A Food-web Approach to Economic Thresholds : A Sequence of Pests /Predaceous Arthropods on California Cotton

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1 Entomophg 7 (Specil ssue) : (198) A Food-web Approch to Economic Thresholds : A Sequence of Pests /Predceous Arthropods on Cliforni Cotton by D. GONLEZ nd L.T. WLSON' Deprtment of Entomology, University of Cliforni, Riverside (USA) The vlue of n dequte food bse for predceous rthropods s provided by Terrcirryckirs is relevnt for the development of economic thresholds for this group nd lso for pests tht occur lter in the seson. A higher predceous potentil ws reflected in greter levels of Geocoris nd of Oriirs on untreted plnts with bundnt food, s compred with lower predtor numbers on insecticide-treted plnts hving lower food levels. The higher numbers of Geocoris nd Oriirs \vere significnt becuse they persisted through July-August. n the Sn Joquin Vlley, n increse in predtors during this period is highly beneficil, becuse L.yspr.s nd lepidopterous worms my dmge cotton t this time. ' Policy guidelines on effects from chemicls on rthropods need re-evlution. The ssessment of disruptive effects of chemicls must include their impct on the rthropods tht constitute the principl sources of food for predceous rthropods. The indirect effect of chemicls ginst beneficil-predceous insects vi disruption of their food chin is highly significnt becuse considerbly longer time-intervl is required to replenish the food supply, thn is needed for beneficil-predceous dults to reinvde field. Results from this study support our contention tht t lest four components must be considered sesonlly in developing economic thresholds for pest mngement progrms : 1) the plnts. ) the complex of key pests. 3) the complex of beneficil rthropods, nd 4) sources of food in the form of minor (( pests )) required to support significnt numbers of predceous rthropods in griculturl crops. ntroduction A sequence of rthropod pests occurs on cotton in the Sn Joquin Vlley of Cliforni, with their bundnce regulted to gret extent by insect predtors, Geocoris pllem Stihl nd Orius fristicolor White (fig. 1 ). Generlly, the popultion curves for the more bundnt rthropods follow the cotton flower or fruit formtion ptterns, lthough there is phenologicl sequence of pest occurrence generlly in the order : Tetrnychus/ Lygus slsh worms. Lepidopterous lrve nd especilly Heliorltis ze (Boddie) generlly re more bundnt during the erly fruit mturtion period in erly August (Gonzlez et /., 1967). The reltionships mong the cotton plnt nd the ccompnying rthropod fun cn be described s food-web or n energy pyrmid. n the web concept, the underlying premise is tht the components re interrelted. The energetics pproch focuses on those reltionships in terms of their energy/nutritionl ( = food) requirements t vrious trophic or energy levels (fig. ). Under this scheme, it is obvious tht the bundnce of predceous rthropods depends to gret extent on their food sources. From preliminry results on cloric vlues for Geocoris, Orius nd Tetrrtycltus urrice Koch (unpubl. dt of Bonnie Ruth Ptterson), we find tht on simple cloric or biomss rtio, Geocoris would require more thn 5 times the number of T. urtice thn Oriiis to mintin their cloric levels. Energy budgets re essentil to more clerly delinete the specific food needs of ech of these predtors. Nevertheless, these preliminry cloric requirements support field dt in which we find Geocoris is more effective predtor thn Oriirs ginst Tetrri.vchus spp. on Cliforni cotton. 1) University of Cliforni, Dvis. 31

2 PLANT ESTABLSHMENT -1- / FRUT FORMATON-~-FRUlT MATURATON- ORUS c HELOTHS. TRCHOPLUSA, S PODPTE R A * TETRANYCHUS, THRPS, LYGUS, GEOCORS 1 SQUARES JUN GREEN BOLLS JUL SE PT AUG Fig. 1. Phenologies on cotton : fruiting ptterns, pests, nturl enemies nd food rthropods. We believe tht in Cliforni cotton, nd perhps in most cotton-growing res, food sources for predtors consist of complex of rthropods numericlly bundnt nd redily vilble becuse of their concurrence in time nd spce with predceous rthropods. Within plnts, the verticl distribution of most of these rthropods is generlly concentrted within the upper 1 / 3 of the plnt. Verticl nd sesonl rthropod distribution ptterns re closely correlted with plnt nitrogen levels. These levels re higher in the erlier s compred with lter portions of the seson nd lso on younger leves closer to the pex of the plnt s compred with older leves further from the pex. n Cliforni cotton, the principl food sources include 1 or more species of complex of cow PASTURE FOOD FOR PREDATORS MTES, THRPS, WF'S, APHDS PLANT COTTON NOTE: "KEY" PESTS ARE NOT A CRTCAL PART OF THE FOOD NSECTS = "DRTY FELD" "FELD NSECTARY" "PREDATOR UNTS" Fig.. Schemtic digrm of energy pyrmid in cotton groecosystem. 3 FOOD CHAN

3 Tetrnychiis consisting of T. urtice, T. tiirkestni Ugrov & Nikolski nd T. pcificus McGregor ; Frnkliniell occidentlis (Pergnde) nd, to lesser extent, Aphis gossypii Glover nd Trileurodes vporriorum (Westwood). n cloric terms, the key pests, such s LjJgus nd Heliofhis, do not generlly become sufficiently bundnt to serve s prime source of food for predceous rthropods. Predtors re generlly in greter bundnce in (( dirty fields )), i.e. those chemiclly undisturbed nd contining n bundnce of prey species/diversity. Such fields provide fvorble conditions for (( field insectries N in which survivl, longevity nd fecundity of nturl enemies re enhnced pprecibly. Dt re presented in this pper to support our contention tht economic thresholds need to be structured ccording to the plnt s growth stgedfruiting ptterns, its interreltionships with the complex of key pests nd their nturl enemies, nd the sources of food in the form of minor pests required to support significnt numbers of predceous rthropods in griculturl crops. Methods /Mterils Studies were conducted on Acl SJ- cotton in Cliforni s southern Sn Joquin Vlley. Experiments were crried out in : ) individully cged plnts in which the number of predtor/prey plced in the bg ws controlled (Grci-Escobr et /., 198) ; nd ) field trils in which vrious chemicls (tble 1) were used to chieve vrious levels of predtor/prey on uncged plnts (Gonzlez et /., 198). n second yer of these trils, methyl prthion ws dded (t the rte of 1 lb ctul/cre) s seprte tretment in ddition to the tretments used the first yer (fig ). Ech cge consisted of sleeve of sheer-mist cotton linen with Velcro sewn to ech end. The Velcro served s sels round the plnt stem t the lower portion of the plnt terminl nd bove the terminl. Before predtors nd prey were introduced into the cges, ech plnt ws fully inspected nd ny rthropods found were removed with brush. n ddition, young plnts (c. cm tll) nd lrge cges were spryed twice t 3-dy intervls to run-off with TEPP (tetrethyl pyrophosphte) t.8 liters of 4 % emulsifible concentrte/4 liters of wter one week before the introductions were mde. Ech tretment consisted of 4 cges. Four femle nd two mle dults of G. pllens were introduced into ech cge in lte June. Cges were rrnged in rndomized complete-block design. Prey were replenished ech week with numbers estimted to be more thn miniml for predtor survivl bsed on our unpublished dt of prey consumption, predtor development rtes nd preference of G. pllens. Tretment response ws mesured by collecting four plnt cges per tretment ech week, over period of 1 weeks. Plnts were severed below the cge nd tken to the lbortory where they were shken, brushed nd vcuumed within 15 litre continer to remove rthropods. Dislodged rthropods were collected by vcuuming the bottom of the continer. Plnts nd cges were visully inspected, nd 1/ of the plnts were further processed in Berlese funnels to remove the remining rthropods. Dt used in nlyses re bsed on the combined numbers obtined from bgs + Berleses. Field trils with uncged plnts were set in plots 4 m wide nd 6 meters long. Arthropods (insects, Tefrnycltus nd spiders) were collected from ech tretment re by system of bgging individul plnts. Bgged plnts were then tken to lbortory for processing s described bove, fter which rthropods were counted. Ten plnts per tretment were smpled ech week, nd thus ech dt point for rthropods represents men vlue from 1 smples. 33

4 Results nd Discussion Dt re presented only for GeocorislTetrnychus nd only for totl numbers becuse of limittions of spce/time. Dt were recorded by ge groups, nd by sex for dults (excepting for Frnkliniell) for Geocoris, Orius, Frnkliniell nd Tetrnychus. Geocoris, Orius nd Frnkliniell were the only predtors collected consistently t levels of >, l/plnt. The Tetrnychus complex nd Frnkliniell were the only prey collected consistently t levels > S/plnt. Predtor-food Web n field cge trils (fig. 3,4, 5), the number of eggs, nymphs nd dults produced by G. pllens when fed on three types of food followed the pttern T. pcificus > F. occidentlis > cotton plnt only (from Grci-Escobr et l., 198). n lbortory preypreference studies with G. pllens, A. Grci (unpubl. dt) found T. pcificus ws preferred to F. occidentlis. 3r Tetrnychus W V u v, (3 loo- (3 '$1 W 5 - ) (3 U d - L l - J JUL AUG SEPT Fig. 3. Accumultive men number of eggs produced by G. pllens dults on cotton plnts (n = 4) only (control tretment), nd on cotton plnts plus ech of three prey species fl. pcificus, F. omidentlk nd L. hesperus) in field cges. Tretments followed by different letters re significntly different t the.1 VO level when tested by Duncn's Multiple Rnge Test. Although G. pllens my survive for severl dys/ weeks on cotton plnts lone, it fils to mintin significnt popultion. The ddition of rthropods s source of food incresed fecundity nd survivl of G. pllens significntly with T. pcificus providing the gretest response. The decrese in numbers nd biotic potentil of G. pllens fter mid- August, in spite of the vilbility of rthropod prey under open-field nd field cge conditions, indictes tht fctors other thn food re regulting the numbers of G. pllens during tht prt of the seson. n the first yer of field plot trils with uncged plnts, numbers of Geocoris were closely ssocited with the numbers of Tetrnychus (fig. 6, 7). The plot with the highest numbers of Tetrnychus lso hd exceptionlly high numbers of Geocoris. A seprte plot, "OM (tble ), with pproximtely hlf s mny Tetrnychus s in HNOM, lso contined bout 5 % fewer Geocoris. 34

5 Tble 1. Tretments', nd their designtions, used to crete differences in pest nd beneficil rthropod bundnce in cotton fields. Designtion NNOM Tretment Actul rtes norml nitrogen 8 bs (1 % nitrte nitrogen + 1 O/O phosphorus) miticides/insecticides NN5M HNOM norml nitrogen 5 miticide pplictions dicofol (Kelthne) 1 June s in NNOM 19 My 1, 18 June 1 July 8 Aug. 8 Aug. only 19 My 1, 18, 8 June 4 qts proprgite (Comite) 4 insecticide pplictions cephte (Orthene) pts 1 b high nitrogen 16 bs (1 % nitrte nitrogen + 1 % phosphorus) miticides/insecticides HN5M Appliction dtes high nitrogen 5 miticide pplictions 4 insecticide pplictions 1 June s in HNOM s in NNSM s in NNBM nsecticides/miticideswere pplied to plnts by stndrd high clernce ground rig. - Tetronychus pcificus A F r o n k l i n i e l l o occidentlis - \ o Lygus h e s p e r u s X W 1 - m 15- Cotton plont only - ) r' 14 6 AUG JUL SEPT Fig. 4. Accumultive men number of G. pllens nymphs recovered fter dults were cged outdoors on cotton plnts (n = 4) (control), nd with cotton plnts plus ech of three prey species T. pdficus, F. occidentlis nd L. hesperm). Tretments followed by different letters re significntly different t the.1 % level when tested by Duncn's Multiple Rnge Test. n the miticide-treted plot NNSM, where Tetrnychus were t low levels, less thn one Geocoris per plnt ws found. These low levels were sustined for two months fter the lst ppliction of insecticide on 8 June. Considering this two-month intervl nd the 35

6 known short residul effect of the insecticide used, it is likely tht the low levels of Geocoris were sustined becuse of lck of food nd not becuse of ny direct effect from the chemicl. k 4- -J \ Tetrnychus pcificus w 3- A Fronkliniell o c c i d e n t l l s X - C o t t o n plnt only J 3 3 W U in - b AUG JUL SEPT Fig. 5. Accumultive men number of G. pllens dults recovered fter dults were cged outdoors on cotton plnts (n = 4) only (control). nd with cotton plnts plus ech of two prey species (T. pcificus nd F. occidentlis). Tretments followed by different letters re significntly different t the.1 Ol level when tested by Duncn's Multiole Rnge Test. 1 e n u < c 4 z 1c 1 1 MA1 UN: JUL" Fig. 6. Tetrnvchus from plnts subiected to three tretments in AUG

7 Orius lso were most closely ssocited with high numbers of Tetrnychus (Gonzlez et l., 198). Unusully high numbers of Orius were collected from plnts where Tetrnychus were most bundnt. At lower Tetrnychus levels, Orius were reduced correspondingly. A mjor difference from the pttern found with Geocoris ws the increse of numbers of Orius lter in the seson, closely ssocited with high numbers of Frnkliniell, from plnts with low numbers of Tetrnychus. Frnkliniell ws consistently the single most bundnt species from plnt emergence through the end of June (Gonzlez ef l., 198). Their feeding on plnts hs frequently resulted in their clssifiction s pests nd in subsequent insecticide pplictions. They were found, prticulrly in My-June, ctively feeding on Tetrnychus eggs wherever the ltter were congregted. Frnkliniell my be considered beneficil lso in tht they provide n essentil food source for predceous rthropods, especilly for Orius. When flowers becme vilble, most Frnkliniell were found in those structures, nd their popultion trends in July-August were most closely ssocited with flowering ptterns, not with Tetrnychus. n the second yer of the trils (1979) with uncged plnts, there were more predceous rthropods over longer period of time thn in the previous yer (fig. 7-9). There were corresponding differences in the totl numbers of mites collected (fig. 6, 1, 111, nd in yields. Numbers of Geocoris nd Orius reflected the chemicl tretments, the food vilble nd the plnt fruit stge within phenologicl frmework tht ppers to be primrily determined by temperture (tble ). Within the temperture frmework, chemicl tretments pper to be the single fctor most ffecting bundnce of predtors in three wys : chemicls 1) kill predtors, ) nd their food sources ; 3) they cn increse the nitrogen levels in lef folige (C. Lbnuskus, UCR, unpubl. dt), thus incresing mite fecundity nd survivl without corresponding effect on predtors. Figures reflect the interreltionship between predtors/ Tetrnychus/chemicls. At low food levels (Orthene + Kelthne nd NNSM tretments), Geocorislorius numbers were uniformly reduced. The highest levels of predtors over n extended time period were from plnts not receiving chemicl tretments nd hving high numbers of Tetrnychus. Tble. Temperture thresholds Cr) nd dys to develop (DD) for three cotton rthropods. Geocoris Orius T. urtke T DD T DD T DD Min Mx (?) Results in 1979 were notbly different from those in 1978, principlly in the Tetrnychus/predtor rtio from the methyl prthion treted plnts. The combined effect of methyl prthion in killing predtors nd stimulting Tetrnychus fecundity nd survivl produced increses of the ltter to high levels -3 weeks erlier thn occurred in untreted plots where predtor levels were much higher. n 1979, in the bsence of insecticides/miticides but under excess nitrogen fertiliztion, the Tetrnychus growth rte ws gretly ccelerted wheres tht of Geocoris remined essentilly similir s compred with plnts under norml nitrogen fertiliztion rtes (fig ). The inbility of predtors to 37

8 regulte Tefrnychus numbers beyond certin upper limit is in prt relted to their different developmentl rtes (tble ). During the fruit formtion period, GeocorislOrius require bout -4 weeks to complete genertion wheres Tetrnychus only need 5-7 dys. During this period, Tefrnychusmy increse geometriclly in numbers nd there is lso n incresing surfce re for predtors to serch in order to cpture their prey. - NNOY A....& h..\ug lull JUNf YAY Fig. 7. Geocuris from plnts subjected to three tretments in O---- METHYL PARATHON () NO CHEMCAL TREATMENT 8ooo r A- -A ORTHENE t KELTHANE 1 t K 1 K ONLY 4O JULY r - z AUG t v) W - d z t 4 JUNE 1 7 h4 1 JULY AUGUST Fig. 8. Geocoris from plnts subjected to three tretments in i98, under norml nitrogen fertiliztion rtes. 38

9 K ONLY 8K AUG 4 JULV OBK \1 JUNE JULY AUGUS Fig. 9. Geocoris from plnts subjected to three tretments in 198, under x norml nitrogen fertiliztion rtes. 1 - Q MtTHYl. PARATHON J NO CHEMCAL T R E A T M E N T A--A ORTHENE + KELTHAhE im (O+Kl & K J. K ONLY 1 AUG 4 JULY 6 JUNE \ V z d / t JUNE AUGUST Fig. 1.TefrmYChUS from Plnts subjected3 three tretments in 198, under norml nitrogen fertiliztion rtes. JULY Dmge mpct The most significnt impct of Tetrnychus feeding on cotton yields ppers to be during reltively short period of perhps -4 weeks during the erly to pek period of fruit formtion (fig. 1-13). Highest Tetrnychus levels in tretments clerly hd the lowest yields (fig.1). But plnts with high nitrogen (HNJ lso hd pprecibly lower yields 39

10 HGH 6- N < h-4 O8K "7 K ONLY 4 JULY u.1 8K OBK OL Lu - 8 K c z NT Aooo31 P 6 lune lune,j / \ JUNE lh \ '1 1 JULY AUGUST Fig. 11. Tetrnychus from plnts subjected to three tretments in 198, under x norml nitrogen fertlllztion rtes, C---U 5- \ ORTHENE t KELTHANE b - - d _ METHYL PARATHON NO CHEMCAL TREATMENT 4- n 3- W 5 - _ 1-4 JUNE JULY 4 AUGUST 8 5 O NOV t NO. O P E N BOLLS Fig. 1. Pttern of lrge green bolls set on cotton plnts subjected to 3 tretments under norml nitrogen fertiliztion rtes. thn plnts with norml nitrogen (NNJ tht otherwise hd corresponding tretments (fig. 13). Yields were lower in 1979 ( + K. NP, ) thn in 1978 (NNOM, NNSM, HNOM, HNSM), including plnts treted with miticides/insecticides on which Tetrunychus levels were low in both yers. n 1979, when predtor levels were high, there were no differences in yields between plnts untreted nd plnts treted with 5 miticide pplictions. Dt in fig. 3 re bsed on sesonl totls for both yields nd Tetrui~yclz~is. The criterion of 1 femle Tetrur~~vchus/plnt/time s threshold is bsed on dt points t levels beyond which popultion growth curves incresed rpidly. This level corresponds 9 % infested (mobile Terrulz.vchus, ll stges) on presence/bsence to level of - 4

11 criterion (Wilson et f.,198 1). Bsed on comprtive counts between bg + Berlese nd those on min-stem leves, the efficiency of the bg + Berlese method for estimting Tetrunychus numbers ws found to be 5 %. The 1 femledplnt level thus 4 femles/plnt. corresponds to n ctul level of - - "e v vv 5 > t HN N.p A O t K NO PESTCDES A ORTHENE KELTHANE t HN5M V 3 5 V NNJM * METHYL PARATHON NNOM HNOM 4 NUMBER OF WEEKS MTE FEMALES 6 7 > OO/PLANT 8 Fig. 13. Cotton yields vs. popultion density of femle Tetrnychus from cotton plnts subjected to 1 tretments over -yer period. Reserch mplictions n gthering the dt presented in this pper, we becme more wre of wht we believe re limittions in the experimentl design/nlysis commonly used in economic threshold reserch. On experimentl design, we propose tht fctoril experiments using split-plot designs pper to be more pproprite thn the more commonly used rndomized complete-blocks or Ltin squre designs. This is prticulrly relevnt for experiments in which dt on pest enemies re being gthered in reltion to vrious pest levels with the ltter in turn relted to plnt fruiting ptterns. The obvious limit is in blnce between tretments/replictions becuse of the underlying limit of resources in lnd, equipment, personnel, etc. As n exmple, if resources re vilble for 4 totl plots, in rndomized block design one my evlute six tretments ech replicted 4 times. With fctoril splitplot pproch, 4 tretments cn be evluted if repliction is not employed. n terms of experimentl points involving predtor /prey interctions ech impcting on fruiting ptterns, 4 points re obviously more useful thn six. The second re of concern to us involves nlyses of dt. We believe tht in cses of restricted resources combintion of regression nlyses of dt from fctoril split-plot experiments my yield more relible predictive vlues thn nlyses of vrince from dt on rndomized block experiments with few tretments/ more replictions or vice vers with more tretments/ few replictions. With regression techniques, the tendency is to relte yields/totl (sesonl) pest numbers. Dt on fruiting ptterns cn ssist in clrifying the sesonl impct from one or more pests t vrious densities ech t one or more specific time periods. We suggest tht stepwise regression nlyses be crried out regressing yields vs. pest numbers t specific time intervls in n dditive mnner, nd tht resulting r vlues be compred s n estimte of predictive vlues for the regression equtions. Regression 41

12 nlyses of pest/predtor interctions my yield more efficient predictive vlues if time-lg comprisons re mde to llow for the ctul time-dely tht occurs in predtor/prey interctions under field conditions. Plot size nd its loction in reltion to beneficil insect sources cn lso present mjor limittion on effectiveness of predtor/prey dt collected. We found tht experimentl prcels need to be minimum of 4 rows (8 1 meter/row) wide x 6 m long to prevent confounding effects mong tretments resulting from insecticide drift nd dilution from migrting rthropods. n our situtions, the experimentl res were surrounded by vrious insecticide-free crops other thn cotton (lflf, brley, sfflower) serving s reservoirs of nturl enemies with no pplictions of insecticides by ircrft in the immeditely djcent res. Plot size for experiments incorporting beneficil insects into economic threshold progrms obviously requires tht reservoir of beneficil insects be vilble within or ner to the experimentl re to replce those individuls destroyed by insecticide tretments. Under conditions where reservoirs of beneficil insects re low nd/or where there is insecticide drift into plots, minimum plot size should be lrger thn tht determined by us under more fvourble conditions. Our Tetruriychus studies clerly show two different growth potentils, one with higher levels nd the other with lower levels of predtors. Both of these conditions need to be evluted in economic threshold studies. Definition of dequte or minimum levels of predtors for effective regultion of pest numbers in reltion to yields is crucil priority. n our studies, (< high )) levels of predtors (combined Geocoris/Orius/thrips) were /plnt during the fruit formtion period (4 June-8 July) from untreted plnts with norml nitrogen levels where yields were comprble to those from plnts protected by pesticides ( 1979). Summry Our findings indicte tht guidelines for the ssessment of pests on cotton should consider lso their essentil role s primry food sources t ll popultion levels, s well s their potentil for yield reduction. The vlue of n dequte food bse for predceous rthropods s provided by Tetrnychus is relevnt for the development of economic thresholds not only for this group, but lso for pests tht my occur lter in the seson. A higher predceous potentil ws reflected in the greter levels of Geocoris nd of Orius found on untreted plnts with bundnt food, s compred with much lower ( + K tretment) or higher ( tretment) food (mite) level on insecticide-treted plnts. The incresed potentil for regultion of pest numbers through higher numbers of predtors ws especilly significnt, becuse the higher numbers were present over n extended period, including July-August. n the Sn Joquin Vlley, n increse in predtion potentil during this period is highly beneficil, becuse Lygus nd lepidopterous worms lso occur on cotton t this time. We strongly urge tht policy guidelines on effects from chemicls on rthropods be re-evluted. The long-stnding concern over disruptive effects of chemicls must be extended to include their impct on the rthropods tht constitute the principl sources of food for predceous rthropods. t is not sufficient to know the direct mortlity impct of chemicls ginst beneficil-predceous insects. The indirect effect ginst them vi disruption of their food chin is more significnt becuse considerbly longer timeintervl is required to replenish the food supply thn is needed for beneficil-predceous dults to reinvde field. Results from this study support our contention tht t lest four components must be considered sesonlly in developing economic thresholds for pest mngement progrms : 1) the plnts, ) the complex of key pests, 3) the complex of beneficil rthropods, nd 4) sources of food in the form of minor (( pests )) required to support significnt numbers of predceous rthropods in griculturl crops. 4

13 REFERENCES GARCA-ESCOBAR, A., GONZALEZ D. & LEGH, T.F. (198). Evlution of Frnklinielle occidentelis nd Tetrenychus pecifiws s primry food sources for Geocoris pellens nd Orius tristicolor on cotton. Entomophege (in press). GONZALEZ D., PATTERSON, B.R., LEGH, T.F. & WLSON, L.T. (198). Mites: primry food source for two predtors in Sn Joquin Vlley cotton. Clif. Agric. 36 ; 18-. GONZALEZ D., VAN DEN BOSCH, R., ORPHANDES, G., DAWSON, L.H. & WHTE, C. (1967). Popultion ssessment of Cotton bollworm in reltion to pest control prctices. Clif. Agric. 7 ; WLSON, L.T., LEGH, T.F. & MAGG, V. (1981). Presence-bsence smpling of spider mite densities on cotton. Clif. Agric. 35 : 1. 43