H. Van Den Berg

Evaluating the role of predation in population fluctuations of the soybean aphid Aphis glycines in farmers' fields in Indonesia.

The soybean aphid Aphis glycines can build up to high densities in young soybean, causing farmers to apply insecticides early in the season. The seasonal phenology of A. glycines was studied at 15 sites on Sumatra. Aphis glycines densities peaked in the vegetative plant stage and declined rapidly afterwards or towards the flowering stage of soybean. A clear statistical dependency of coccinellid predator densities on aphid densities was found. Age-specific survival and reproduction of A. glycines were determined in clip-cages on plants 3, 5 and 7 weeks of age. The growth rate was highest on 3-week-old plants and declined linearly with plant age. Predation rates were measured through direct observations of individual coccinellid and staphylinid predators in the field at different times during daylight hours. Predator observations were made at various field densities of A. glycines, and thus a functional response of predation rate to prey density was obtained for each predator type. A simple deterministic model was developed to calculate population changes of A. glycines based on field-measured diurnal predation functions and aphid growth rates. The model was adopted to evaluate whether aphid population declines observed in the field could be ascribed to predation. In young soybean crops ( 40 days), the observed population declines to low levels were attributable to predation. This pattern was observed on subsequent occasions in the same crop, and was observed concurrently in crops of three different ages planted in randomized blocks. The influence of other mortality factors (competition and migration of A. glycines, rainfall, plant age) on A. glycines dynamics is discussed. Coccinellid predators play a role in suppressing A. glycines in soybean. Therefore it is important that predators are conserved early in the season by avoiding unnecessary insecticide applications. Exercises developed from this study have been added to integrated pest management training curricula in Indonesia, in order that farmers learn to make more appropriate pest management decisions.

Stage-specific mortality of Helicoverpa armigera in three smallholder crops in Kenya.

Partial life tables of Helicoverpa armigera were constructed for three crops, sunflower, maize and sorghum, commonly grown on smallholder farms in Kenya. Oviposition coincided with early flowering of the crops, and was due to the preference of ovipositing moths for flowering plants rather than to narrow periods of oviposition activity. Consequently, single cohorts of H. armigera developed on each crop. The partial life tables show that mortality during development was generally high (82-99.3%) on sunflower, maize and sorghum, but stage-specific mortality varied greatly from season to season. Mortality was highest on maize, particularly during the young stages. Mortality was generally higher in the short rainy seasons than in the long rainy seasons (...)

Predation and parasitism on eggs of two pod‐sucking bugs, Nezara viridula and Piezodorus hybneri in soybean

Abstract Cohorts of newly laid egg masses of two pod‐sucking bugs, Nezara viridula and Piezodorus hybneri (Hemiptera: Pentatomidae), were exposed to naturally occurring predators and parasitoids on soybean in unsprayed fields at four sites in Northern Sumatra. The fate of the young stages was recorded until the late first instar, before the nymphs start to disperse and feed. Partial life tables showed that mortality of N. viridula until the late first instar was 50–87%, of which 18–65% occurred during the egg stage and was mainly caused by predation; 2–26% of the eggs died of parasitism. P. hybneri which was present at only two sites, showed a 73–89% mortality until the late first instar, of which 69–85% was during the egg stage and, again, mainly caused by predation; 15–24% of the eggs died of parasitism. To evaluate the role of individual egg predator species in the field, we conducted hourly observations of exposed egg masses of the two pests. Dolichoderus sp., Solenopsis geminata (Hymenoptera: Formici...

Natural Control of Helicoverpa armigera in Cotton: Assessment of the Role of Predation

Irreplaceable mortality of Helicoverpa armigera due to natural enemies was studied in cotton in western Kenya. Field populations of H. armigera were followed in three types of subplots: where crawling predators were suppressed, where both crawling and flying predators were suppressed and where predators were unaffected. Ants were the predominant crawling predators, whereas anthocorids were the predominant flying predators. H. armigera mortality from egg to late larval stage was very high (96.4-99.7%) and was greater in the second than in the first generation. Suppression of the different groups of predators did not increase the density of the pest. It is argued that the high level of background mortality obscured the role of predators. The possible role of host-plant condition on background mortality is discussed.

Biological control of Helicoverpa armigera in Africa.

Andy Cherry,1 Matthew Cock,2 Henk van den Berg3 and Rami Kfir4 1International Institute of Tropical Agriculture, Cotonou, Bénin; 2CABI Bioscience Switzerland, Delémont, Switzerland; 3Laboratory of Entomology, Department of Plant Sciences, Wageningen University, Wageningen, The Netherlands; 4Agricultural Research Council – Plant Protection Research Institute, Division of Insect Ecology – Biological Control, Pretoria, South Africa

Exclusion cage studies on the impact of predation on Helicoverpa armigera in cotton

The impact of predation on Helicoverpa armigera was studied in four field cage exclusion trials on cotton in Kenya. H. armigera egg cohorts were introduced inside predator‐free and open control cages, and the impact of local predator populations on the cohort was examined. Fourteen days after inoculation, exclusion cages had four times more larvae than controls, indicating a strong impact of predation. Ants and Anthocoridae were the predominant predator groups. Exclusion cages had more damaged fruiting plant parts (squares, flowers and bolls) than the control. In the absence of predators, natural mortality of H. armigera was greater as cotton matured, and is likely to be linked to the host plant condition.

Soybean stem fly, Melanagromyza sojae (Diptera: Agromyzidae), on Sumatra: Seasonal incidence and the role of parasitism

Abstract The density of M. sojae and percentage parasitism was studied in unsprayed soybean at 18 farmers’ field sites in Aceh, North Sumatra and West Sumatra provinces, Indonesia, during 1992 and 1993. M. sojae generally infested soybean throughout the season; infestation was initially low, reached its peak from the fifth till the eighth week after planting and declined towards the end of the season. A complex of seven hymenopterous parasiloids had a high impact on the pest. Parasitism levels built up alongside with host density and remained high until just before harvest. The eucoilid Gronotoma sp. was the most prevalent species during the early and mid‐season; a complex of pteromalids became dominant towards the end of the season. In soybean planted successively at 2‐week intervals, the parasitism level in the first‐planted crop built up slowly, whereas the parasitism level in the third‐planted crop was high from the early crop stage onwards, which suggests that the third‐planted crop benefited from pa...

Spatial association between Helicoverpa armigera and its predators in smallholder crops in Kenya.

1. This work is part of a research project on the population dynamics of a polyphagous noctuid pest, Helicoverpa armigera, on crops commonly grown in small-scale agriculture in Kenya. The between-plant and within-plant distributions of H. armigera eggs and larvae are presented for sunflower, maize, sorghum and cotton. The association of H. armigera with its predominant predators, which are anthocorids and ants, was analysed both within plants and between plants to assess the extent to which predators occur on the same plant parts as H. armigera. 2. The distribution of H. armigera eggs between plants was slightly aggregated, but the degree of aggregation tended to decline as H. armigera larvae matured. 3. The distribution of predators was more aggregated than that of H. armigera. Generally, the number of predators per plant was not associated with the number of H. armigera per plant, but associations may have been obscured by the foraging strategy of ants. Only on sunflower were predatory ants associated with H. armigera larvae4. Oviposition and larval feeding of H. armigera were concentrated on the flower head of sunflower and the panicle of sorghum. On maize and cotton, however, the majority of eggs were deposited away from the soft plant parts suitable for larval feeding. Implications for survival of hatchlings are discussed. 5. Anthocorids were concentrated in the same types of microhabitat as H. armigera eggs on sorghum, but regression analysis showed that their association within plants was low, mainly because anthocorid populations increased after the H. armigera oviposition had peaked. 6. Ants (Pheidole spp. and Myrmicaria spp. combined) were generally more closely associated with H. armigera stages within plants than were anthocorids