dys lter, up to eight S. gemint were flushed from the ground per hill ut did not ffet spider numers (fig. 4). Susequently, one dy fter further dringe on 23 Mrh nd 3 April, fewer nts ontinued forging on the hills though re-flooding (25 Mrh nd 8 April) showed tht, overll, there were still mny nts in the field (fig. 4). uring 2 dys ontinuous Tle 4. Chnges in numers of initilly introdued 4 Bph on ten hills given ess to S. gemint nd other predtors or to other predtors only or with ll predtors exluded. Wter Regime y nd time(hrs) Bph nd predtors on ten hills. All predtors Other Predtors only No predtors Bph S.gemint Other predtors Bph Other predtors Bph rined (y ) 1 (8:) 4 nr nr 4 nr 4 1 (12:) 12 96 35 35 31 4 1 (16:) 43 24 41 286 33 395 4 (7:3) 3 6 17 15 18 247 looded (pm on dy 4) nr, not reorded. 8 (7:3) 1 18 27 25 177 15 (7:3) 4 39 166 38 11,6 18 (7:3) 42 25 48 21,5
51 M.J. Wy nd K.L. Heong 1 e 9 8 d de d,e Men numers / hill 7 6 5 4, d, 3 2 1, 1 8 14 2 24 25 3 4 8 11 14 28 8 15 22 27 Mrh April My, ig. 4. Men numers per hill of S. gemint nd spiders in lok of 36 rie hills in periodilly flooded nd drined onditions throughout dry seson rie rop seson., flooded;, drined. Histogrm numers with the sme letters re not signifintly different t P <.5 (, S. geminte; K, spiders). rtifiil flooding required for rop ripening in April, the numers of nts strnded on hills notly deresed. Some returned to the unds ross the rie nopy, ut others seemingly eme dormnt nd twie were seen eing ught y the spider L. pseudonnult. Reltively few nts were ounted forging on the hills fter dringe prior to hrvesting on 22 My; lso few were on the ground nd stule fter hrvesting, s indited y ount mde on 27 My fter they hd een flushed onto the stule y temporry re-flooding (fig. 4). urther ounts were mde on the stule until 16 June when, in riny prtly flooded onditions, mens of only 1.4 S. gemint were strnded per hill. Regrdless of dringe or flooding, spider numers initilly remined t out two per hill in Mrh nd then inresed to pek during rop ripening in April when they rehed 9.8 per hill on 28 April. This inrese ws no dout ssoited with the gret inrese in size of territories provided y the growing hills, whih lso ttrted muh supplementry prey other thn pest speies, whih remined sre. Two speies of spider predominted initilly out 3% L. pseudonnult deresing to 1% of the totl, nd A. formosn rising from out 6% to over 8% t the time of pek spider numers on 28 April. Throughout, the pest popultion, minly hoppers, remined very low with Bph t.39 per hill on 1 Mrh nd etween.16 nd 1.91 from 8 to 28 Mrh. Susequently, their numers flututed etween zero nd.25 per hill until 28 April, nd then remined t zero. Reltionship etween S. gemint nd spider numers on individul hills The reltive numers of nts nd spiders on individul hills in the 36-hills experiment (fig. 4) were nlyzed in order to ssess the possile impt of S. gemint on the spiders whih were hosen s the nturl enemies most likely to e interfered with y the nts. Overll, in Mrh nd April, the men numers of spiders did not differ signifintly in reltion to inresed nt numers (P >.5). S. gemint were never seen ttking spiders lthough the ltter, like the ug C. lividipennis nd dult prsitoids, when onfronted y n nt rwled side or moved elsewhere on the hill. The impt of notly more nturlly reruited nts ws determined on totl of 33 hills in five drined fields where 5 9 Bph were rtifiilly estlished on single hills. As lredy desried, ggressively predtory S. gemint workers were quikly reruited. Overll, on the first dy, the men numers of 23 S. gemint nd.4 spiders per hill ontrsted with 2.2 nts nd 4.8 spiders on 3 rndomly exmined hills not rtifiilly infested with Bph (P <.1). As efore, spiders were not seen eing ttked y the nts, nd most moved wy to peripherl prts of the hill nd to neighouring hills s the reruited nts rrived. Ten dys lter, fter most nts hd deprted, there were 5.6 spiders per hill whih hd returned from djoining hills or rrived erilly. In these irumstnes, the ggression nd hypertivity of the reruited S. gemint ontrsted shrply with
Signifine of the tropil fire nt Solenopsis gemint 511 the ehviour of wndering forgers towrds individul Bph prey on nturlly infested hills, whih they ttked ut whih did not eliit nt reruitment. isussion Whilst S. gemint n undoutedly e very effetive predtor ginst some pests in uplnd (drylnd) rie (Wy et l., 22), its impt on irrigted rie pests must depend on onditions tht enle it to forge into the rop from the very lrge olonies tht n flourish on the unds. One the rie nopy is losed nd in ontt with the unds, workers n forge ross the folige of flooded fields, though reltively few reh more thn few metres inwrds (Wy et l., 1998; Islm et l., 2). Otherwise, this pper shows tht they n very quikly spred into fields ross the wet mud t times when some frmers regulrly prtise periodi dringe for out three to four dys efore re-flooding (S. Msjo, personl ommunition). In these irumstnes, S. gemint n forge out 2 m into field nd eliminte lrge popultions of Bph t lest 1 m from nest. However, even in drined onditions, the impt of S. gemint is impired diretly y wet seson rinfll nd indiretly y the semiflooded onditions tht rinfll n rete. We reognize tht our experimentl ging systems to ompre the reltive importne of different nturl enemies in pest omplex my not urtely represent relity. or exmple, our experiments did not lwys distinguish the effet of nts from the full effet of other nturl enemies though they nevertheless demonstrted tht S. gemint n uniquely nd rpidly suppress lrge infesttions of Bph. This ontrsts with the reltive indequy of other predtors nd prsitoids ginst suh infesttions. Otherwise, S. gemint mde no dded ontriution to the vitl erly ontrol of the normlly few initil immigrnt dult Bph nd other hoppers in insetiide-untreted fields, s these were still effetively suppressed y the other nturl enemies in oth drined nd flooded onditions. However, it is signifint tht undnt popultions of S. gemint flourished in unds of irrigted fields frequently spryed with insetiides t IRRI nd lso in our experiments using rod spetrum ypermethrin nd rofurn (Wy & Heong, in prep.). In these irumstnes, nturl enemies within flooded fields were killed; ut, fter dringe, S. gemint quikly spred into the rops killing inoming Bph nd Glh when most other predtory speies were still sent or sprse, prt from wind-lown spiderlings, whih re too smll to ttk hopper pests. As our results onfirm, erly seson mortlity of immigrnt Bph is ruil euse the non-nt nturl enemy omplex n normlly prevent their multiplition to otherwise tstrophilly dmging popultions (tle 2). Therefore, when dringe permits S. gemint to enter trnsplnted rop, it is vitl tht the nt should not impir the impt of erly seson predtors. This is lso importnt in reltion to the inresingly used tehniques of diret seeding rie, whih must remin unflooded for out two weeks fter germintion. The nt s sfety ws demonstrted y our experiments, whih showed tht the normlly very few S. gemint tht in drined fields re regulrly seen forging on hill do not derese spider numers. Exeptionlly, more thn out 2 S. gemint per hill were ssoited with mny fewer spiders. This hppened when nt reruits egn ggressively preying on initilly lrge popultions of Bph. However, this seems irrelevnt euse suh immeditely reruited nts were highly effetive ginst the lrge Bph popultions whih re indequtely ontrolled y the delyed nd density responsive predtors suh s spiders, C. lividipennis nd lso prsitoids (Clridge et l., 22). One the nts hd deprted, spiders re-olonized the hills erilly nd from neighouring plnts. Although Bph ws tstrophi pest of tropil Southest Asin rie during the 196 198 er of exessive insetiide usge nd non-resistnt rie, it n still e lolly severe (Wy & Heong, 1994). In these irumstnes, dringe would no dout enle the often huge und popultions of S. gemint to mke mjor impt sed not only on the nts immeditely density dependent reruitment ut lso euse suh nts n ontinue predtion eyond the immedite needs of olony (Wy & Khoo, 1992). urthermore, S. gemint prey on other pests, notly Lepidopter inluding leffolders (Wy et l., 22) nd sometimes stem orers. It is lso mong the few nturl enemies of eggs nd young of the golden pple snil Pome nliult, whih n e devstting pest of newly estlished rie plnts in Southest Asi (Wy et l., 1998; Yus, 21). In this ontext, it is noteworthy tht erly seson dringe hs een reommended empirilly for ontrol of the snil. No dout predtion y S. gemint helps explin the vlue of suh dringe. Until reently, onventionl dvie hs ssumed tht ontinuous flooding of irrigted rie is needed to mximize irrigted rie yields. Yet previous evidene hd lredy shown tht periodi dringe my not limit yield nd indeed n fvour it (Singh et l., 1996; Guerr et l., 1998). Moreover, in reent yers, there hs een disturing yield deline in some ontinuously flooded intensive rie monoultures (we et l., 2), whih despite the dvent of new higher yielding ultivrs hs slowed overll rie yield growth in prts of tropil Asi nd is onfirmed y deresing yields of the lssil Green Revolution IR8 ultivr. Now, the evidene from long term experiments in Southest Asi shows tht ontinuous flooding is detrimentl to soil fertility (we et l., 2), whih n e ountered y periodi dringe nd lessened volume of irrigtion wter y s muh s 4 7% in southern Chin, pprently without yield detriment (Guerr et l., 1998). This seems ruilly importnt not only for mintining soil fertility ut lso for mking etter use of delining wter for irrigtion. Indeed, in view of the looming wter risis, lternte wetting nd drying prties my eome neessry even where it might inur some yield loss (Boumn, 21; Boumn et l., 27). The present pper shows tht suh prties n lso onfer notle iologil pest ontrol enefits from S. gemint. Aknowledgements We re muh indeted to Mr S. Msjo for llowing us to work on his frm nd for invlule dvie. We wish to express our grtitude espeilly to G. Jvier nd lso to J. Angeles,. izon, E.T. Rio nd. Vsquez for tehnil ssistne, to Isoel Wy for exhustive work on the mnusript, to M.E. Cmmell for illustrtions, nd to the IRRI Experimentl rm stff for field experiment filities. Referenes Boumn, B.A.M. (21) Wter-effiient mngement strtegies in rie prodution. Interntionl Rie Reserh Notes 26, 17 22.
512 M.J. Wy nd K.L. Heong Boumn, B.A.M., Lmpym, R.M. & Tuong, T.P. (27) Wter Mngement in Irrigted Rie Coping with Wter Srity. 54 pp. Los Bnos, Philippines, Interntionl Rie Reserh Institute. Clridge, M.., Morgn, J.C., Steenkiste, A.E., Imn, M. & mynti,. (22) Experimentl field studies on predtion nd egg prsitism of rie rown plnthopper in Indonesi. Agriulturl nd orest Entomology 4, 23 29. we,., oermnn, A., Moy, P., Adulrhmn, S., Bijy Singh, Ll,., Li, S.Y., Lin, B., Pnullh, G., Srim, O., Singh, Y., Swrup, A., Tn, P.S. & Zhen, Q.-X. (2) How widespred re yield delines in long-term rie experiments in Asi? ield Crops Reserh 66, 175 193. Guerr, L.C., Bhuiyn, S.I., Tuong, T.O. & Brker, R. (1998) Produing More Rie with Less Wter from Irrigted Systems.19 pp. Los Bños, Philippines, Interntionl Rie Reserh Institute. Islm, Z., Wy, M.J. & Heong, K.L. (2) Methods for smpling nd monitoring nts in the rie environment. Bngldesh Journl of Entomology 1, 97 114. Kenmore, P.E., Crino,.O., Perez, C.A., yk, V.A. & Guitierrez, A.P. (1984) Popultion regultion of the rie rown plnthopper Nilprvt lugens Stäl within rie fields in the Philippines. Journl of Plnt Protetion in the Tropis 1, 19 37. Sheprd, B.M., Brrion, A.T. & Litsinger, J.A. (1987) Helpful Insets, Spiders nd Pthogens. 136 pp. Los Bños, Philippines, Interntionl Rie Reserh Institute. Singh, C.B., Aujl, T.S., Sndhu, B.S. & Kher, K.L. (1996) Effet of trnsplnting dte nd irrigtion regime on growth, yield nd wter use in rie (Oryz stiv) in northern Indi. Indin Journl of Agriulturl Siene 66, 137 141. Sunderlnd, K.., Axelsen, J.A., romph, K., reier, B., Hemptinne, J.-L., Holst, N.H., Mols, P.J.M., Petersen, M.K., Powell, W., Ruggle, P., Triltsh, H. & Winder, L. (1997) Pest ontrol y ommunity of nturl enemies. pp. 271 326 in Powell, W. (Ed.) Arthropod Nturl Enemies in Arle Lnd. Vol. III. At Jutlndi 72(2). enmrk, Arhus University Press. Wy, M.J. & Heong, K.L. (1994) The role of iodiversity in the dynmis nd mngement of inset pests of tropil irrigted rie review. Bulletin of Entomologil Reserh 84, 567 587. Wy, M.J. & Khoo, K.C. (1992) Role of nts in pest mngement. Annul Review of Entomology 37, 479 53. Wy, M.J., Islm, Z., Heong, K.L. & Joshi, R.C. (1998) Ants in tropil irrigted rie: distriution nd undne, espeilly of Solenopsis geminte (Hymenopter:ormiide). Bulletin of Entomologil Reserh 88, 457 476. Wy, M.J., Jvier, G. & Heong, K.L. (22) Role of nts, espeilly the ire Ant Solenopsis gemint, in the iologil ontrol of tropil uplnd rie pests. Bulletin of Entomologil Reserh 92, 431 437. Yus, Y. (21) Predtion on eggs of the pple snil Pome nliult (Gstropod:Ampullriide) y the fire nt Solenopsis gemint. Journl of Mollusn Studies 67, 275 279.