ONION SEEDLING PROTECTION AGAINST THRIPS TABACI LINDEMAN AND
ITS COMPARATIVE ECONOMIC BENEFIT OVER PESTICIDE SPRAY
M.M. Waiganjo1, S. Sithanantham2, J.M. Mueke3 and L.M. Gitonga4
1
KARI-Thika, P.O Box, 220, Thika
2
International Centre of Insect Physiology and Ecology (ICIPE). P.O Box 30772, Nairobi
3
Department of Zoology, Kenyatta University. P.O Box 43844, Nairobi
4
Jomo Kenyatta University of Agriculture and Technology, P.O Box 62000, Nairobi
Abstract
The onion thrips, Thrips tabaci Lindeman is a major constraint in onion production worldwide. Two field trials
were conducted to assess the potential of onion seedling management options against onion thrips and their
economic benefits in comparison with routine pesticide application. The design of the trials was randomised
complete block design of 8 treatments replicated 4 times. The treatments were onion seed coating with
Thiamethoxam 350g/l a.i (Cruiser®), seedling drenching with Neem oil Azadirachtin 0.15%w/w (Achook®),
seedling drenching with neem powder Azadirachtin, 0.03% w/w (Neemros®), seedling drenching with
Imidacloprid 200g/l a.i (Confidor®), overnight seedling dipping in Confidor® and soil placement with
Carbofuran 5% a.i (Furadan®) during transplanting. These single dose treatments were compared with
Profenofos 400g/l a.i plus Cypermethrin 40g/l a.i (Polytrin®) fortnightly foliar spray and unprotected control.
All the preventive treatments reduced thrips infestation significantly (P=0.05) during the first and second
month after transplanting in both trials, except Neemros® seedling drenching which was not effective during
the second month in the second trial. Confidor® seedling drenching and Furadan® soil placement had the
longest residual effect and significantly increased onion bulb yield and quality. There was significant negative
relationship between onion yield and thrips infestation, which was best described by a third order polynomial
regression model. Confidor® seedling drenching gave the highest economic benefits among the treatments and
resulted in comparable yield increase to routine fortnightly spray with Polytrin®. The results suggest that onion
thrips management may be optimised through systemic pesticide seedling drenching application.
Introduction
The bulb onion Allium cepa L. is an important source of income for smallholder farmers and businessmen in
Kenya. Despite there being favourable conditions for onion production, the demand exceeds the supply thus
necessitating importation. Onion yields in Kenya range between 5 and 20 t ha-1, compared to over 30 tonnes ha-
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in countries such as Spain, Japan and China (FAO, 1999). The low yields are attributed to low yielding
cultivars, poor agronomic techniques, diseases and insect pests such as onion thrips, Thrips tabaci Lindeman.
Yield losses ranging from 18 to 60% have been reported in Kenya due to thrips damage (Waiganjo, 2004).
Onion protection against thrips infestation is therefore essential for economic production of the crop. Control of
onion thrips in Kenya and other sub-Saharan countries is mostly by pesticide sprays. Although pesticides
provide the most potent means of suppressing pests, their use should be minimised due to their numerous
hazards. To protect the resource- poor farmers from pesticide contamination and high production costs, there is
need to develop low-cost technologies that will reduce pesticide exposure and the high cost of onion
production. The objective of this study therefore was to determine the potential of single dose preventive
pesticides in controlling onion thrips and to assess their economic benefits with the aim of identifying
promising options for incorporation in integrated pest management (IPM).
Materials and methods
Two field trials were conducted at the Kenya Agricultural Research Institute (KARI) Mwea farm in Kirinyaga
District. The onion seed variety Red creole was sown on 15th February and 26th May, 2001 for the first and
second trial respectively. The trial plots of 3x3m each were arranged in randomised complete block design of 8
treatments, replicated 4 times. The 8 treatments tested were seed coating with Thiamethoxam 350g/l a.i
(Cruiser®) during planting, seedling drenching with Imidacloprid, 200g/l a.i. (Confidor®), overnight seedling
dipping in Confidor® solution, seedling drenching with neem extracts consisting of Azadirachtin, 0.15%w/w
(Achook®) and Azadirachtin, 0.03% w/w (Neemros®), soil treatment with Carbofuran 5% a.i (Furadan®)
during transplanting and an unprotected control. The preventive treatments were compared with Profenofos
400g/l a.i plus Cypermethrin 40g/l a.i mixture (Polytrin®) sprayed fortnightly as the standard. Recommended
rates on the manufacturer’s label were used for application of all the pesticides. Absolute thrips density
estimation was carried out by whole plant cutting and bagging (Freuler and Fischer, 1984), starting from 14
days after transplanting and at weekly intervals thereafter, until the onion crop matured. Thrips extraction from
the cut onion plants was done in the laboratory, using 70% alcohol and counting under a dissecting
microscope. The onion crop was harvested when more than 50 % of the leaf bundles had lodged. Bulb yield
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and quality was assessed from 2 inner rows per plot. Onions were sorted in 3 grades based on the bulb diameter
(grade 1=>5 cm, grade 2=≤ 5 >3cm and grade 3=≤3 cm) (Nguthi et al., 1994)
Data were analysed using SAS (2000) software. Analysis of variance (ANOVA) procedure was performed to
compare the thrips infestation and onion bulb yield and quality among the eight treatments and means
separated using Student-Newman-Keuls (SNK) Multiple Range test. Data on thrips numbers were log
transformed (log10 (n+1) during analysis, to harmonise the variance observed in insect counts. Regression
analysis was performed for relating onion bulb yield and thrips pressure, expressed as the cumulative thrips-
days per plant (Fournier et al., 1995). Economic analysis was carried out using a method described by
Zawedde et al., (2001). In this method, yield gains due to the protection regimes were calculated as the
difference between yield from each treatment and the unprotected control and marginal returns as the income
of yield gain divided by the cost for the control operations.
Results
Effect of treatments on thrips infestation during the onion crop development
Thrips infestation differed among the treatments during the first month after transplanting (MAT) in the first
(F=51.54; d.f=7, 950; P5 cm, Grade 2=≤ 5 >3cm and Grade 3=≤3 cm, bulb diameter.
Within a column, means marked with the same letter were not significantly different (P=0.05, SNK test).
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70
y = 2E-08x 3 - 9E-05x 2 + 0.1098x + 17.335
60 R2 = 0.7721
50
Y ld (to s/h .)
n a
40
30
ie
20
10
0
0 500 1000 1500 2000 2500 3000
A. Cumulative thrips-days/plant
70
60
50
y = 1E-08x 3 - 5E-05x 2 + 0.0431x + 57.216
Y ld( n /h .)
R2 = 0.976
ie to s a
40
30
20
10
0
0 500 1000 1500 2000 2500 3000
B. e
Cumulativ thrips-days/plant
Fig. 1–The relationship between thrips infestation pressure in cumulative thrips–days/ plant and onion bulb
yield (tons/ha.) during the first (A) and second (B) field trials at Mwea-Tebere, Kenya
Table 3–Economic analysis of thrips protection treatments in onion grown in 2 field trials during year 2001 at
Mwea-Tebere, Kenya
Protection regime *Protection Bulb Yield (t ha-1) **Yield value (US$).) Total yield Marginal
cost/Ha. gain (US$.) returns
Grade1&2 Grade3 Grade1&2 Grade3
Control - 26.95 4.68 5649.39 549.46 -
Cruiser® 6.58 33.10 2.19 6938.20 257.03 996.28 151.43
Achook® 7.57 28.85 3.11 6046.85 365.87 213.77 28.25
Neemros® 7.57 29.93 2.26 6273.49 265.11 339.65 44.89
Confidor® drench 11.18 36.89 2.65 7732.60 311.52 1845.18 164.98
Confidor® dip 11.18 30.21 3.50 6332.18 410.81 544.04 48.64
Furadan® 63.16 38.89 1.98 8151.29 232.50 2184.85 34.59
Polytrin® 281.26 42.36 2.75 8879.40 322.98 3003.44 10.68
®Treatment trade name
*Insecticide price was provided at the retail agrochemical shops, Knapsack sprayer cost @US$ 111.84 and
Protective clothing @ US$ 41.17 payable in 3 years.
**Farm gate price of onions estimated at US$ 0.21/Kg for grade1&2 and 0.12/Kg for grade 3. 1(1US$=76KES .
Discussion
In the present study Confidor®, seedling drench and Furadan® soil placement showed the longest preventive
residual effect against thrips infestation and significantly increased the onion bulb yield and quality. Confidor®
seedling drenching also gave the highest marginal returns among the treatments. However, overnight seedling
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dipping with the same product (Confidor®) was not equally effective. The ineffectiveness of the same product
(Confidor®) was due to the mode of application. Apparently, the short overnight dipping did not allow
adequate amounts of the active ingredient to be assimilated in the plant system. Similar differences in pesticide
effectiveness due to the mode of application were reported by Allan (1964). In the present study, Furadan®
exerted over 2 months residual effect against onion thrips. Similar findings of long residual effect (56 days
after transplanting) of Carbofuran granules with no residues detected on the onion produce were reported in
eastern Washington, USA (Getzin, 1973). Significant negative relationship between onion bulb yield and thrips
infestation was observed in both trials, confirming that increase in thrips pressure impacted negatively on the
onion bulb yield. This is in agreement with earlier reports that, onion bulb yield and bulb quality correlate
negatively with increased thrips numbers (Zaman, 1989). All the preventive treatments did not control thrips
infestation during the late onion growth stage (fourth MAT). However, delay in thrips population build-up
during the early growth period apparently contributed to significant yield increase and high quality bulbs. The
results of the present study are in agreement with Sato and Nakano (1990) and Waiganjo (2004), who reported
that thrips infestation during the first and second MAT was most critical for onion protection while the crop
was insensitive to thrips infestation in the fourth MAT. The study also showed preventive seedling treatments
were more cost effective than routine foliar spray. Similar findings on cost effectiveness were reported in
cotton using Gaucho® (Imidacloprid) seed dressing against early season cotton pests (Graham et al., 1995).
Conclusion and recommendations
Seedling protection using Confidor® seedling drenching and Furadan® soil placement had the longest residual
effect against onion thrips and resulted in comparable increase in onion bulb yield and quality to routine foliar
spray with Polytrin®. The preventive options also resulted in the highest marginal returns due to the yield
increase and low protection cost associated with single application. Since the preventive pesticides are
available in the market and less cumbersome to use than routine foliar spraying, they would be more beneficial
for the onion farmers. The single dose application would benefit resource-poor farmers especially women who
are more vulnerable to pesticide contamination.
Further studies using other horticultural crops to assess the effects of the preventive treatments on the major
pests and their natural enemies are recommended and on-farm validation using the farmer field school
approach.
Acknowledgement
The authors acknowledge the financial support of the International Centre of Insect Physiology and Ecology
(ICIPE), under the African Regional Programme in Insect Science (ARPPIS). We also thank the Director,
KARI for facilitation and the Centre Director, KARI Thika, Dr. C. N. Waturu for support and encouragement.
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