Effect of Intercropping Pattern on Stem Borer Infestation in Pearl Millet ( Pennisetum glaucum L . ) Grown in the Nigerian Sudan Savannah

Fields experiments were conducted at the Department of Crop Protection, Faculty of Agriculture, University of Maiduguri Teaching and Research Farm, Maiduguri during 2010 and 2011 cropping seasons to investigate the effects of intercropping pattern on stem borer pest infestation in pearl millet (Pennisetum glaucum L.) intercropped with ground nut (Arachis hypogea L.). A split-plot design was used to test the intercrop pattern of 1:0 ratio (sole millet), 1:1 ratio (1 millet row to 1 ground nut row), 2:1 ratio (2 millet rows to 1 ground nut row) and 1:2 ratio (1 millet row to 2 ground nut rows). The results obtained showed that the intercrop pattern of 1:2 ratio and 1:1 ratio yielded less stem borer infestation and abundance in pearl millet, and as well supported high panicle weight and grain yield. In addition to recommending either of these two intercropping patterns to pearl millet farmers for more effective stem borer pest management, results further show the need for identification of effective intercropping patterns in other cropping systems.


Introduction
Studies carried out in both tropical and temperate zones reported lower crop pest densities in more diversified agricultural ecosystems [1]- [3].In Africa, such crop diversification practices include mainly pest diversion or trap cropping and inter-or mixed cropping [4].Intercropping, an important cultural practice in crop pest management, primarily involves increasing the plant diversity of a given agro-ecosystem to aid reducing insect pest populations, and consequently, their attack [5] [6].The practice seeks to alter the environment to support favourable crop growth and yield, rather than encourage insect population increase [7] [8].Some plant combinations, for instance, with non-hosts lower the spread of pests within crops [9] [10].Non-host plants in such mixtures may emit chemicals or odours that adversely affect the pests, thereby conferring some level of protection to the host plant [6] [11].Studies indicate that crop diversification through intercropping, such as cereals with legumes, is effective in reducing insect pest damage [12]- [14].Even plant diseases are believed to be less in intercropped agroecosystems due to increased crop diversity than those in sole crops [15] [16].Also, the intercropping of groundnut with pearl millet (Pennisetum glaucum L.) has particularly been found to increase the population of Goniozus sp., a parasitoid species that effectively manages leaf miner pest populations in ground nut (Arachis hypogaea L.) [17].
Yield reduction due to stem borers occur as a result of leaf feeding, stem tunnelling, direct damage to cereal grain [18]- [20].Depending on the season and nutritional status of plant, crop yield reduction by stem borer feeding and tunnelling activities in Africa can fall between 10% -100% [21]- [23].Three to eight times more stems tunnelling and one to three times more cob damage were recorded in monocropped maize with high stem borer larval densities (21% -48%) and yield loss (1.8 -3.0 times greater) than in the intercropped counterparts [23].In contrast however, work by [23] in West Africa, found a considerably reduced amount of noctuid eggs laid by Sesamia calamistis Hampson and Busseola fusca Fuller due to reduced host found by the ovipositing adult moths in maize intercropped with grain legumes or cassava than those in the monocrop.
Excessive tillering of pearl millet gives the plant a unique structure for exploitation to manipulate crop ecosystems with different possible intercrops for reduction of insect pests' incidence [24] [25].Intercropping patterns for example paired row, paired-wider row or skip-row should preferably allow for minimal competition amongst component crops [26].Although paired row sowing of pearl millet is reported not to alter the yield of component crops [7] [27], it is essential to identify the effect of different intercropping patterns on insect infestation of this crop.Adoption of effective intercrop practices for natural regulation of insect pests including stem borers remains crucial [28] [29], especially by resource-poor farmers that lack the capacity of input-intensive plant protection measures.Groundnut is a short-duration legume crop grown by farmers in the savannah regions of Africa, and is readily intercropped with other medium duration crops such as pearl millet [30].

Seed Material Collection
The pearl millet (variety: SOSAT 888) and ground nut (variety: Ex-Dakar) seeds used in this study were obtained from the Lake Chad Research Institute, Maiduguri.

Location and Experimental Procedures
Field experiments were conducted during 2010 and 2011 cropping seasons at the Department of Crop Protection, Faculty of Agriculture, University of Maiduguri Teaching and Research Farm, Maiduguri (11˚51'N and 13˚15'E) situated in the Semi-Arid region of Nigeria.The experiments were conducted in a split-plot design whereby groundnut was introduced as component crop to pearl millet in the ratios of 1:0 (sole pearl millet); 1:1 (1 row of millet to 1 row of groundnut), 2:1 (2 rows of pearl millet to 1 row of groundnut) and 1:2 (1 row of pearl millet to 2 rows of groundnut).The experimental field was divided into 3 blocks each containing 16 subplots replicated 3 times to give 48 subplots.Each subplot size was 4.0 m by 3.0 m (12 m 2 ) with 1.0 m space between blocks and 0.5 m space between subplots.The experimental fields were ploughed and leveled before sowing pearl millet and groundnut seeds using a hand hoe.The seeds were sown in June at the recommended spacing of 30 cm × 15 cm for pearl millet, while ground nut seeds were sown at the spacing of 20 cm × 10 cm.The reason for the selection of intercrops is to quantify the impact of intercrops on stem borer infestation.Ground nut is short-duration legume grown by farmers in the area while pearl millet is a medium duration crop grown by farmers.The experimental field was weeded using hand hoe whenever weeds appeared.

Data Collection
Twelve millet plants attacked by stem borers per plot were counted and recorded.The numbers were then used to calculate the percentage (%) plant infestation.The abundance of stem borers was obtained from 10 randomly dissected plants found infested at harvest in each sub-plot.The numbers of larvae found inside the nodes, internodes and peduncles of each plant were counted and recorded.
The pearl millet panicles (heads) from randomly selected plots were weighed and recorded immediately after harvest using Jenway top loading balanced (2000 model) scale with a maximum capacity of 25 kg.
Following harvest, grain yield was obtained from threshed panicles and winnowed grains placed in well labelled polythene bags.The weight of grains obtained from each subplot was then taken and recorded separately.
Data collected on the number of pearl millet plants infested and percentage plant infestation, abundance of stem borers, panicle weight and grain yield were subjected to the analysis of variance (ANOVA).Means of the intercropping patterns (treatments) were separated using the least significant difference (LSD) at 5% level of probability.

Effects of Intercropping Pattern on the Number and Proportion of Infested Pearl Millet Plants
Pearl millet plants found infested and percentage plant infestation per plot in this study are presented in Table 1.
Results showed that plots with the intercropping pattern of 1:2 ratio and 1:1 ratio had significantly lower (6.58 and 9.60 respectively) number of infested plants than in plots with the intercropping pattern of 2:1 ratio (11.41 and 10.17 respectively) and the sole crop (1:0 ratio).The results of percentage plant infestation followed the same trend.The lower number or proportion of infested plants found under 1:2 ratio and 1:1 ratio intercropping patterns than that of the sole crop (1:0 ratio) indicates that two-thirds of the intercropping patterns tested support less stem borer pest incidence in pearl millet [31]- [33].Thus, concurring with the results of [34], who likewise found the incidence of stem borer pests in the humid forest areas of Cameroon to be lower in maize intercropped with cassava, cowpea and soybean to be lower than in monocropped maize.Less stem borer pest incidence, in turn, should allow for low or negligible crop damage levels [12] [34].

Effects of Intercropping Pattern on Stem Borer Abundance in Pearl Millet
The results of stem borer abundance per plant (found within the nodes, internodes and peduncles) are presented in Table 2. Stem borer abundance in pearl millet intercropped with ground nuts at 1:2 ratio (3.83) and 1:1 ratio (5.75) was significantly lower compared to that obtained from the intercropped pattern of 2:1 ratio (6.08) and the sole crop (1:0 ratio) (9.17).Intercropping has been reported to reduce insect pest populations due to colonization deterrence in crop fields with increased plant diversity that creates unsuitable habitat or unfavourable environment to some pest specis [35]- [38].Oviposition by adult female stem borers may thus have been seriously reduced as a result of host finding difficulties experienced in pearl millet plots intercropped with ground nut using the above two effective intercropping patterns [34] [39], and thereby limiting their population build up.

Effects of Intercropping Pattern on Panicle Weight and Grain Yield of Pearl Millet
Results in Table 3 show the effect of intercropping on panicle weight and grain yield of pearl millet intercropped using different patterns.For pearl millet intercropped with ground nut using the pattern of 1:1 ratio and 1:2 ratio panicles weight (1049.20 kg and 1249.33 kg) and grain yield (975.62 kg and 1209.33 kg/ha) were significantly higher than from crops intercropped using the pattern of 2:1 ratio as well as the sole crop (1:0 ratio).Altogether, by giving lower stem borer infestation rates and abundance as well as higher panicle weight and grain yield, the intercrop pattern of one row of pearl millet to one (1:1 ratio) or two (1:2 ratio) rows of ground nut tested in this study proved to contribute greatly in managing the attack and destruction or losses caused by these pests in pearl millet.Stable cereal-legume intercrop systems with adequate or effective feed-back mechanism or synchronization of insect pest populations can maintain good crop yields in spite of pest attacks [36] [40] [41].As with [17] who found increased numbers of Goniozus sp.against leaf miner pest populations in ground nut intercropped with pearl millet, the presence and attack (parasitism and predation) of stem borer pests by natural enemies could also be increased in pearl millet under intercrop conditions [13] [32] [38] [41].Up to 30% crop pest reduction due to "natural enemy effect" have been observed by [4] in intercropped systems.In addition to natural enemy effects, the groundnut component crop as non-host plants could have further constituted a physical barrier to stem borer pests in pearl millet that possibly inhibits their inter or intra row migration in intercropped systems [1] [34] [42] [43].

Conclusion
Intercropping, particularly with one row of pearl millet to one row of ground nut and one row of pearl millet to two rows of ground nut seems to encourage less stem borer infestation and abundance in pearl millet, whilst additionally support high panicle weight and grain yield.As such, the cultural practice is greatly encouraged over monocropping for stem borer pest management in pearl millet grown in the Nigerian Sudan Savannah.Being an uncomplicated method of control and not capital-intensive, the practice should be readily adopted especially by resource-poor-farmers.

Table 1 .
Percentage of infested pearl millet plants per plot.

Table 2 .
Stem borers per plant in pearl millet.

Table 3 .
Panicle weight and grain yield of pearl millet.