Change in acceptability of barley plants to aphids after exposure to allelochemicals from couch-grass (Elytrigia repens)

The response of the bird cherry-oat aphid, Rhopalosiphum padi, to barley plants was investigated following exposure of the plants to root allelochemicals from the aggressive weed couch-grass, Elytrigia (Agropyron) repens. Plants were treated either with root exudates from living couch-grass plants or with previously identified couch-grass root compounds [5-hydroxyindole-3-acetic acid, DL-5-hydroxytryptophan, L-5-hydroxytryptophan hydrate, and 6-hydroxy-1,2,3,4-tetrahydro--carboline-3-carboxylic acid (carboline)] either separately or in mixtures. In choice and no-choice settling tests, aphid acceptance of barley plants was significantly reduced following treatment with root exudates, and the carboline when tested alone or in combination with the other compounds. In contrast, the other compounds without the carboline were less active in reducing aphid acceptance. In a probing bioassay, individual substances were either neutral or stimulatory to aphids, indicating that the reduced settling was probably not due to direct effects on aphids, but rather due to effects on the plant. This was confirmed in olfactometer assays, in which aphids were repelled by odors from barley plants following treatment with a mixture containing all four chemicals.     

Profitability of agro-forestry based soil fertility management technologies: the case of small holder food production in Western Kenya

Persistent food insecurity accompanied by low and declining farm household incomes are a common feature of many small holder maize and bean producers in western Kenya. This has been largely attributed to soil nutrient depletion, among other factors. One way of addressing soil fertility problems in many maize-based cropping systems is the use of agro-forestry based technologies. We carried out a survey in western Kenya (Vihiga and Siaya districts) aimed at analyzing the financial and social profitability of use of agroforestry based (improved tree fallows) and other soil fertility management technologies among smallholder farmers. The Policy Analysis Matrix (PAM) was used to determine the financial and social profitability of different production systems, which were categorized on the basis of the technology used to address soil fertility. Farm budgets were first prepared and in turn used to construct the PAMs for six production systems namely: maize–bean intercrop without any soil fertility management inputs; maize–bean intercrop with chemical fertilizers only; maize–bean intercrop with a combination of chemical fertilizers and improved fallows; maize–bean intercrop with improved fallows only; maize–bean intercrop with a combination of improved fallows and rock phosphate; and maize–bean intercrop with Farm Yard Manure (FYM) only. Results revealed that use of chemical fertilizers with improved fallows was the most profitable technology and thus the study recommended that farmers be encouraged to intensify the use of chemical fertilizers. To make chemical fertilizers more accessible to farmers, the study also recommended that good linkages be made between farmers and micro credit institutions so that small scale farmers are not actually biased against due to lack of collateral when credit is being advanced to clients.     

Oviposition Induced Volatile Emissions from African Smallholder Farmers’ Maize Varieties

Maize (corn), Zea mays, is a genetically diverse crop, and we have recently shown that certain open pollinated varieties (OPVs) of Latin American origin possess a trait not present in mainstream commercial varieties: they produce volatiles in response to stemborer oviposition that are attractive to stemborer parasitoids. Here, we tested whether a similar tritrophic effect occurs in the African OPVs ‘Nyamula’ and ‘Jowi’. Herbivore induced plant volatiles (HIPVs) were collected from plants exposed to egg deposition by the stemborer Chilo partellus. In a four-arm olfactometer bioassay, the parasitic wasp Cotesia sesamiae preferred samples containing HIPVs from plants with eggs to samples collected from plants without eggs. EAG-active compounds, including (E)-4,8-dimethyl-1,3,7-nonatriene (DMNT), were released in higher amounts from the egg induced headspace samples. Our results suggest that this oviposition trait is not limited to S. American Z. mays germplasm, and that it could be used to increase indirect defense against attack by stemborers.     

Responses of Parasitoids to Volatiles Induced by <Emphasis Type="Italic">Chilo partellus</Emphasis> Oviposition on Teosinte,a Wild Ancestor of Maize

Maize, a genetically diverse crop, is the domesticated descendent of its wild ancestor, teosinte. Recently, we have shown that certain maize landraces possess a valuable indirect defense trait not present in commercial hybrids. Plants of these landraces release herbivore-induced plant volatiles (HIPVs) that attract both egg [Trichogramma bournieri Pintureau & Babault (Hymenoptera: Trichogrammatidae)] and larval [Cotesia sesamiae Cameron (Hymenoptera: Braconidae)] parasitoids in response to stemborer egg deposition. In this study, we tested whether this trait also exists in the germplasm of wild Zea species. Headspace samples were collected from plants exposed to egg deposition by Chilo partellus Swinhoe (Lepidoptera: Crambidae) moths and unexposed control plants. Four-arm olfactometer bioassays with parasitic wasps, T. bournieri and C. sesamiae, indicated that both egg and larval parasitoids preferred HIPVs from plants with eggs in four of the five teosinte species sampled. Headspace samples from oviposited plants released higher amounts of EAG-active compounds such as (E)-4,8-dimethyl-1,3,7-nonatriene. In oviposition choice bioassays, plants without eggs were significantly preferred for subsequent oviposition by moths compared to plants with prior oviposition. These results suggest that this induced indirect defence trait is not limited to landraces but occurs in wild Zea species and appears to be an ancestral trait. Hence, these species possess a valuable trait that could be introgressed into domesticated maize lines to provide indirect defense mechanisms against stemborers.     

What role can planted fallows play in the humid and sub-humid zone of West and Central Africa?

Crop management without fertilizer input, which is commonly practiced by most farmers in the humid forest zone of West and Central Africa, requires soil fertility replenishment during a fallow period. Hypothetical relationships between fallow length and crop yields assume, that after the cropping phase replenishment starts with high annual increments, leading to an early recovery of most soil fertility, then slowly approaching a maximum level. The few available empirical data, however, indicate that this assumption is wrong. Within the first 8 years of fallow, biomass and nutrient accumulation is either progressive (low initial increments) or linear. Planted fallows are supposed to replenish soil fertility faster or to higher levels than natural regrowth and should thus lead to higher crop yields. Two major types of planted fallow are distinguished: tree-based and herbaceous fallows. Data from West and Central Africa do not confirm that tree based fallows are generally capable of attaining higher crop yields than natural regrowth or other planted fallows. The majority of experiments with tree-based fallows showed no differences to the control (60.0%). Crop yield declines were found in 15.7% of cases, and only 24.3% resulted in significant yield increases. Changes in soil properties were more frequently positive (34.3%) than negative (9.8%), yet, most often (55.9%), there was no effect. Herbaceous fallow had dominantly positive effects on crop yields (52.5% of cases), with only 3% of cases in which significant reductions were observed. Positive features of some herbaceous fallows, such as easy establishment, rapid weed suppression, and labor efficient slash-and-burn crop establishment make the technology more likely to be accepted and adopted by farmers. It appears that fallows have to be specifically designed for responsive crops, i.e. maize. It is unlikely that one type of fallow can serve the multitude of crops and intercrops grown in the region. Depending on the major constraints to crop production or income generation, planted fallows have to be specifically designed to address these constraints. This may require de-emphasizing the soil fertility aspect and focusing on marketable fallow by-products, weed and pest suppression and reduced labor requirements. Thus, future impact through research on planted fallows will depend on exact targeting of specific fallow types and species to the most responsive crops and to explicit farmer circumstances.     

Micro-dosing as a pathway to Africa’s Green Revolution: evidence from broad-scale on-farm trials

Next to drought, poor soil fertility is the single biggest cause of hunger in Africa. ICRISAT-Zimbabwe has been working for the past 10 years to encourage small-scale farmers to increase inorganic fertiliser use as the first step towards Africa’s own Green Revolution. The program of work is founded on promoting small quantities of inorganic nitrogen (N) fertiliser (micro-dosing) in drought-prone cropping regions. Results from initial on-farm trials showed that smallholder farmers could increase their yields by 30–100% through application of micro doses, as little as 10 kg Nitrogen ha⁻¹. The question remained whether these results could be replicated across much larger numbers of farmers. Wide scale testing of the micro-dosing (17 kg Nitrogen ha⁻¹) concept was initiated in 2003/2004, across multiple locations in southern Zimbabwe through relief and recovery programs. Each year more than 160,000 low resourced households received at least 25 kg of nitrogen fertiliser and a simple flyer in the vernacular explaining how to apply the fertiliser to a cereal crop. This distribution was accompanied by a series of simple paired plot demonstration with or without fertiliser, hosted by farmers selected by the community, where trainings were carried out and detailed labour and crop records were kept. Over a 3 year period more than 2,000 paired-plot trials were established and quality data collected from more than 1,200. In addition, experimentation to derive N response curves of maize (Zea mays L.), sorghum (Sorghum bicolor (L.) Moench) and pearl millet (Pennisetum glaucum (L.) R.Br.) in these environments under farmer management was conducted. The results consistently showed that micro-dosing (17 kg Nitrogen ha⁻¹) with nitrogen fertiliser can increase grain yields by 30–50% across a broad spectrum of soil, farmer management and seasonal climate conditions. In order for a household to make a profit, farmers needed to obtain between 4 and 7 kg of grain for every kg of N applied depending on season. In fact farmers commonly obtained 15–45 kg of grain per kg of N input. The result provides strong evidence that lack of N, rather than lack of rainfall, is the primary constraint to cereal crop yields and that micro-dosing has the potential for broad-scale impact on improving food security in these drought prone regions.     

Effects of molasses grass,Melinis minutiflora volatiles on the foraging behavior of the cereal stemborer parasitoid,Cotesia sesamiae

Olfactory responses of the cereal stemborer parasitoid Cotesia sesamiae to volatiles emitted by gramineous host and nonhost plants of the stemborers were studied in a Y-tube olfactometer. The host plants were maize (Zea mays) and sorghum ( Sorghum bicolor), while the nonhost plant was molasses grass (Melinis minutiflora). In single-choice tests, females of C. sesamiae chose volatiles from infested and uninfested host plants and molasses grass over volatiles from the control (soil). In dual-choice tests, the wasp preferred volatiles from infested host plants to those from uninfested host plants. There was no discrimination between molasses grass volatiles and those of uninfested maize, uninfested sorghum, or infested maize. The wasp preferred sorghum volatiles over maize. Combining uninfested maize or sorghum with molasses grass did not make volatiles from the combination more attractive as compared to only uninfested host plants. Infested maize alone was as attractive as when combined with molasses grass. Infested sorghum was preferred over its combination with molasses grass. Local growth conditions of the molasses grasses influenced attractiveness to the parasitoids. Volatiles from Thika molasses grass were attractive, while those from Mbita molasses grass were not. Growing the Thika molasses grass in Mbita rendered it unattractive and vice versa with the Mbita molasses grass. This is a case of the same genotype expressing different phenotypes due to environmental factors.     

Manipulation of Chemically Mediated Interactions in Agricultural Soils to Enhance the Control of Crop Pests and to Improve Crop Yield

In most agro-ecosystems the organisms that feed on plant roots have an important impact on crop yield and can impose tremendous costs to farmers. Similar to aboveground pests, they rely on a broad range of chemical cues to locate their host plant. In their turn, plants have co-evolved a large arsenal of direct and indirect defense to face these attacks. For instance, insect herbivory induces the synthesis and release of specific volatile compounds in plants. These volatiles have been shown to be highly attractive to natural enemies of the herbivores, such as parasitoids, predators, or entomopathogenic nematodes. So far few of the key compounds mediating these so-called tritrophic interactions have been identified and only few genes and biochemical pathways responsible for the production of the emitted volatiles have been elucidated and described. Roots also exude chemicals that directly impact belowground herbivores by altering their behavior or development. Many of these compounds remain unknown, but the identification of, for instance, a key compound that triggers nematode egg hatching to some plant parasitic nematodes has great potential for application in crop protection. These advances in understanding the chemical emissions and their role in ecological signaling open novel ways to manipulate plant exudates in order to enhance their natural defense properties. The potential of this approach is discussed, and we identify several gaps in our knowledge and steps that need to be taken to arrive at ecologically sound strategies for belowground pest management.     

Greenhouse gas emissions from soil under maize–soybean intercrop in the North China Plain

Intercrop systems can exhibit unique soil properties compared to monocultures, which influences the microbially-mediated processes leading to greenhouse gas emissions. Fertilized intercrops and monocultures produce different amounts of N₂O, CO₂ and CH₄ depending on their nutrient and water use efficiencies. The objective of this study was to compare the fluxes and seasonal emissions of N₂O, CO₂, and CH₄ from a maize–soybean intercrop compared to maize and soybean monocultures, in relation to crop effects on soil properties. The experiment was conducted during 2012, 2013 and 2014 at the WuQiao Experimental Station in the North China Plain. All cropping systems received urea-N fertilizer (240 kg N ha^?1 applied in two split applications). The cropping systems were a net source of CO₂ and a net sink of CH₄, with significantly (P < 0.05 in 2012) and numerically (2013 and 2014) lower N₂O flux and smaller seasonal N₂O emissions from the maize–soybean intercrop than the maize monoculture. The proportion of urea-N lost as N₂O was lower in the maize–soybean intercrop (1.6% during the 3-year study) and soybean monoculture (1.7%), compared to maize monoculture (2.3%). Soybean reduced the soil NO₃^?–N concentration and created a cooler, drier environment that was less favorable for denitrification, although we cannot rule out the possibility of N₂O reduction to N₂ and other N compounds by soybean and its associated N₂-fixing prokaryotes. We conclude that maize–soybean intercrop has potential to reduce N₂O emissions in fertilized agroecosystems and should be considered in developing climate-smart cropping systems in the North China Plain.     

The role of legume fallows in intensified upland rice-based systems of West Africa

Traditional upland rice-based cropping systems in West Africa rely on periods of fallow to restore soil fertility and prevent the build-up of insect pests and weeds. Demographic growth and increased demand for land is forcing many farmers to intensify their rice production systems. Declining fallow length and increasing number of crops before leaving the land to extended fallow result in a significant yield reduction. Promising cropping system alternatives include the use of site specific, weed-suppressing, multi-purpose cover legumes as short duration fallows. Constraints to rice production related to intensification were determined in 209 farmers' fields in four agro-ecological zones during 1994 and 1995. Nitrogen accumulation and weed suppression were evaluated in 54 legume accessions, grown for six months during the dry season, under a range of hydrological and soil conditions in 1994/95. Their effect on the yield of upland rice was determined in 1995. To increase benefits from improved fallow technology, the timing of legume establishment in relation to rice and the effect on crop and weed growth of removing, burning, mulching, or incorporating fallow residues prior to the rice crop were determined. Intensified land use resulted in a significant plot-level yield reduction that was highest in the derived savanna and the bimodal forest zones where it was associated with a doubling of the weed biomass in rice and a significant reduction in soil N supply. Legume fallows appear to offer the potential to sustain rice yields under intensified cropping. Legume biomass was in most instances significantly greater than in the weedy fallow control and several legume species suppressed weed growth. Nitrogen accumulation by legumes varied between 1–200 kg N ha-1 with 30–90% Ndfa. Rice grain yield following legume fallows increased by an average of 0.2 mg ha-1 or 29% above the weedy fallow control. Relay establishment substantially increased legume biomass. However, seeding of the legume at 28 days or earlier significantly reduced grain yield due to interspecific competition. Incorporating or mulching of fallow residues provided no significant yield advantage as compared to burning. Absolute effects varied as a function of site, legume species, and management practice.     

Farm‐scale assessment of maize–pigeonpea productivity in Northern Tanzania

Little is known about productivity of smallholder maize–pigeonpea intercropping systems in sub-Saharan Africa. We conducted a survey of 277 farm households in Northern Tanzania to assess socio-economic factors, field management characteristics, and their association with productivity of maize–pigeonpea intercrops. On each farm, crop assessments were focused on a field that the farmer identified as most important for food supply. Variables associated with yields were evaluated using linear regression and regression classification. Biomass production ranged between 1.0 and 16.6 for maize, and between 0.2 and 11.9 t ha^?1 for pigeonpea (at maize harvest). The corresponding grain yields ranged between 0.1 and 9.5 for maize, and between 0.1 and 2.1 t ha^?1 for pigeonpea. Plant density at harvest, number of years the field had been cultivated, slope, weeding, soil fertility class, fertiliser and manure use were significantly associated with variation in maize grain yield, with interactions among the factors. Fields on flat and gentle slopes with plant density above 24,000 ha^?1 had 28% extra yields when fertiliser was applied, while less than 24,000 plants ha^?1 yielded 16% extra yield when manure was applied. Plant density at harvest was the key factor associated with pigeonpea yield; fields with densities above 24,000 plants ha^?1 yielded an average of 1.4 t ha^?1, while less than 24,000 plants ha^?1 yielded 0.5 t ha^?1. We conclude that performance of intercrops can be enhanced through application of organic and inorganic nutrient sources, and agronomic interventions including weeding, implementing soil conservation measures on steep slopes and optimising plant density.

     

Experience-Induced Habituation and Preference Towards Non-Host Plant Odors in Ovipositing Females of a Moth

In phytophagous insects, experience can increase positive responses towards non-host plant extracts or induce oviposition on non-host plants, but the underlying chemical and behavioral mechanisms are poorly understood. By using the diamondback moth, Plutella xylostella, its host plant Chinese cabbage, and a non-host plant Chrysanthemum morifolium, as a model system, we observed the experience-altered olfactory responses of ovipositing females towards volatiles of the non-host plant, volatiles of pure chemicals (p-cymene and α-terpinene) found in the non-host plant, and volatiles of host plants treated with these chemicals. We assessed the experience-altered oviposition preference towards host plants treated with p-cymene. Naive females showed aversion to the odors of the non-host plant, the pure chemicals, and the pure chemical-treated host plants. In contrast, experienced females either became attracted by these non-host odors or were no longer repelled by these odors. Similarly, naive females laid a significantly lower proportion of eggs on pure chemical-treated host plants than on untreated host plants, but experienced females laid a similar or higher proportion of eggs on pure chemical-treated host plants compared to untreated host plants. Chemical analysis indicated that application of the non-host pure chemicals on Chinese cabbage induced emissions of volatiles by this host plant. We conclude that induced preference for previously repellent compounds is a major mechanism that leads to behavioral changes of this moth towards non-host plants or their extracts.     

Effect of K on nitrogen fixation of intercrop groundnut and the competition between intercrop groundnut and maize

Application of adequate level of K has shown to improve the competitive ability of the legume in legume/grass mixtures. However, the effect of K on the competitive ability of grain legumes in legume/cereal intercropping systems has not been adequately studied. Hence, studies were made to ascertain if the effects of K could be exploited in improving the performance of groundnut (Arachis hypogaea L.) cv. No. 45 when intercropped with maize (Zea mays L.) cv. Badra. The study was conducted at the Faculty of Agriculture, University of Ruhuna, Kamburupitiya, Sri Lanka in 1988 in basins filled with 36 kg of soil. It involved establishing maize and groundnut as monocrops and as intercrops at three K levels viz. 0, 20 and 40 mg of K kg^–1 of soil. Monocrop maize and groundnut had 2 and 5 plants/basin, respectively while the intercrop had 1 maize plant and 3 groundnut plants/basin. The soil used was Red Yellow Podzolic which was tagged by incorporating¹⁵N-labelled plant material. When grown as a monocrop, K had no effect on the percent N derived from atmosphere, amount of N₂ fixed, dry matter production, pod yield and total N content of groundnut. However, when intercropped with maize lack of K application affected the above parameters significantly which was overcome by improving K level. Thus, the optimum level of K for groundnut was greater when intercropped than monocropped. A significant interaction between K level and cropping system was evident with regard to N₂ fixation, pod yield and total dry matter production of groundnut. Intercrop maize derived 30–35% of its N content from the associated groundnut plants which amounted to 13–22 mg N/plant. The amount of N supplied by groundnut to associated maize plant was not affected by K level. It appears that there is scope for alleviating growth depression of the legume component in legume/cereal intercropping systems by developing appropriate K fertilizer practices.     

Appropriate technologies to replenish soil fertility in southern Africa

In southern Africa, soil nutrient reserves are being depleted because of continued nutrient mining without adequate replenishment. The consequent downward spiral of soil fertility has led to a corresponding decline in crop yields, food insecurity, food aid and environmental degradation. The central issue for improving agricultural productivity in southern Africa is how to build up and maintain soil fertility despite the low incomes of smallholder farmers and the increasing land and labour constraints they face. Under this review five main options namely: inorganic fertilizers, grain legumes, animal manures, integrated nutrient management and agroforestry options appropriate to smallholder farmers are presented. Issues addressed in the use of inorganic fertilizers are reduction in fertilizer costs, timely availability and use efficiency. Legumes can be used to diversify farm system productivity but this requires P and lime application to support better legume growth and biological nitrogen fixation (BNF) as well as development of markets for various legume products. Manure availability and quality are central issues in increasing smallholder farm productivity and increasing its efficiency through proper handling and application methods. Integrated nutrient management of soil fertility by combined application of both inputs will increase use efficiency of inputs and reduce costs and increase profitability; but the challenge is often how to raise adequate amounts of either inorganic or organic inputs. Issues such as quality of inputs, nutrient balancing, labour to collect and transport organic inputs and their management need to be optimized. These are the challenges of adoption as are the scaling up of these options to millions of small-scale farmers.     

The comparison of nitrogen use and leaching in sole cropped versus intercropped pea and barley

The effect of sole and intercropping of field pea (Pisumsativum L.) and spring barley (Hordeum vulgareL.) and of crop residue management on crop yield,NO₃ ^– leaching and N balance in the cropping systemwas tested in a 2-year lysimeter experiment on a temperate sandy loam soil. Thecrop rotation was pea and barley sole and intercrops followed by winter-rye anda fallow period. The Land Equivalent Ratio (LER), which is defined as therelative land area under sole crops that is required to produce the yieldsachieved in intercropping, was used to compare intercropping performancerelative to sole cropping. Crops received no fertilizer in the experimentalperiod. Natural ¹⁵N abundance techniques were used to determine peaN₂ fixation. The pea–barley intercrop yielded 4.0 Mg grainha^–1, which was about 0.5 Mg lowerthan theyields of sole cropped pea but about 1.5 Mg greater than harvestedin sole cropped barley. Calculation of the LER showed thatplant growth resources were used from 17 to 31% more efficiently by theintercrop than by the sole crops. Pea increased the N derived fromN₂fixation from 70% when sole cropped to 99% of the total aboveground Naccumulation when intercropped. However, based upon aboveground N accumulationthe pea–barley intercrop yielded about 85 kg Nha^–1, which was about 65 kg lower thansolecropped pea but about three times greater than harvested in sole croppedbarley.Despite different preceding crops and removal or incorporation of straw, therewas no significant difference between the subsequent non-fertilized winter-ryegrain yields averaging 2.8 Mg ha^–1, indicating anequalization of the quality of incorporated residue by theNO₃ ^– leaching pattern.NO₃ ^– leaching throughout the experimental periodwas61 to 76 kg N ha^–1. Leaching dynamics indicateddifferences in the temporal N mineralization comparing lysimeters previouslycropped with pea or with barley. The major part of this N was leached duringautumn and winter. Leaching tended to be smaller in the lysimeters originallycropped with the pea–barley intercrops, although not significantly differentfromthe sole cropped pea and barley lysimeters. Soil N balances indicated depletionof N in the soil–plant system during the experimental period, independent ofcropping system and residue management. N complementarity in the croppingsystemand the synchrony between residual N availability and crop N uptake isdiscussed.     

Nitrogen balances of smallholder farms in major cropping systems in a peri-urban area of Beijing,China

An in-depth understanding of nutrient management variability on the regional scale is urgently required due to rapid changes in cropping patterns and farmers’ resource use in peri-urban areas of China. The soil surface nitrogen (N) balances of cereal, orchard and vegetable systems were studied over a 2-year period on smallholder fields in a representative peri-urban area of Beijing. Positive soil surface N balances were obtained across all three cropping systems. The mean annual N surplus of the vegetable system was 1,575 kg N ha⁻¹ year⁻¹, or approximately 3 times the corresponding values in the cereal (531 kg N ha⁻¹ year⁻¹) and orchard systems (519 kg N ha⁻¹ year⁻¹). In the vegetable system, animal manure (1,443 kg N ha⁻¹ year⁻¹ on average) was the major source of N input (65 % of the total N input) and the factor with strongest impact on the N surplus. In the cereal system, however, about 74 % of the total N input originated from mineral fertilizer application which was the major contributor to the N surplus, while in the orchard system, the N surplus was strongly and positively correlated with both mineral fertilizer and animal manure applications. Furthermore, within each cropping system, N fertilization, crop yields and N balances showed large variations among different smallholder fields, especially in orchard and vegetable systems. This study highlights that differences in farming practices within or among cropping systems should be taken into account when calculating nutrient balances and designing strategies of integrated nutrient management on a regional scale.     

Allelopathic interactions in crop-weed mixtures

A very important component of the aggressive nature of weeds is allelopathic interference, the full potential of which is just being realized in the management of agroecosystems. Research results are presented which demonstrate the allelopathic interactions involved in a wide range of cropweed combinations occurring in a great variety of habitats. This includes crops planted in weed control, crops with allelopathic potential, and noncrop plants of beneficial use for weed control as a result of allelopathic interference. Allelopathy can play a beneficial role in multiple cropping systems, crop rotations, and cover cropping. The potential role for allelopathic interactions in the design of biological weed control is proposed.     

Electrophysiological Responses of the Lepidopterous Stemborers Chilo partellus and Busseola fusca to Volatiles from Wild and Cultivated Host Plants

The stemborers Chilo partellus and Busseola fusca are major pests of subsistence cereal farming in Africa. Volatiles released by two cultivated hosts, sorghum and maize (Sorghum bicolor and Zea mays), and two wild grass hosts, Pennisetum purpureum and Hyparrhenia tamba, were collected by air entrainment. Electrophysiologically active components in these samples were detected by coupled gas chromatography-electroantennography (GC-EAG), and the active peaks identified by gas chromatography-mass spectrometry. A total of 41 compounds were identified from the four plant species, all of which, as well as two unidentified compounds, elicited an electrophysiological response from one or both of the stemborers. The compounds included a number of green leaf volatiles and other aliphatic aldehydes, ketones, and esters, mono- and sesquiterpenoids, and some aromatic compounds. EAG studies with authentic samples, conducted at two discriminating doses for all compounds, and dose–response curves for 14 of the most highly EAG-active compounds, showed significant differences in relative responses between species. The compounds that elicited large responses in both species of moths included linalool, acetophenone, and 4-allylanisole, while a number of compounds such as the aliphatic aldehydes octanal, nonanal, and decanal elicited a large response in B. fusca, but a significantly smaller response in C. partellus. Furthermore, the wild hosts produced higher levels of physiologically active compounds compared with either of the cultivated hosts. These differences are discussed in relation to the differential attraction/oviposition of the two stemborers observed in the field and, particularly for eastern African small-scale farming systems, in the context of using a push–pull strategy for their control.     

Maize-bean intercrop response to nutrient application relative to maize sole crop response

Maize-bean intercropping is important in sub-Saharan Africa. Maize sole crop (MSC) nutrient response has been much studied but data is scarce for determination of intercrop functions. A procedure for adapting MSC functions for the maize-bean intercrop was developed. Maize sole crop and intercrop responses were near parallel with notable exceptions for P in high potential areas for maize and for K. Mean intercrop bean yield with no nutrient application was about 0.4 Mg ha^?1 and increased on average by 24, 11 and ?3% with N, P and K application, respectively. Response function coefficients for MSC adjusted with the ratio of bean to maize grain value as the dependent variable accounted for nearly all variation in intercrop response coefficients providing the basis for determining intercrop response functions from MSC functions. Maize grain yield equivalent was less with MSC compared with intercrop; exceptions were for response to N in high potential areas and for bean to maize value ratios of two or less. The economically optimal rate of N and P were on average about 15% more but less for K with intercrop compared with MSC but with inconsistency. The economically optimal rate ranged widely with variation in the cost of nutrient use relative to grain value but generally without great effect on yield; an exception was a great effect on MSC yield response to N for high potential areas. Intercrop nutrient response functions can be reliably determined once maize sole crop functions are determined for a recommendation domain.     

Inhibition of seedling growth of crop species by recirculating root exudates ofBidens pilosa L.

The allelopathic effects of root exudates ofBidens pilosa L. on seedling growth ofLactuca sativa L.,Phaseolus vulgaris L.,Zea mays L., andSorghum bicolor (L.) Moench were studied using a root exudate recirculating system that allows continuous exposure of crop plants to allelopathic chemicals. This system maintains an undisturbed rhizosphere and eliminates competition and physical contact between the donor and acceptor plants. Comparison of responses to hydrophobic and hydrophilic root exudates is made possible by removal of hydrophobic compounds using XAD-4. Treatments consisted ofB. pilosa, B. pilosa with an Amberlite XAD-4 resin column attached to the donor pot to remove hydrophobic allelochemicals, and a donor pot without weeds.B. pilosa significantly inhibited seedling growth of all crop species tested. The crop species varied in response to the root exudates, withL. sativa being most sensitive. Larger and olderB. pilosa plants caused greater inhibition of seedling growth ofL. sativa andP. vulgaris than did smaller (younger)B. pilosa plants.B. pilosa with XAD-4 caused significantly less inhibition to all crop species, exceptZ. mays, thanB. pilosa without XAD-4, indicating that the hydrophobic exudates played an important role in the allelopathic growth inhibition. Variability in species response toB. pilosa with and without XAD-4 was probably due to differences in sensitivity to hydophobic and hydrophilic allelochemicals.Supported by a grant under USDA Agreement No. 83-CRSR-2-2293. Journal Series No. 2887 of the Hawaii Institute of Tropical Agriculture and Human Resources.