|Archived at http://orgprints.org/3883|
Host potential of insects from cruciferous crops to entomopathogenic nematodes and augmentation of nematodes through oil seed rape growing
Holger Philipsen, Otto Nielsen
Zoology Section, Department of Ecology, Royal Veterinary and Agricultural University, Copenhagen, Denmark
Abstract:Larvae of the pollen beetles (Meligethes spp.), the brassica pod midge (Dasyneura brassicae), the cabbage seed weevil (Ceuterrhynchus assimilis), and the cabbage moth (Mamestra brassicae) were collected in the field and exposed to entomopathogenic nematodes in the laboratory. Pollen beetle larvae, seed weevil larvae and cabbage moth larvae where all relatively easy infected while infections of pod midge larvae only were observed twice. The number of nematodes produced in the cadavers was positively related to the size of the insects. The recovery of D. brassicae and Meligethes spp. was further studied after exposure to entomopathogenic nematodes during the pupating process. The recovery of D. brassicae was almost unaffected by the nematodes whereas recovery of Meligethes spp. decreased with increasing number of nematodes. The susceptibility of oil seed rape insects to entomopathogenic nematodes was indirectly demonstrated under field conditions. Here soil grown with oil seed rape for three years was compared to soil grown with grain by baiting with Tenebrio molitor. While entomopathogenic nematodes were almost absent in soil grown with grain, almost all samples were positive for nematodes in soil grown with oil seed rape.
Keywords: Host potential, soil-pupating insects, entomopathogenic nematodes, Ceuterrhynchus assimilis, Dasyneura brassicae, Mamestra brassicae, Meligethes spp..
Entomopathogenic nematodes of the genera Steinernema and Heterorhabditis occur in the soil as non-feeding infective juveniles. Since they are obligate parasites of insects, they depend on the presence of these for completion of their life cycle. The focus of this work was on the host potential of soil-pupating insects living in oil seed rape, ranging from tiny gall midges to large noctuid larvae.
The main focus was on the pollen beetle (Meligethes spp.), and the brassica pod midge (Dasyneura brassicae). Both of these species are in most years very numerous in oil seed rape and even though the insects are relatively small, the sum of biomass passing through the soil may be of significance to a nematode population. The larvae of the pollen beetle feed on pollen of oil seed rape plants until it has reached a size of approximately four millimetre. Feeding by the adults may cause damage to the plants, dependent on the developmental stage of the plant, as open flowers are not damaged. Dasyneura brassicae larvae live within the pods and may cause severe damage due to the destruction of the pod walls. The larvae enter the soil for pupating when they are approximately two millimetre long.
Additional insects studied were the cabbage seed weevil (Ceuterrynchus assimilis) and the cabbage moth (Mamestra brassicae). The cabbage seed weevil larvae live within the pods of oil seed rape and reach a size of approximately 5 milimetre. The cabbage moth is a relatively large insect (approximately 40 millimetre) and has as such a great potential for propagation of nematodes. The cabbage seed weevil and the cabbage moth are normally of minor importance as pest insects.
Little information is so far available about the interactions of entomopathogenic nematodes and the insects studied in the present work. Also information about soil-pupating insects are in general rare, but several experiments have been made especially with the saw flies (Battisti, 1994; Georgis & Hague, 1988; Mrárek & Spitzer, 1983). Most of the insects in our work were relatively small. The knowledge on the success of nematode infections in small insects is limited. The smallest insects studied are to the authors knowledge Franklieniella occidentalis (Tomalak, 1994), sciarid flies (Gouge & Hague, 1995), second instar Otiorrhyncus sulcatus larvae (Kakouli-Duarte & Hague, 1999) and first and second instar Delia radicum larvae (Bracken, 1990).
The aim of our work was therefor to study the host potential of these soil-pupating insects to entomopathogenic nematodes. This included the estimation of mortality and infection rates and nematode producing capacity of the different larvae. Further was the effect of oil seed rape growing on entomopathogenic propagation studied under field conditions
Materials and methods
Sampling of larvae
Pollen beetle larvae, cabbage stem weevil larvae and pod midge larvae were sampled by the collection of oil seed rape plants in June and July. The plants were laid in heaps of approximately 30-40 plants on large pieces of plastic. After a period of time, larvae could be collected from the plastic surface by lifting the plants. Cabbage moth larvae were sampled directly in the field. For all species, only the most active larvae with maximum sizes were used.
Mortality rates, infection rates and nematode production in insects
The larval stage of the insects were exposed single or in groups to different levels (0-200 nematodes per larva) of entomopathogenic nematodes. The tests were carried out at room temperature (20-24 ° C) in either petri dishes (10-cm-diameter) lined with filter paper or in small cups (height×diameter = 42×30mm), jars (height×diameter = 9.5×6.5cm) or boxes (height×width×length = 6×17×22 cm) with soil (heat sterilised sandy loam with a water content of 11 %). Petri dishes were kept separately in plastic bags to avoid evaporation, while cups, jars and boxes were closed with lids. Larvae that died during the experiments were placed individually on water traps and emerging nematodes were counted.
Recovery of pod midges and pollen beetles after exposure to nematodes
The larval stage of the insects were placed individually in cups with 10 ml soil (see above) and different numbers of nematodes. The cups were closed with lids and left at room temperature (20-24 ° C) for approximately three weeks after which they were stored at 5 ° C until extraction of the insects from the soil. Cups with pod midges had been supplied with a piece of sticky material on the inside of the lid to catch emerging adults. The soil from each individual cup from where no adults were visible was then rinsed on a sieve and poured into a tray with water.
The effect of oil seed rape growing on entomopathogenic nematode propagation
These experiments were carried out at the experimental site Højbakkegård. An area of 114´ 36 meters was grown with oil seed rape for three subsequent years. In the autumn of the last year 64 soil samples were taken. Each of these samples consisted of 40-50 soil cores (1´ 10 cm) sampled within an area of 2´ 2 metres. In addition, another 64 similar samples were taken on both side of the area grown with oil seed rape in an area that had been grown with grain. The samples were taken to the laboratory and a subsample from each sample was baited with six Tenebrio molitor larvae in a 10-cm-petri dish for a period of one week. Dead larvae were placed on water traps to reveal infections with entomopathogenic nematodes.
Results and discussion
An overview of the results from the host potential experiments is given in Table 1. The highest mortality rates were observed with seed weevil larvae (93 %) followed by cabbage moth larvae (81 %) and pollen beetle larvae (56 %). The pod midge larvae were apparently unsusceptible to entomopathogenic nematode with mortality rates of 0-2 %. Also infection rates for the pod midges were low (0-1 %, only two infected individuals observed) while infection rates reached 81 % for the cabbage moth larvae, 75 % for the seed weevil larvae and 40 % for the pollen beetles. The production of nematodes in the cadavers was related to host size. A maximum of 2675 (mean around 1300) was observed in the seed weevil and a maximum of 2150 (mean around 1000) in the pollen beetle. The maximum production in the cabbage moth was 95750 (mean of 47000).
Table 1. Mortality rates, infection rates and nematodes produced per larva for different insect species collected from oil seed rape (M. brassicae was collected on cabbage). The numbers are minimum and maximum numbers obtained in different experimental designs (petri dishes with filter paper or small containers with soil) with different isolates and numbers of entomopathogenic nematodes (Steinernema and Heterorhabditis).
Insect specie Mortality Infection Nematodes
rates (%) rates (%) per larva_ _
Pod midge (D. brassicae) 0-2 0-1 2-71
Seed weevil (C. assimilis) 55-93 22-75 150-2.675
Pollen beetles (Meligethes spp.) 16-56 0-40 100-2.150
Noctuids (data for M. brassicae) 81 81 4.800-95.750
Figure 1. Recovery of pod midge (left) or pollen beetle (right) pupae or adults after exposure to different levels of S. feltiae during the pupation process.
The recovery experiments were based on experiments in small cups with moist soil and different numbers of nematodes. The recovery of the pod midges was more or less unaffected by the presence of the nematodes The recovery of the control larvae was around 80 % while the minimum recovery of exposed larvae was around 60 % (Figure 1, left). The recovery of pollen beetles was, however, dramatically affected (Figure 1, right). The lowest exposure level reduced the recovery from around 75 % in the control to around 50 % and less than 10 % of the insects were recovered at the highest exposure level.
Pollen beetles are often very numerous in oil seed rape and the high susceptibility to entomopathogenic nematodes could have an effect on nematode augmentation. This is indicated by the results obtained from the baiting of soil grown with oil seed rape or grain (Figure 2). Entomopathogenic nematodes were almost absent in the area grown with grain while almost all samples were positive in the area grown with oil seed rape. The overall conclusion of the present work is thus that several insect species from oil seed rape are potential host for entomopathogenic nematodes and may contribute to nematode augmentation in the field.
Figure 2. The effect of oil seed rape growing on entomopathogenic nematode propagation in the field as estimated by baiting with Tenebrio molitor. The two middle sections of the field had been grown with oil seed rape for three years prior to baiting while the surrounding area was grown with grain. A bar represents that one or more bait larvae was infected by nematodes. The total size of the experimental area was 114´ 72 meters.
We wish to thank E-Nema, Kiel, Germany and Reitzel, Copenhagen, Denmark for providing nematodes. The technical assistance of Pernille Frandsen is highly appreciated. The work was financially supported by the Danish Ministry of Agriculture (Contract no. BIO 96-KVL-1).
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