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									    The use of blue sticky traps in management of Thrips palmi (Karny)

       (Thysanoptera: Thripidae) in eggplant cultivation in Mauritius

                                          K. Naojee*

                            Entomology Division, AREU, Réduit.

                                    entoareu@gmail.com

                                         D. Abeeluck

                            Entomology Division, AREU, Réduit

                                    entoareu@gmail.com

                                       T. Rawananshah

                            Entomology Division, AREU, Réduit

                                     areuento@intnet.mu

                                       Dr. S. Facknath

                       Faculty of Agriculture, University of Mauritius

                                      sunif@uom.ac.mu



Abstract

Thrips palmi Karny (Thysanoptera: Thripidae) is a major pest of eggplant in Mauritius and is

being controlled by insecticides. The potential of two types of blue sticky traps (Pherobank,

Plant Research International (PRI) and locally prepared) to either monitor or mass capture

adults of T. palmi was investigated in eggplant cultivation



Both types of traps were effective in attracting adults of T. palmi. Catches were significantly

higher in local traps than those in PRI traps. Trap catches and adult infestation on leaves

were strongly correlated.




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The distribution pattern of adults and nymphs of T. palmi was determined on eggplants for a

period of four months. Within plant distribution, significantly higher numbers of T. palmi

(adults and nymphs) were found on upper leaves (t = 13.47, P< 0.01) compared to lower

leaves (t = 7.87, P< 0.01) of eggplants. The adults and nymphs population on both upper and

lower leaves increased during flowering and fruiting stage of the plant but decreased by the

end of crop cycle.



The local blue trap has a high potential for monitoring and mass capturing of adult T. palmi

and could be used as an indicator to time insecticide application in eggplant fields.



Key words: Thrips palmi, eggplant, blue sticky trap, Pherobank, mass capture



Introduction

Eggplant (Solanum melongena L.) is an important perennial crop grown on about 160

hectares in Mauritius with an annual production of about 1800 tonnes (Anon, 2008). The

melon thrips, Thrips palmi (Karny) is one of the three pests commonly encountered in

eggplant cultivation (Abeeluck et al., 2004). It was first recorded on eggplant at Baie du

Tombeau (North West of Mauritius) in 1985 (Anon, 1986) and subsequently became well

established on many horticultural crops (e.g., bean, chilly, cucumber, potato, watermelon)

and even on weeds.       Nymphs and adults live underneath leaves, in flowers and feed

gregariously on them and fruits as well. Heavily infested leaves dry up, plant growth and

yield is greatly reduced and finally plants can even die (Young et al., 1998). They also feed

under calyxes of young fruits that develop silvery patches. Harvested fruits become russetted

in appearance and their market value is considerably reduced. Thrips palmi occurs in low

numbers during the cool and dry winter but are very abundant during the hot summer. Many



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crops are adversely affected and their yield is very much reduced (Abeeluck D., pers. comm.,

2007).



Since 1985, a chemical control regime is being advocated against T. palmi in eggplant

cultivation and other susceptible crops. Formetanate, methiocarb and profenofos are

recommended against T. palmi (Abeeluck et al., 2004). In 2005, vegetable growers observed

a rapid pest build-up in their field within a short interval during summer and they claimed

that these recommended insecticides were no longer effective against T. palmi. The inability

of growers to control T. palmi was related to their insecticide application practices (Naojee,

2007). However, effective control is not often achieved because of its cryptic behaviour that

renders early pest detection difficult (Kawai, 1990).       This unilateral use of insecticides

against T. palmi can result into adverse effects on the environment, beneficial organisms and

even consumer. The Government of Mauritius strongly supports development of Integrated

Pest Management systems in horticultural crop production and the safe use of insecticides

(Anon, 2003-2007). The search for alternatives methods for T. palmi control had become

highly desirable. The use of sticky traps to monitor and control T. palmi in protected

cultivation and open fields has been studied in Japan but not in Mauritius (Murai, 2001).



This study, therefore, aimed at investigating into the potential of blue sticky traps to monitor

and mass capture T. palmi adults in eggplant cultivation.



Materials and Methods

Study site and Experimental set up

The experiment was conducted on an eggplant plot (15m x 20m) on the Crop Research

Station (CRS) of the Agricultural Research and Extension Unit (AREU) at Richelieu in the

North West of Mauritius during September-December 2006.               Seeds of eggplant (var.

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Barbentane) treated with Thiram (3g/kg of seeds) were raised to seedlings in trays. At 6-leaf

stage, 300 seedlings (free from pests and diseases) were transplanted at 90cm apart in the

plot. Fertilization and disease control were effected as per recommendation (Abeeluck et al.,

2004). Fenazaquin (Pride 200 SC @ 1mL/L of water) and acetamiprid (Mospilan 200 SP @

0.25g/L) were applied to control red spider mites and aphids respectively. No treatment was

effected against thrips.



Source of blue sticky traps

Two types of blue traps were evaluated. The first one (24cm x 10cm) was purchased from

Pherobank, Plant Research International (PRI), The Netherlands. The second one was

constructed with a 4mm plywood board covered with blue plastic sheet and was similar to the

PRI trap in size. To avoid trap damage, transparent plastic sheets were fitted to PRI traps.

The wavelength of the blue colour of both traps was determined with a UV/VIS/NIR

spectrophotometer (Perkin Elmer, Lambda 19). An adhesive (insect glue-Tanglefoot) was

purchased from PRI and applied on both sides of blue traps.



At three weeks after transplantation (WAT), eight sticky traps (4 PRI and 4 local) were

placed randomly on wooden stakes at 2.5m apart just above plant canopy. They were

removed every week and replaced by new ones. The PRI and local traps were rotated

clockwise during replacement. The trial was a randomized block design with 12 weeks as

blocks and 2 treatments (PRI and local traps). Removed traps were brought to the laboratory.

Their plastic sheets were removed, placed on grids and examined under a microscope (x13).

Caught insects were identified and counted.



Eighty plants from the plot were selected at random and tagged. On each plant, 10 leaves (5

from new flushes and 5 lower ones from plant base) were examined every week and nymphs

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and adults of T. palmi were recorded. Trap catches and T. palmi numbers were recorded

during 12 weeks. Temperature, rainfall and humidity at the Richelieu CRS were recorded

during the study period.



Statistical Analysis

Counts of T. palmi from traps were expressed as the numbers of thrips per trap per week.

Data were log transformed (log (x)) to meet assumption of homogeneity of variance and then

analysed by General Linear Model (PROC GLM) (SAS Enterprise Guide 3, 2004).



Counts of T. palmi from plants were expressed as the numbers of nymphs and adults per leaf

and then analysed by t-test (SAS Enterprise Guide 3, 2004). The relationship of trap catches

and plant infestation was analysed by the Pearson’s product–moment correlation (SAS

Enterprise Guide 3, 2004).



Results


Evaluation of blue sticky traps

During the 12-week study, 34,666 adult insects (thrips, flies and moths) were caught on the

PRI and local traps. T. palmi constituted the highest proportion (85 %) of trap catches that

also included unidentified flies (10 %) and moths (5 %) that were not of economic

importance. No bees, coccinellids, lace wings or parasitoids and other species of thrips were

recorded.



29,466 T. palmi adults were caught by both traps during the study with highest catches

(19,318) on the local traps. Weekly catches on the local trap were significantly higher (402.5

± 47.5/trap) than those on PRI trap (211.5 ± 34.2/trap) (F = 99.37; df = 1, 11; P < 0.01).


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As from 3 WAT, catches increased significantly and attained peak at 9 WAT (1061/local

trap) and (741/PRI trap) during flowering and fruiting stage of eggplants. Towards the end of

crop cycle, catches on both traps gradually dropped to an average of 224.8/local trap and

48.8/PRI at 15 WAT (Figure 1).



There was a significant interaction between trap types and week (F = 8.54; df = 1, 11; P <

0.01), indicating that differences in catches between the traps varied on a weekly basis.




                               1200
                                                                                               PRI

                               1000                                                            Local
  Mean No. of T. palmi/ trap




                               800


                               600


                               400


                               200


                                 0
                                      4   5   6   7     8     9    10     11    12   13   14   15
                                                  Week after transplantation (WAT)


Figure 1: Mean number of T. palmi adults per blue sticky trap (PRI and local) in eggplant plot

during September to December 2006. Values represent means ± SE


Abundance of T. Palmi on plants

A significantly higher number of nymphs and adults were observed on upper leaves

compared to lower leaves (Table 1).


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Table 1: Mean number of T. palmi on upper and lower eggplant leaves during September to

December 2006

     Mean number of                 Upper leaves   Lower leaves       Test statistics

       T. palmi/leaf

           Adult                     18.1 ± 2.5     10.4 ± 1.4     t = 13.47; P < 0.01

          Nymph                      7.8 ± 1.3       5.2 ± 0.9      t = 7.87; P< 0.01



Nymphs and adults were low in numbers (< 6.0/leaf) on upper and lower leaves during the

first six weeks after transplantation. However, during fruiting stage (9 WAT) their numbers

rose on upper (24.3 nymphs/leaf and 45.9 adults/leaf and) and lower leaves (18.9 nymphs

and 23.9 adults/leaf). Thereafter, a gradual decrease in their numbers was observed. At 11

WAT, the average number of nymphs and adults per leaf dropped to 7.2 and 18.2 on upper

leaves and 8.7 and 4.3 on lower ones. The number of nymphs on upper and lower leaves

varied from 3.1 to 8.1 for the last 4 weeks of study. During that period, the number of both

nymphs and adults T. palmi on upper leaves peaked at 12 WAT (8.7 and 21.4 respectively).

At 15 WAT, the mean number of adults on both upper and lower leaves reached a minimum

level of 10.1 and 9.6 (Figure 2).




                                                                                          7
                                                                                                  Upper leaves- Adult
                                                                                                  Upper leaves- Nymph
                             50                                                                   Lower leaves- Adult
                                                                                                  Lower leaves- Nymph
                             45

                             40                        Fruiting
   Mean No. T. palmi /Leaf




                             35                                                            End of crop cycle

                             30

                             25

                             20           Flowering
                             15

                             10
                             5

                             0
                                  4   5   6      7      8         9   10    11        12     13     14      15
                                                  Weeks after transplantation (WAT)




Fig 2: Catches of T. palmi adults per eggplant leaf from September to December 2006.

Values represent means ± SE


Trap catches and infestation of T. palmi on plants

There was a strong correlation between mean catches on local blue traps and mean number of

adults (R2 = 0.9006) and nymphs (R2 = 0.8552) on upper leaves. Moreover, correlation also

existed between local trap catches and nymphs (R2 = 0.7465) and adults (R2 = 0.7619)

infestation on lower leaves (Figure 3).




                                                                                                                        8
                        Upper Leaves




                        Lower Leaves




Fig. 9: Relation between mean number of T. palmi and its abundance on leaves of
eggplant




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Discussion

The blue sticky trap (PRI and local one) were found to be attractive to T. palmi adult. This

substantiates findings of Kawai (1990), Layland (1994), Murai (2001) and Anon (2005). In

contrast, Caullychurn (2002) did not find that blue colour was effective to capture thrips in

his study when he compared the attractiveness of white, pale green, red and yellow colour.

Blue sticky traps in the eggplant plot showed a high level of specificity with 85% of the

catches constituting T. palmi adults. Insects are attracted to coloured objects and usually

coloured traps are not known to be highly specific to target insects (Dent, 1991). The local

blue sticky trap was found to outperform PRI trap probably because the former has a broader

wavelength (400-600 nm) than the PRI trap (400-550 nm). The difference in wavelength

between the two types of traps (550-600 nm) could partly explain highest captures on local

ones.     The strong correlation (R2 > 0.7) of adult infestation on leaves and on trap catches

indicates that the blue sticky trap can be used to monitor T. palmi population in eggplant

fields.



Both nymphs and adults of T. palmi were predominant on upper leaves compared to lower

leaves. The high incidence of T. palmi on upper leaves can be a result of movement of

nymphs and adults from old leaves to new ones in search for better feeding sites. Cho et al.

(2000) also made similar observation on potato plants.



Infestation of T. palmi in an eggplant crop cycle can be demarcated in 3 distinct phases: (1)

colonization on newly transplanted seedlings by migrating thrips from adjoining fields during

3 WAT to 6 WAT, (2) multiplication of T. palmi during flowering and fruiting of eggplants,

from 7 WAT to 10 WAT and (3) emigration from fields at end of crop cycle from 12 WAT

to 15 WAT..



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Abiotic factors, temperature (22.2 ºC), humidity (63 %) seem to influence T. palmi

population at Richelieu CRS. High temperature and humidity during the study period also

contributed to a subsequent rise in T. palmi population. Indeed, Capinera (2004) had reported

that development of T. palmi is favoured by high temperature. The low incidence of T. palmi

at 10 and 11 WAT coincided with heavy rainfall. Heavy rainfall, as such, has been reported

to wash thrips from leaf surfaces by Young et al. (1998). Moreover, by the end of crop cycle,

plants had fewer new, tender leaves and flowers and probably could not support further

development of thrips. Capinera (2004) reported that thrips development declines in relation

to crop maturity.



The local blue trap was cheaper (Rs 4) than the PRI trap (Rs 16) and can be readily adopted

by growers for monitoring and also mass trapping of T. palmi in eggplant fields. However,

the type of adhesive used in the trap is expensive and alternatives need to be sought.



Conclusion

Non-chemical methods must be promoted for a sustainable agriculture. In line with this

study, the use of blue sticky traps against T. palmi can assist in reducing the excessive use of

insecticides in eggplant fields. Thus, there is a potential in its use for either monitoring or

mass capturing T. palmi in eggplant cultivation and also in other crops susceptible to the pest

in open field and closed shed.



The use of sticky traps will no doubt help growers to reduce their heavy dependency on

insecticides and contribute towards a proper management of T. palmi in eggplant cultivation.

However, investigation should be done in order to find an alternative to stickem adhesive

(Tanglefoot- an expensive product)



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Acknowlegdement

We are grateful to Dr D. Dumur, Director of AREU for giving the opportunity to undertake

this study. We extend our gratitude to Mr J. Chan Sip Siong (Station Manager, Richelieu

Research Station) and members of the Entomology Division for their assistance during

research. Special thanks to Dr A. Manrakhan and Dr A. Ruggoo for their statistical advice.



References

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