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African Crop Science Journal
African Crop Science Society
ISSN: 1021-9730 EISSN: 2072-6589
Vol. 11, Num. 1, 2003, pp. 35-42

African Crop Science Journal, Vol. 11. No. 1, 2003,  pp. 35-42



Department of Horticulture, Egerton University, P. O. Box 536, Njoro, Kenya

Received 23 August, 2001;
Accepted 17 December, 2002

Code Number: cs03005


Collard (Brassica oleracea var. acephala D.C) is an important source of income to many smallscale farmers in Kenya and also is a major dietary component for many Kenyans.  The production of collards is, however, constrained by several pests, with diamondback moth (Plutella xylostella) larvae being the major pest.  Control of the diamondback moth larvae has mainly been by chemical sprays, which has resulted in the problem of pest  resistance and pollution to the environment.  This study was, therefore, an attempt to look into alternative environmentally friendly ways of controlling diamondback moth larvae in collards.  Two studies were conducted in Egerton University, Tatton Farm, Njoro, Kenya to determine the effects of intercropping and nitrogen application on the diamondback moth larvae population and damage,  and yield of collard.  Intercropping collard with either beans (Phaseolus vulgaris L.) or onions (Allium cepa)  significantly lowered moth larvae populations and damage on collard leaves compared to unsprayed  collard monocrop.  Overall, the lowest larvae population and damage was recorded in insecticide sprayed collard monocrop.  Nitrogen application increased diamondback moth damage on collard, but did not affect larvae populations.  Intercropping increased crop output per unit area of land, measured by land equivalent ratios.  Considering the harmful effects associated with synthetic insecticides, these results indicate that intercropping collard with either beans or onions can be effectively used to suppress diamondback moth larvae population and damage in collards.

Key Words:  Cropping systems, IPM, LER, Plutella xylostella


Le collier (Brassia oleracea var. acephala D.C) est une importante source de revenue pour les nombreux petits fermiers au Kenya et aussi un majeur composant alimentaire pour beaucoup des Kenyans. La production du collier est cependant contrainte par plusieurs insectes, avec les papillons nocturnes Plutella xylostella comme l'insecte majeur. Le contrôle des larves de Plutella xylostella  a été principalement par pulvérisation des particles chimiques laquelle a engendré le problème de résistance des insectes et la pollution de l'environnement. Ainsi donc, cette étude était une tentative de voir des voies alternatives, environnementalement favorables, de contrôle de population et dégâts des larves de Plutella xylostella dans les colliers. Deux études étaient conduites à l'Université d'Egerton, et dans les fermes de Talton et de Njoro au Kenya pour déterminer les effets d'intercultures et l'application de l'azote sur la population et les dégâts des larves de plutella xylostella et le rendement de colliers. L'interculture de collier avec d'autres haricot (Phaseolus vulgaris L.) ou les oignons (Allium cepa) a signficativement baissé les populations et les dégâts des larves de Plutella xylostella sur les feuilles de colliesr comparé à la monoculture de collier de collier non pulverisé. Globalement, la faible population et dégâts était enregistrés dans la pulvérisation d'insecticide en monoculture de collier. L'application d'azote a augmenté les dégâts de Plutella xylostella sur les colliers, mais n'avait pas affecté les populations des larves. L'interculture a augmenté la production des plantes par unité de surface de terre, mesurée par rapports à l'équivalent de terre. Considérant les effets nuisibles associés avec les insecticides synthétiques, ces résultats indiquent que l'interculture de collier avec les autres haricots ou oignons peut être effectivement utilisée pour supprimer la population et dégâts des larves de Plutella xylostella dans les colliers.

Mots Clés: Systèmes de cultures, IPM, LER, Plutella xylostella


Diamondback moth is a major pest of Brassica species in many parts of the world (Messian, 1992), including eastern and southern Africa (Nyambo and Pekker, 1995; Mwaniki et al., 1998; Oduor et al., 1998). Diamondback moth control is largely by synthetic pesticides, but the efficacy of pesticides (Plutella xylostella) has declined (Mwaniki et al., 1998).  Rates of application have increased leading to increased problems of pesticide resistance, residues in the harvested product, toxicity to farmers, and loss of beneficial insects.  Pest management alternatives, which are appropriate for use by smallscale producers, are needed.

Low diamondback moth populations have been reported in intercropped cabbage fields   (Trevor, 1990; Madriaga, 1994).  In Kenya, collards are mostly intercropped with beans (Phaseolus vulgaris L.), maize (Zea mays L.), potatoes (Solanum tuberosum), and tomato (Lycopersicon esculentum), but the effects of these intercrop systems on insect pests populations have not been established. Farmers have, therefore, continued to apply routine sprays of insecticides in an effort to lower pest attack and damage to collard crops.

Collards and other Brassica are responsive to high nitrogen (M.O.A., 1979), but the nitrogen rate effects on pest populations are not well understood.  This study was conducted to determine the effects of intercropping collards with beans or onion, and nitrogen application on diamondback moth larvae population and damage on collards, and collard yields.


Two studies were conducted at Egerton University Horticultural demonstration field, Tatton farm, in Kenya at approximately 0°23' south 35°35' east, and 2200m above sea level.  The soils are well drained silty clay loams, classified as Haplic Phaeozen. It has organic matter content of 2.17% in the upper 0 - 30 cm, pH (H20) 6.15 and 0.23% total nitrogen.

The experiment was conducted for two rainy seasons (May - September, 1999 and September 1999 to February, 2000). The amount of rainfall received was 241 mm and 196 mm during the first and second rainy seasons, respectively, with its distribution as shown in Table 1.  The temperatures ranged from 15 to 22° during the first season, and 16°C to 24°C during the second season, with the monthly mean temperatures as shown in Table 1.

Treatments included N application at  0, 145, 289 and 361 kg N ha-1; cropping regimes; collard-bean intercrop (KB), collard-onion intercrop (KO), monocrop collard sprayed with insecticide 'Bulldock' (KD), monocrop collard unsprayed (K), monocrop beans (B) and monocrop onions (O).  The experimental design was a split plot, arranged in a randomised complete block, with three replications.  Nitrogen levels comprised the main plots and cropping regimes were the sub-plots.  Main plots measured 9 m x 12.8 m.  Sub-plots measured 4 m x 3.6 m.  Sub-plots were separated by a one meter path and main plots in a block were separated by two meters path.  Harvested areas were of 3.6 m x 2.7 m plot leaving the outer plants as guard rows.

One  Collard or kale cultivar, one bean cultivar 'GLP2' and one onion cultivar 'Red Creole' were used in the study.  Beans were sown at a spacing of 45 cm x 10 cm, onions transplanted at a spacing of 30 cm x 10 cm, while collard seedlings were transplanted at a spacing of 90 cm x 30 cm.  Planting of beans and transplanting of onions were done two weeks prior to transplanting the  collard.  Monocrop plots of each crop were maintained as controls and for purposes of computing land equivalent ratios.  Monocropped and intecropped spacings were the same.  Nitrogen was broadcasted in half-rate splits, with the first split applied two weeks after collard transplanting and the second split three weeks later.  Calcium ammonium nitrate (26% N) was the nitrogen source.  All other agronomic practices were applied uniformly to all the plots.  No pesticide was applied except in the control plots (KD), where 'Bulldock' (a.i. - Beta-cyfluthrin, 025 EC) supplied by Bayer Ltd. was sprayed biweekly until termination of each experiment.  

Diamondback moth count and damage assessment on collard plants were done  weekly, beginning the first week after the second split application of nitrogen.  Diamondback moth larvae were physically counted from three randomly selected plants in each sub-plot for each data collection date.  At each harvest, ten leaves were randomly picked from each plot harvest for damage assessment.  Damage caused by diamondback moth larvae on the leaves was rated on a scale of 1 to 5 where 1 = no damage; 2 = <10% of leaf damage; 3 = 10 - 50% of leaf damage, 4 - 50 - 75% of leaf damage; 5 = >75% of leaf damage.

Weekly collard leaf picking began six weeks post-transplanting and continued until termination of the study.  The parameters assessed were fresh marketable leaf yields both by leaf numbers and fresh leaf weight.  The marketable fresh leaf yields were later expressed as a percentage of the total fresh leaf yields.

Bean grain yield and onions were harvested at physiological maturity.  Onion yields were determined on the basis of marketable bulbs, and bean yields on the basis of seed dry weight, adjusted to 12% moisture.  Each season's data were subjected to analysis of variance and  significant treatment means were separated  using the least significant difference method.  Data on diamondback moth counts were subjected to square root transformation prior to analysis of variance.  Data on percentage marketability of collard leaf numbers and fresh leaf weights were subjected to arc sine transformation prior to analysis of variance.  Values presented are, however, the original means.  Land Equivalent Ratios (LERs) for the different cropping regimes were computed using the formula:

LER     =   Y intercrop 1   + Y intercrop 2
                  Y monocrop 1 + Y monocrop 2

Where Y is the yield per unit area of either crop 1 or crop 2 in either monoculture or interculture.  The highest yields of insecticide treated collard, beans and onions in monocrop for each season were used as denominators for computing the LERs.


Diamondback moth larvae populations on collard plants. Nitrogen levels had no significant (P<0.05) effect on diamondback moth larvae populations in both seasons.  Intercropping suppressed larvae populations when compared to the unsprayed monocropped collard treatment.  In both seasons, the lowest larvae populations were recorded in collard sprayed with Bulldock (Table 2).  There was a significant (P<0.05) interaction between nitrogen fertility levels and cropping regime in influencing diamondback moth larvae population per collard plant in season 1, but the interaction was not significant during the second season (Table 2).  In season 1, diamondback moth larvae populations were lowest in monocrop collard sprayed with Bulldock and highest in unsprayed monocropped collard at all nitrogen fertility levels. Among intercrop treatments, diamondback moth larvae populations were  significantly lower in collard intercropped with onions at nitrogen levels 145 and 289 kg N ha-1, but the populations in these treatments were not any different at nitrogen  levels  0 and 361  kg N ha-1.

Collard leaf damage by diamondback moth larvae. The damage caused by diamondback moth on collard leaves was more with nitrogen fertilisation during the second season.  In season 1, leaf damage also tended to be higher in collard plants that received supplemental nitrogen (Fig. 1).

Intercropping reduced collard leaf damage compared to the unsprayed monocropped collard treatment (Table 2).  The least collard leaf damage was recorded in monocropped collard sprayed with Bulldock in both seasons.  There was a significant interaction between nitrogen level and cropping regime on collard leaf damage in season 2 of the study due to relatively more leaf damage at higher nitrogen rates in collard intercropped with onion.  The highest collard leaf damage was recorded in unsprayed monocrop collard.  Less leaf damage was observed in collard leaves from collard-onion intercrop compared to leaves from collard-bean intercrop.

Collard yields. Intercropping collard with beans or onion significantly (P<0.05) reduced collard marketable leaf numbers in season 1 (Table 2), but not in season 2.  Nitrogen level had no significant (P<0.05) effect on marketable collard leaf numbers.

Intercropping with either onions or beans significantly reduced collard fresh leaf weight (Table 2), except in season 2, when intercropping collard with onions did not reduce yield.  Nitrogen level had no effect on the marketable fresh leaf weight of collard.

In season 1, spraying with 'Bulldock' produced the highest percentage of both marketable leaf numbers and fresh leaf weight (Table 2).  In season 2, both spraying and intercropping had no effect on the percentage of marketable collard leaves.  In both seasons, nitrogen level had no effect on the percentage of marketable collard leaves and the percentage of marketable collard fresh leaf weight.

Bean yields. Bean seed yield response to intercropping varied with seasons (Table 3).  Bean seed yield was more with intercropping during the first season, but less in the second season.  Monocropped beans suffered more severe attack by charcoal rot a fungal disease caused by Microphomina phasiolina than intercropped bean in season 1.  Nitrogen level had no significant effect on bean yield.

Onion yields. Onion bulb yields were significantly less with intercropping in season 2 (Table 3), probably due to competition for moisture, which was more limiting in season 2 than 1.  Season 1 received 241 mm of rain while season 2 received 196 mm (Table 1).  Nitrogen level did not affect onion bulb yields.

Land equivalent ratios.  Intercropping collard with beans or onions was beneficial in increasing food output per unit land area (Table 4), but the benefits were less consistent with onion.  The LERs for intercropping collard with beans ranged from 1.5 to 1.89 in season 1 and from 1.07 to 1.44 in season 2.  The LERs for intercropping collard with onions were 1.02 to 1.45 in season 1 and 0.85 to 1.08 in season 2.  Intercropping collard with beans was more beneficial than with onions.  The LERs were higher in season 1 than in season 2, probably due to the better rainfall received during the first season.


Control of the diamondback moth larvae population in collard was better by intercropping with onions than with beans.  Similar reduction in diamondback moth larvae population due to intercropping was reported by Trevor (1991) on head cabbage (Brassica oleracea var. capitata) intercropped with tomato (Lycopersicon esculentum) and by Madriaga (1994) on head cabbage intercropped with sweet pepper (Capsicum annum), peanut (Arachis hypogea), and cowpea (Vigna unguiculata).

According to William (1981), weeds or rotational plants may produce oduors or organic compounds that modify pest behaviour.  The ability of an onion intercrop to suppress diamondback moth more than beans is attributed to the strong aroma that is due to allyl-propyl-disulfide, a volatile sulfur compound that is characteristic of the onion family that may have interfered with the host finding ability and fecundity of the diamondback moth.  The less damage observed in intercropped collard compared to unsprayed monocropped collard is attributed to the lower diamondback moth larvae populations observed in the intercropped plots.  Correlation analysis showed a significant (P<0.05) positive correlation (r = 0.44 and 0.24 for season 1 and 2, respectively), between diamondback moth larvae populations and collard leaf damage rating.  Similarly, Qfuya (1991) reported reduced insect damage in cowpea intercropped with tomato.

The lack of significant influence by nitrogen fertility on diamondback moth larvae populations in this study is in agreement with the observations made on head cabbage by McHugh and Foster (1995).  The higher collard leaf damage associated with high nitrogen levels may be attributed to increased succulence in these plants that may have increased their susceptibility and attractiveness to feeding by the diamondback moth larvae.  Similarly, Ampong-Nyarko and Nyang'or (1991) reported increased insect damage in sorghum with increased nitrogen rates.

The lack of significant response of collard yields to nitrogen application is contrary to observations made on collard (Itulya, 1995) and on cabbage by McHugh and Foster (1995).  The limiting moisture conditions experienced during the period of the current study may have resulted in reduced nitrogen uptake and plant growth, and thus, plants were unable to utilise the applied nitrogen to the maximum.

The differential response of collard yields to intercropping with onions or beans observed in the individual seasons of this study is attributed to the differences in rainfall distribution pattern experienced in the two seasons and its subsequent effects on the growth of the plants.

Most of the rainfall was received later in the season during season 1, while in season 2, most of the rainfall was received early during the season.  In season 2, beans had more vegetative growth with a large canopy early in the season, resulting in more shading of collard plants and reduced the vegetative growth of collard plants.  In season 1, rainfall was low early in the season, and therefore beans were unable to form a large canopy, resulting in less shading on collard plants.

In the collard-onion intercrop plots, the growth of collard and onion plants was reduced early during the first season due to water deficits.  When rainfall increased in the middle of the season, onion plants recovered and grew more vegetatively, imposing more competition for moisture.  On the other hand, during the second season, moisture was abundant early in the season.  Collard plants took advantage of the available moisture and grew fast producing more leaves that boosted the leaf numbers and fresh leaf weights recorded for this treatment compared to bean intercrop.

In addition to suppressing diamondback moth larvae populations and damage in collard, the results of this study show that intercropping collard with beans or onions is beneficial in increasing food output per unit land area.  In both seasons, higher benefits were realised from intercropping collard with beans that collard with onions at most nitrogen levels.  Generally, higher benefits were realised in season 1 than in 2, which is attributable to the higher rainfall amount received during this season.  Itulya (1995) and Itulya and Aguyoh (1998) have reported similar results.  In both studies, intercropping collard with beans increased food output per unit land area with higher benefits recorded in seasons that received more rainfall than in those that received less rainfall.

In conclusion, intercropping collard with beans or onions is beneficial in suppressing both diamondback moth larvae populations and damage in collard, and that it results in increased food output per unit area of land.  Diamondback moth damage, but not larvae numbers, may be increased with nitrogen application.


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©2003, African Crop Science Society

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