Injury and Control of Onion Thrips(Thysanoptera: Thripidae) on Edible Podded Peas

A. M. SHELTON ANDR. C. NORTH

Department of Entomology, New York State Agricultural Experiment Station,Cornell University, Geneva, New York 14456

J. Econ. Entomol. 80(6): 1325-1330 (1987)ABSTRACT Harvests of edible podded peas from commercial fields in upstate New Yorkduring 1984 contained two types of injury previouslyunreported in the United States.Damageto pods consisted of either small raised bumps or scars appearing as silver mottling. Field-sampling indicated the presence of onion thrips, Thrips tabaci Lindeman, sogreenhouse andfield tests were designed to assessits ability to cause this injury and strategies for control.Resultsfrom greenhouse and field trials demonstrated that the scarswere caused by T. tabaci.In greenhouse trials, T. tabaci reproduced on peas and caused scars. A single application ofparathion at the prebloom or bloom stage was effective in eliminating all thrips and scarring,whereas an application at pod initiation allowed a low but noticeable level of damage. Infield trials, applications of parathion and dimethoate provided comparable control of scars.In one field trial it was necessary to apply an application at either the prebloom or bloomstage, followedby an additional application at the next crop stage (i.e.,bloomor pod-initiationstage, respectively), to achieve >70% scar-free pods. Counts of total thysanopteran larvae intreated plots before harvest indicated that both insecticides were effective in reducing larvalpopulations. In another field trial in which thrips populationswere much higher, no treatmentswere effective in reducing scarring injury. In greenhouse and field trials the occurrence ofbumps was not reduced by the use of insecticides,nor did it appear related to thrips numbers,thus indicating that they were not the cause of this blemish.

KEY WORDS Insecta, Pisum sativum, Thrips tabaci, control.

PEAS,Pisttm sativttm L., are a major vegetable cropin western New York, grown on ca. 5,000 ha in1985 (Anonymous 1985). Peas are normally shuckedbefore processing but recent trends in consumertastes have increased processor interest in growingedible podded varieties used in stir-fry cooking.Processors demand defect-free pods when peas aregrown for this market. Insect problems on peas inthis area are normally confined to aphids whichtransmit viruses such as pea enation mosaic virus.However, beginning in 1984, harvested loads wererejected by processors because of scars (silvery mot-tling or blotching on the epidermal layer) and con-spicuous small raised bumps on the exterior of thepod. Although no published reports indicated thecause of this injury in the United States, both typesof injury were similar to injury we have observedon cabbage frame leaves (Shelton et al. 1983, An-daloro & Shelton 1983). Because of the type ofinjury and the fact that thrips were present on thefoliage, we suspected that thrips were responsiblefor both types of injury, since thrips are known toscar developing strawberries, citrus, grape, andother fruits. Herein, we report the results of studiesto determine the cause and control of these defectson peas in western New York.

Materials and MethodsField studies were conducted during the grow-

ing season of 1984, and greenhouse studies were

conducted during the fall and winter of 1984. Allstudies were conducted using the pea variety SPL24.

Field Studies. In samples of buds, flowers, andpods in early-season pea fields, the only arthropodsfound were thrips larvae and two species of adultthrips, Thrips tabaci Lindeman and Frankliniellatritici (Fitch), with T. tabaci being the predomi-nant species. Damage to pods consisted of eithersmall raised bumps or scars (Fig. 1). We suspectedthat oviposition or feeding by thrips was causingthese blemishes, but it was also possible that patho-gens may have caused them, so field trials includedthe use of a broad-spectrum fungicide. No treat-ments were included that could have eliminatedthe possibility of the blemishes being caused byphysiological disorders or pathogens other thanfungi. In 1984, trials were conducted at two lo-cations in upstate New York-one near Geneva inOntario County and the other near Springwater inYates County. At each location the treatments andplot design were identical. The experimental areaconsisted of a 0.8-ha section within a larger field.Treatments were arranged in a randomized com-plete block design with three replicates; each plotwas 9 by 6 m and was bordered by 3 m on all sides.Treatments are listed in Table 1. Treatments con-sisted of an application of parathion 8E (0.28 kg[AI]/ha) or dimethoate 2E (0.20 kg [AI]/ha) andall possible combinations for timing these sprays at

0022-0493/87/1325-1330$02.00/0 © 1987 Entomological Society of America

1326 JOURNAL OF ECONOMIC ENTOMOLOGY Vol. 80, no. 6

Fig. 1. Injury at harvest on edible podded peas. (a)Arrow denotes scarring injury caused by T. tabaci. (b)Arrows denote small raised bumps which are not relatedto T. tabaci injury.

the prebloom, bloom, or pod initiation stage of thecrop. Both insecticides are registered for peas inNew York. Included as treatments were sprays ofa combination of Benlate 50W (0.84 kg [AIl/ha)plus Bravo 500 (0.59 kg [AIl/ha) applied at all threecrop stages, this fungicide combination plus eitherinsecticide applied at all three crop stages, and twountreated controls. Treatments were applied witha Solo backpack sprayer with a flat-fan nozzle de-livering 168liters/ha at a pressure of 1.03 kg/cm2•Treatments were applied at Geneva and Spring-water, respectively, on 12 and 18 July (prebloom),17 and 23 July (bloom), and 24 and 30 July (pod

formation). Harvests consisted of examination of50 mature pods per plot and rating them on a scaleof 0 (no injury) to 3 (severe injury) for both scarringinjury and bumps. The ratings of 1, 2, and 3 in-dicated the blemish covered ca. 5, 20, and >50%of the pod, respectively. Although there are nostrict guidelines, processors accepted some per-centage of pods with a rating of 1. An analysis ofvariance on the angular transformed percentage ofuninjured pods (both scarring and bumps) was per-formed to determine the effect of location, andorthogonal contrasts were made within locations toseparate the effects of insecticide, time of appli-cation, and use of fungicide.

To determine the effect of treatments on thripspopulations, foliage samples were taken on 30 July(Geneva) and 6 August (Springwater) in the un-treated plots and in those plots which had receiveda single application of parathion or dimethoate atthe prebloom, bloom, or pod initiation stage. Be-cause our past experience with thrips indicates auniform distribution within small fields, our foliagesamples consisted of randomly selecting only oneplot per treatment, harvesting five whole plants,and placing them in a Berlese funnel to extract thethrips. Harvests were taken at Geneva and Spring-water, respectively, on 31 July and 6 August.

Greenhouse Studies. Experiments were con-ducted to document the effect of thrips infestationon pod injury and to determine if such injury couldbe reduced by insecticides. Experiments used

ba

Table 1. Mean percentage of pea pods within a damage category" for scars or bumps under different lreatmenlregimes, Geneva, N.Y., 1984

Scars Bumps

Treatment % of pods in damage category % of pods in damage category

0 2 3 0 2 3Parathionb at pre bloom 51 42 7 0 76 20 3 1DimdhoateC at prebloom 52 36 9 3 64 25 9 2Parathion at bloom 66 26 6 2 71 24 4 IDinll'thoate at bloom 64 26 7 3 75 17 7 1Parathion at pod initiation 45 42 11 3 71 20 7 1Dimethoate at pod initiation 45 48 5 2 61 27 11 IPmathion at prebloom, bloom, and pod initiation 80 19 1 0 72 23 5 0Dimethoate at pre bloom, bloom, and pod initiation 74 25 I 0 69 23 7 1Parathion at prebloom and bloom 80 19 1 0 84 13 3 0Dimethoate at prebloom and bloom 70 27 3 1 69 22 8 0Parathion at prebloom and pod initiation 60 36 4 0 71 24 3 IDimethoate at prebloom and pod initiation 61 37 2 0 55 37 8 0Parathion at bloom and pod initiation 71 25 3 0 76 17 6 0Dimethoate at bloom and pod initiation 71 27 3 0 72 18 8 Ientreated 31 50 15 4 69 22 7 3t.:ntreated 29 49 14 7 43 34 21 3Fungicides

and pod initiation 85 13 2 0 82 17 0Fungicides plus dimethoate at prebloom, bloom,

and pod initiation 60 36 12 0 64 27 8

a Category scale: 0 (no injury) to 3 (severe injury) (see text).b Rate of 0.28kg (AI)/ha.C Rate of 0.20kg (AI)/ha.d Benlah" at 0.84kg (AI)/ha plus Bravo at 0.59kg (AI)/ha.

December 1987 SHELTON & NORTH: THRIPS INJURY TO PEAS 1327

Tnble2. Menn percent of pen pods within a dnmnge category" for scnrs or bumps under differenttrentment regimes,Springwater, N. Y., 1984

Scars Bumps

Treatment % of pods in damage category % of pods in damage category

0 2 3 0 2 3I'arathionh at prehloom 19 58 22 1 62 25 13 0Dimethoate< at prebloom 17 60 20 3 54 33 11 3Parathion at bloom 10 55 30 5 44 35 19 1Dimethoate at bloom 15 51 31 3 50 25 23 2Parathion at pod initiation 13 60 23 3 57 33 10 0Dimethoate at pod initial ion 22 51 27 1 71 13 15 1Par ••thion at prebloom, bloom, and pod initiation 22 65 13 0 58 25 17 0Dilllt'th" ••te at preblo"m, bloom, and pod initialion 28 58 13 1 43 36 20 1Pamthinn at prebloom and bloom 31 55 14 0 49 31 21 0Dimeth"ate at prt'bl"om and bloom 19 58 22 1 51 33 14 1P••ralhioll at prehloom aod pod initiation 19 61 18 2 54 30 15 1Dilllt'tho"te "t pr"bloom and pod initiation 19 49 29 3 45 30 25 1Parathion at bloom and pod initiation 30 58 11 1 52 29 17 1Dimeth""te "t bloom and pod initiation 32 51 17 0 62 21 17 1l'ntrl'ated 8 54 36 2 51 30 18 1llntre ••ted 7 57 31 5 65 19 14 1FOllgieidl'Sd "t prebloom. bloom, and pod initiation 24 52 21 2 59 25 14 1Fungiddes plus p"r"thion al prebloom, bloom,

and pod initiation 33 55 10 55 27 16 3FUllgkid,'s plus dinwthoate "t prebloom, bloom,

and pnd initiation 25 59 15 51 29 18 2

a C"tegory 5",,1,·, 0 (no injury) to 3 (severe injury) (see text).h Rate of 0.28 kg (A1)/h".

1328 JOURNAL OF ECONOMIC ENTOMOLOGY Vol. 80, no. 6

Table 3. Total number of thysanopteran larvae andThrips tabaci adults extracted from pea foliage of fiveplants under different treatment regimes in two areas orwestern New York, 1984

Table 4. Mean number of adult Thrips tabaci per peaplant after initial infestation of five adults per plant on 14December 1984 in a greenhouse

percentage of pods without scars in treatments withinsecticides versus no insecticides (t = 6.18, P =0.0001), but not between the two insecticides. Thecontrast between using one or two insecticide sprayswas highly significant for the percentage of podswithout scars (t = 4.09, P = 0.0002). To illustratethe influence of timing and number of insecticidesprays, in the Geneva trial with parathion, >70%of the pods were free from scars when plants weretreated at all three stages, or before bloom andthen again at bloom, or at bloom and pod initiation.Thus, at least two adjacent sprays were needed toachieve this level of control. If only one spray wasapplied, fewer pods were scarred when it was ap-plied at bloom than at pod formation (t = 3.03,P = 0.0044). When no spray was applied, only 29%of the pods were free from scars. Treatments withthe broad-spectrum fungicides did not significantlyreduce the amount of scarring compared with in-secticide-treated plots, although they reduced theamount of scarring compared with untreated plots(t = 2.54, P = 0.0155). This last result could be duesimply to the three applications of water, regardlessof the fungicide, washing the thrips from leaves ordrowning them.

The occurrence of bumps did not follow anyapparent trends, as, for example, parathion appliedat anyone crop stage provided >70% pods withoutbumps, an amount similar to one of the controls.The application of fungicides alone or in combi-nation with an insecticide did not influence thepercentage of pods without bumps. Thus, thesedata suggest that neither an arthropod nor a path-ogen susceptible to these pesticides was causing thebumps.

Treatment

Geneva locationParathion" at pre bloomParathion at bloomParathion at pod initiationUntreated

Dimethoateb at prebloomDimethoate at blossomDimethoate at pod initiationUntreated

Springwater locationParathion at prebloomParathion at bloomParathion at pod initiationUntreated

Dimethoate at prebloomDimethoate at bloomDimethoate at pod initiationUntreated

" Rate of 0.28 kg (AI)/ha.b Rate of 0.20 kg (AI)/ha.

No. thripslarvae

37117

14549388

215

245198213368276251218414

No. T.tabaciadults

979

1316179

19

796

1318138

15

Mean no. adults

17 December" 20 Decemberb

Leaves Blos- Leaves Blos-soms soms

Parathion at prebloom 4.2 0.3 0 0.2Parathion at bloom 3.5 0.6 1.5 2.7Parathion at pod initiation 4.0 0.3 2.7 1.5Control 3.8 0.5 2.3 1.3LSD (P = 0.05) NS NS 1.1 1.3

" Evaluation made before any treatment.b Evaluation made after prebloom treatment (17 December) but

before any other treatment.

In the Springwater trial, greater scar damagewas evidenced, in that only 7-8% of the pods inthe controls were free from scars. No clear trendsare evident from this trial, as the highest percent-age of pods free from scar injury was only 33%(treatment at all three crop stages with fungicidesand parathion) and the fungicides alone provided24%. It appears that none of the treatments wasable to reduce the high thrips population and thesubsequent injury as well as in the Geneva trial.This difference may be attributed to the Spring-water field being surrounded by wheat and oats.These crops are known to harbor high populationsof T. tabaci, which leave these hosts about the timethe pea trial was being conducted (North & Shelton1986). As in the Geneva location, no clear trendsare evident from the Springwater data on bumps.

Data on the effectiveness of the treatments againstthrips larvae and T. tabaci adults in the foliage areshown in Table 3. Because larvae could not beidentified to species, they were combined into asingle total. In the Geneva test, parathion and di-methoate, applied at any crop stage, reduced thenumber of thrips larvae by >74% compared withthe control. In the Springwater test, larval popu-lations were higher, and the best treatment reducedthe population by only 33% compared with thecontrol. More than 71% of the total adults capturedin either area were T. tabaci, but even these den-sities were low compared with the total larvae.Adult T. tabaci did not seem to be markedly in-fluenced by any treatment, but this may be due toa reinfestation by adults into both treated and un-treated areas.

Greenhouse Studies. Documentation of Injury.On 21 November, all 28 pods from the infestedplants and all 27 pods from the noninfested plantswere examined. Pod size averaged 4 em at harvest.Pods from plants infested with T. tahaci had amean damage rating of 2.6, whereas the plantswhich were not infested had a significantly lowermean damage rating of 0.1 (t = 22.09, P = 0.0005).The rating of bumps was low (0.1) in the infestedplants and 0 for the noninfested plants. From thesedata, it appears that T. tabaci can cause severe

December 1987 SHELTON & NORTH: THRIPS INJURY TO PEAS 1329

Table 5. Mean number of Thrips tobaci per plant and pod damage rating under different treatment regimes afterinitial infestation of five adults per plant on 14 December 1984 in a greenhouse

Larvae Adults Pod damage rating

Leaves Blossom Pod Leaves Blossom Pod Scars Bumps

24 DecemberParathion at prt'bloom· 0 0 0 0 0 0 0 0Parathion at bloomb 0 0 0 0 0 0 0 0Parathion at pod init.' 3.0 0.3 0 1.8 0.6 0.8 0.8 0Control 2.6 0.3 0 1.8 0.6 0.8 0.8 0LSD (I' = 0.05) 1.5 NS NS 1.2 NS 0.7 0.3 NS

2 JanuaryParathion at prt'bloom 0 0 0 0 0 0 0 0.1Parathion at bloom 0 0 0 0 0 0 0 0.1Parathion at pod init.d 0.1 0.1 0.1 0.1 0.1 0.4 0.3 0Control 31.5 1.5 25.5 9.8 0.7 3.7 2.8 0.1LSD (I' = 0.05) 4.6 0.9 3.3 2.1 0.4 0.7 0.2 NS

• Treatt'd 17 December.b Treatt'd 20 Dect'mber., Evaluation made berore treatment.d Trt'att'd 24 Decemb ••r.

scarring to developing and mature pods similar tothe injury observed in the field during the summerof 1984. However, the bumps that were observedin the field were almost negligible in these trials,again suggesting that they may be caused by somefactor other than thrips. Also noted in this test wasthat ca. 50% of the pods retained a blossom frag-ment at the distal end of the pod, and thrips andtheir injury were often concentrated there.

Reduction of Injury. The results of the 17 De-cember evaluation, before any spray was applied,are shown in Table 4. The number of thrips wasonly slightly reduced from the original five thripsper plant and most were located on the leaves (blos-som buds were just beginning to develop and onlya few thrips were located on them). By 20 Decem-ber, nearly all thrips had been eliminated by theprebloom spray and thrips on the other, as yetunsprayed plants, began infesting the opened blos-soms. By the 24 December evaluation (Table 5),the prebloom and bloom sprays had been applied.Data from this evaluation indicate that T. tabaciwas able to reproduce on peas since larvae werepresent, and that the prebloom and bloom sprayswere effective in reducing thrips populations andinitial scarring by thrips. The final evaluation (2January) indicates no pod scarring on the prebloomand bloom treatments and significantly (P = 0.05)more scarring on the pod-initiation treatment. Se-vere scarring was recorded on the untreated pods,and the number of larvae per plant was 31.5, 1.5,and 25.5 on the leaves, blossoms, and pods, re-spectively. On the untreated plants the number ofadults per plant was lower than the number oflarvae but 3-fold higher than the original numberof adult thrips which were placed on the plants,thus indicating that a complete generation had beenproduced on peas. The incidence of bumps waslow (0-0.1) for all treatments, indicating no rela-

tionship between the occurrence of thrips and thistype of injury. Although we originally thought thatthe bumps were caused by thrips oviposition, thesedata, plus observation of thrips larvae emergingfrom areas of pods which did not have bumps,confirms that T. tabaci was not the cause.

These appear to be the first published data onT. tabaci as a pest of pea pods and methods for itscontrol. Our recommendations from this study areto treat pea plantings which are infested with T.tabaci to reduce scarring on harvested pods. Thebloom stage appears to be the optimal stage fortreatment, but an additional spray either at pre-bloom or pod initiation may also be needed. If T.tabaci is extremely abundant, as in the Springwatertrial, control will be reduced. Especially in thiscase, it would be important to have good coverageof the plant, and this could be achieved by eitherincreasing the pressure or amount of water (Sheltonet al. 1984).

A report from the Netherlands indicates thatanother thrips species, Kakothrips robustus Uzel,damages pea pods in that area (Franssen 1960).Pictures from Franssen's report indicate that scar-ring damage to pods caused by K. robustus wassimilar to the injury we observed with T. tabaci,but there is no mention of injury resembling smallraised bumps. Additionally, Franssen reports thatK. robustus adults appeared just before or at bloomand reproduced in the flowers, and that the bestinsecticide available was parathion.

Acknowledgment

We wish to thank W. T. Wilsey, J. E. Hunter (De-partment of Plant Pathology), and Dennis Larsen (Gen-eral Foods) for their field assistance in this study. Thestatistical help of Casey and Eloise Hoy is greatly ap-preciated.

1330 JOURNAL OF ECONOMIC ENTOMOLOGY Vol. 80, no. 6

References Cited

Anonymous. 1985. New York state agricultural sta-tistics. 1985. New York Crop Reporting Service, Al-bany, N.Y.

Andalaro, J. T. & A. M. Shelton. 1983. Onion thrips:insect identification fact sheet. Vegetable Crops Pub-lication, Cornell Univ., Ithaca, N.Y., 750.75.

Franssen, C. J. H. 1960. Biology and control of thepea thrips. Verslagen van Landbouwkundige Onder-zoekingen, No. 66.4 (in Dutch, with English abstract).

North, R. C. & A. M. Shelton. 1986. Colonization andintraplant distribution of Thrips tabaci (Thysanop-tera: Thripidae) on cabbage. J. Econ. Entomol. 79:219-223.

Shelton, A. M., R. F. Becker & J. T. Andaloro. 1983.Varietal resistance to onion thrips in processing cab-bage. J. Econ. Entomol. 76: 85-86.

Shelton, A. M., W. T. Wilsey & R. C. North. 1984.Coverage of onion foliage as a requirement for con-trol of onion thrips on onions, p. 124. In Insecticideand acaracide tests, vol. 10. Entomol. Soc. Am., Col-lege Park, Md.

Snedecor, G. W. & W. G. Cochran. 1980. Statisticalmethods, 7th ed. Iowa State University Press, Ames,Iowa.

Received for publication 11 February 1987; accepted13 August 1987.

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