Jørgen Aagaard Axelsen

Effect of a copper gradient on plant community structure

Vegetation data including plant cover, biomass, species richness, and vegetation height was sampled on a copper-contaminated field with total copper contents varying from 50 to almost 3,000 mg/kg soil. The field was covered by early succession grassland dominated by Agrostis stolonifera. Plant cover, biomass, species richness, and vegetation height generally decreased with increasing copper content, although the highest biomass was reached at intermediate copper concentrations. Multivariate statistical analyses showed that plant community composition was significantly correlated with soil copper concentration and that community composition at soil copper concentrations above 200 mg/kg differed significantly from community composition at lower copper levels. Comparison of single-species (Black Bindweed, Fallopia convolvulus) performance at the field site and in laboratory tests involving field soil and spiked soil indicates that the laboratory tests conventionally applied for risk assessment purposes do not overestimate copper effects. Interaction between copper and other stressors operating only in the field probably balance the higher bioavailability in spiked soil.

Simulations of the predator-prey interactions in a two species ecotoxicological test system

Abstract A mathematical simulation model (Gutierrez type) has been developed to investigate the predator-prey interactions in a two species ecotoxicological test system. The test system consists of the predacious mite ( Hypoaspis aculeifer Canestrini) and, as prey, the collembolan Folsomia fimetaria L. in a microcosm (5.5 cm high × 6.0 cm in diameter) with 30 g humidified soil. F. fimetaria is provided with bakers yeast as food source. The model simulates (i) the population development of the test animals in synchronous cultures before the test, (ii) the selection of the test animals from the synchronous cultures and (iii) predator-prey interactions with and without the presence of dimethoate during the test and extraction periods. The driving variable is the temperature dependent food demands of the organisms. Both species were simulated with separate male and female populations, which were divided into a number of life stages. Thus the model handles very complicated predator-prey interactions taking the different interactions between different instars into consideration. The simulated test results were found to be very sensitive to changes in predator search rate, capture efficiency, habitat overlap, the amount of stored reserves of H. aculeifer at test start and the survival and reproduction of F. fimetaria . The simulations constitute the foundation for suggestions for further improvement of the test system.

Host-parasitoid interactions in an agricultural ecosystem: a computer simulation

Abstract The interaction between the pod gall midge (Dasyneura brassicae Winn.) and its most important parasitoid (Platygaster oebalus Walk.) in rape fields was investigated by aid of a computer model. The model is driven by temperature and the population dynamics is modelled by a distributed delay procedure. A relatively large program procedure was used to describe the functional response. Simulation of the population dynamics over 10 years demonstrated that the parasitoid gets the highest population if it exploits the host egg masses lightly, i.e., aggregated response would reduce the population size. The model was used to investigate how vulnerable the populations of both host and parasitoid are to insecticide applications, and was able to explain a change in emergence pattern of P. oebalus which seems to have occurred in Danish rape fields.

Using insect traps to increase weaver ant (Oecophylla longinoda) prey capture

Weaver ants (Oecophylla spp.) are managed in plantations to control insect pests and are sometimes harvested as a protein‐rich food source. In both cases, the amount of insect prey caught by the ants is imperative for returns, as more prey leads to more effective biocontrol and to a higher production of ants. Malaise‐like traps placed in trees may catch flying insects without catching ants, as ants may use pheromone trails to navigate in and out of the traps. Thus, ants may increase their prey intake if they are able to extract insects caught in traps. In a mango plantation in Tanzania, we estimated the amount of insects caught by simple traps (cost per trap = 3.9 USD), and whether Oecophylla longinoda was able to collect insects from them. On average, a trap caught 110 insects per month without catching any weaver ants. The number of insects found in traps with ant access was 25% lower than in control traps (ants excluded), showing that ants were able to gather prey from the traps. Ant activity in traps increased over time, showing that prey extraction efficiency may increase as ants customize to the traps. The prey removed from traps by ants constituted 5% of the number of prey items collected by O. longinoda under natural conditions (without traps), potentially increasing to 14% if ants learn to extract all insects. Thus, prey intake may be increased with 5–14% per 3.9 USD invested in traps. These numbers increased to 38 and 78%, respectively, when light was used to attract insects during night time. Combining ant predation with insect trapping is a new approach potentially building increased returns to ant biocontrol and to ant entomophagy.

Analysis of the populations dynamics of the pod gall midge (Dasyneura brassicae Winn.) in winter rape and spring rape by computer simulation.

Abstract The population development of the pod gall midge ( Dasyneura brassicae Winn.) in winter rape and spring rape ( Brassica napus L.) is described by a computer model. The model is temperature driven and the population dynamics is described by a distributed delay procedure. Simulation by the model was found to be a useful tool to gain more information from a life-table investigation than can be obtained by traditional life-table analysis. Further the model was able to quantify the consequence of a density-dependent process in the life cycle. The output from the model was very sensitive to changes in egg-larval mortality. The strong sensitivity was largely due to an inversely density-dependent mortality later in the life cycle. This was the pre-cocoon mortality, that most likely is caused by natural enemies.

Optimizing methods for rearing mated queens and establishing new colony of Oecophylla longinoda (Hymenoptera: Formicidae)

Oecophylla spp. are used as biocontrol agents for many types of insect pests. A large and stable population is essential for effective control of pests. Colonies of Oecophylla spp. can be transplanted from wild habitats into orchards. Transplanted colonies can only survive in the presence of egg laying queens. It is difficult to locate nests with egg laying queens in large colonies that may sometimes contain more than 100 nests. Therefore, the need to explore and develop methods for rearing newly mated queens in nurseries may not be over emphasized, hence the current study. In the first experiment, we tested three rearing methods on queen survival and colony establishment. In the second experiment, we compared feeding techniques of different weaver ants on young colony growth. We observed that queens were best reared under continuous, indirect access to water. The first workers emerged earlier (32 days on average) in indirect and direct continuous access to water methods than on limited access to water (sprinkled) (38 days on average). Moreover, rearing mated queens under continuous indirect access and continuous direct access to water methods saved labour and time, because of limited attendance to the colonies. Availability of water, sugar solution and different sources of protein throughout improved the growth of young colonies. Likewise, the number of workers increased rapidly. Therefore rearing mated queens in nurseries is possible and would minimize hustles in hunting for the colonies and their queens in the wilderness.

Search Rate and Functional Response of a Eusocial Insect (Oecophylla longinoda) in a Tanzanian Mango Orchard

Weaver ants, Oecophylla spp., are famous for being efficient biological control agents as they prey on a variety of insects, and they are capable of suppressing a large number of pest species. Here, the search rate and functional response of Oecophylla longinoda were investigated in a Tanzanian mango orchard using feeding experiments. This was done by following the removal of prey, which constituted the foundation for estimating the search rate by aid of the Nicholson-Bailey and Lotka-Volterra models. The overall mean search rate was and over 30 minutes, when calculating the search rate using the Nicholson-Bailey equation and the Lotka-Volterra equation (modified Holling equation), respectively. The functional response investigations showed a linear relationship between removed prey and available prey, suggesting type I functional response or, alternatively, the initial phase of type II functional response. The results presented here are probably the first attempt to identify the functional response type of a colony of living predatory eusocial insects.