ENERGY ACQUISITION AND ALLOCATION IN PLANTS AND INSECTS: A HYPOTHESIS FOR THE POSSIBLE ROLE OF HORMONES IN INSECT FEEDING PATTERNS

A distributed delay age structure model is presented for plants and insects that describes the dynamics of per capita energy (dry matter) acquisition and allocation patterns, and the within-organism subunit (e.g. leaves, fruit, ova) number dynamics that occur during growth, reproduction, and development. Four species of plants (common bean, cassava, cotton, and tomato) and two species of insects (pea aphid and a ladybird beetle) are modeled. A common acquisition (i.e. functional response) submodel is used to estimate the daily photosynthetic rates in plants and consumption rates in pea aphid and the ladybird beetle. The focus of this work is to capture the essence of the common attributes between trophic levels across this wide range of taxa. The models are compared with field or laboratory data. A hypothesis is proposed for the observed patterns of reproduction in pea aphid and in a ladybird beetl

The influence of postflowering pests on cowpea seed yield with particular reference to damage by Heteroptera in southern Benin

Stage-specific survival rates of cowpea seeds were evaluated in fields planted during both cropping seasons of 1991 and 1992 in southern Benin using Manly's regression model. Considerable seed damage could be recognised in all seed growth stages and reflected the variable pest infestations during a cropping season. An analytical approach based on the concept of competing risks and originating from human demography was used to assess the seed damage by the pod sucking bugs (PSB), Maruca testulalis and Apion varium during different stages of pod formation in the presence and absence of other pests. The damage pattern created by an individual pest acting alone changed considerably when competition was taken into account. The risk analysis revealed that an increase in seed damage by a pest of a factor five was possible when others were excluded. It showed that the control of one pest alone is unlikely to reduce seed damage to an acceptable level. Seed damage by the PSB was observed during all stages of pod formation. Heavy seed attack by this pest group during early pod formation reduced available food for other pests in many fields and consequently masked their real damage potential. Clavigralla tomentosicollis was the most damaging PSB species in 11 out of 16 fields in the study area

Seed damaging field pests of cowpea (Vigna unguiculata) L.Walp.) in Benin: occurance and pest status

Three insect groups, known from the literature, were confirmed to make up a seed pest complex in cowpea fields in Benin Republic (West Africa). Several species of heteropteran pod‐sucking bugs (PSBs), two lepidopteran species, namely Maruca testulalis (Geyer) and Cydia ptyehora (Meyrick) and the weevil Apion varium Wagner occurred throughout southern Benin. Field infestations of PSBs and M. testulalis were always present, whereas A. varium and C. ptyehora occurrence was sporadic. Seed damage assessment at harvest time showed that in 80% of 25 fields, planted between 1990 and 1992, PSBs were the most important pests. PSB damage to seed usually ranged between 40 and 85%. Seed damage from M. testulalis was low, despite occasional high flower infestations. C. ptyehora was a minor pest in all locations. Significant correlations between seed damage and cumulative numbers of insect days (CD) during pod formation was found for all three groups. For PSBs alone, the CD values were significantly correlated with pod numbers. The results suggest that the PSBs are the most important cowpea seed pests in southern Benin, the main cowpea production area of the country. However, probable competition for seeds among pests may mask the true damage potential of any one of them. There is a need to study damage by the different pests during seed formation

Simulation model for maize crop growth based on acquisition and allocation processes for carbohydrate and nitrogen

A population model for maize growth and development driven by temperature, solar radiation, soil water and soil nitrogen is presented. A time-varying distributed delay model is used to describe the dynamics of the attributes number, biomass and nitrogen belonging to leaf, root, stem, grain, rachys and husk populations. Photosynthate production as well as water and nitrogen acquisition was simulated with a demand-driven ratio-dependent functional response model. Carbohydrate, water and nitrogen supply-demand ratios were calculated to control the growth of different populations of plant. The metabolic pool model was used to allocate carbohydrates to plant subunits. The model was validated with published observations and field data from a 95- and a 120-day variety grown at the Research Station of the International Institute of Tropical Agriculture in Calavi, in the southern part of the Republic of Benin (West Africa). It was concluded to satisfactorily represent crop phenology and growth patterns. Moreover, the model permitted the assessment of drought stress, soil nitrogen, and planting density effects on maize growth and development

Analysis of the cowpea agroecosystem in West Africa: II. Modelling the interactions between cowpea and the bean flower thrips Megalurothrips sjostedti (Trybom) (Thysanoptera, Thripidae)

The interactions between cowpea and one of its key pests, Megalurothrips sjostedti (Trybom), were evaluated via a simulation model based on three modules building the core of the cowpea growth model, i.e. the time-varying distributed delay, the functional response acquisition function, and the metabolic pool model. The thrips population model was superimposed on the cowpea model in order to evaluate two-trophic level interactions, and the way the interactions are affected by phosphate and water availability in the soil. In addition, the model was used to evaluate the effect of possible biological control candidates on plant-pest interactions

Sampling Mononychellus tanajoa (Acari: Tetranychidae) on cassava in Africa

Density-specific sampling plans were developed under African conditions for the exotic spider mite, Mononychellus tanajoa (Bondar), a serious pest of cassava, Manihot esculenta. The within-plant distribution of Mononychellus tanajoa was found to favour new foliage, regardless of time of planting or plant age. Consequently, the first developed leaf near the top of the foliage was selected as the sampling unit and related to whole plant populations of M. tanajoa. The relationship between the mite population's variance and mean as measured by Taylor's Power Law proved to be stable over a range of planting dates, seasons and locations. Two binomial sampling plans, one based on Taylor's dispersion parameters and another based on direct field observations, were developed and compared. Binomial sampling, appropriate only for densities below 30 mites per leaf, was replaced by an enumerative procedure based on a 'quick count' protocol at higher mite densities

The influence of the cassava mealybug Phenacoccus manihoti Mat. Ferr. (Hom., Pseudococcidae) on yield formation of cassava, Manifhoti esculenta Crantz

The cassava mealybug (CM), Phenacoccus manihoti Mat.‐Ferr., is a dry season pest of cassava, Manihot esculenta Crantz. Its effects on the patterns of dry matter production and allocation in cassava planted July, September 1982 and May 1983 and grown for 12 months was studied. Peak CM population densities varied greatly (from 600 to 37000 CM per plant) due to plant size at the time of infestation, duration of infestation, and mortality from the introduced parasitoid Fpidinocarsis lopezi (De Santis). CM feeding reduced new leaf production and assimilation and allocation of dry matter to storage roots. Severly infested cassava plants, lost between 9 % and 46 % during the dry season, compared to CM‐free plants. At the beginning of the ensuing rainy season, mobilisation of reserves from storage roots for regrowth caused temporary root yield losses of up to 75%. Yield losses at harvest across planting dates measured 12 months after planting, were of 52 to 58 % in the CM‐infested plants compared to noninfested plants

Trophic influences on survival, development and reproduction of Hyperaspis notata (Col., Coccinellidae)

The coccinellid, Hyperaspis notata Mulsant, is associated with the mealybugs Phenacoccus manihoti Mat.‐Ferr. and P. herreni Williams, on cassava in southern Brazil and the highlands of Colombia. Brought to Africa to help control the accidentally introduced P. manihoti, its range of target prey and plant food sources as well as its performance under conditions of food scarcity were investigated in the laboratory. H. notata showed a moderately narrow food spectrum which should allow survival in periods when P. manihoti is scarce without threatening a broad range of insects species: H. notatasurvived, completed larval development and reproduced on P. madeirensis(Green) and Ferrisia virgata (Cockerell), which are alternate prey species of the family Pseudococcidae and occur abundantly in cassava fields and on ornamental plants in southern Benin. By contrast, development was not possible on less related taxa of the Sternorrhyncha, namely on Aphis craccivoraKoch which occur on cowpea often intercropped with cassava and on the spiralling white fly Aleurodicus dispersus Russel infesting cassava, and many other food crops and ornamental plants. These alternate food sources allowed survival of the larvae and adults for a limited period only and neither moulting nor egg production were observed. Cassava pollen was unsuitable as a food source since it did not allow larvae to develop, females to oviposit, nor did it extend longevity of larvae and adults. Honey prolonged the life span of adults without allowing egg production, and is thus a suitable food substitute for adult H. notata during shipment to release destinations. The coccinellid larvae completed their development to the adult stage when fed from a range of ad libitum supply (consuming up to 6 mg per day) of cassava mealybugs to a minimal daily amount of 1 mg (consuming only ∽0.8 mg per day). Females even laid eggs when fed with a minimal amount of 1.2 mg per couple and day. Larvae of the Colombian strain gained more weight before pupation, and the tolerance of larvae and adults to starving was more marked than in the Brazilian strain