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Biological studies to optimise non-chemical control of insect pests in stored grain - CE0309

Description
Project Code: CE0309

Funded By Department for Environment, Food and Rural Affairs

Lead Research Centre
Central Science Laboratory,
Sand Hutton,
York
YO4 1LZ

Abstract of Research Proposal:
To provide the necessary background knowledge on the biology and population dynamics of storage insect pests in order to improve their control by physical and other non-chemical methods.

Knowledge of insect biology, particularly near the edges of growth limits, is the basis of physical control which is increasingly used as a replacement for pesticide application owing to concerns about residues in food, pesticide use generally and pesticide resistance. Physical methods can be less costly than chemicals and can lead to greater profitability but this does not depend on achieving the same freedom from pests. These non-chemical control measures will be vital to ensuring continued competitiveness in world markets. However, greater knowledge of biology of pest species is needed to replace pesticide use with physical methods. This would also be relevant if organic production, with non-chemical storage requirements, increases.
Biological data allows appropriate store management conditions to be derived for the mixed infestations often present. Information on speed of pest development tells us how quickly grain needs to be cooled or dried. Information on growth limits is used as guidance on target temperatures while information on survival times outside these limits determines how long grain has to be stored for disinfestation. A detailed knowledge of the dynamics of pest development, survival and fitness will be vital to the development of overall strategies from other CE03 projects.

Summary of Objectives

To provide background knowledge on the biology and population dynamics of storage insect pests in order to improve physicl and other non-chemical control of storage pests.

1) To compare background knowledge on the biology and population dynamics of storage insect pests in order to improve physical and other non-chemical control of storage pests.
2) To identify the low oxygen levels required for storage life protection against the range of common pests using modified atmospheres under controlled conditions.
3) To make preliminary assessment of the natural occurrence and potential of biological control agents as part of an integrated storage strategy.
4) To review the field of biological control as part of an integrated cereal storage strategy from 1995-1999.


Executive Summary of Final Report from 98/99 annual report:
Complete review of pest incidence and sources of infestations in UK stores:
Changing patterns of infestations and origins of infestations are important in determining future threats to commodities and tracing their origins so that they can be treated at source. The most recent comprehensive exercises to monitor infestation rates in farm and commercial stores in England and Wales was completed over 10 years ago. However, these inspections were only concerned with incidence of the primary or major cereal pests; a number of other pest species were regarded as incidental and were overlooked. It was apparent that the major pests were five times more common on mixed farms than those that were arable only. Major pests occurred in about 10% of farm stores, 30% of commercial sores and 70% of feed mills. Commercial stores have a higher pest incidence because they accept grain from many farms. Since a substantial proportion of infested commodities are imported, some of which may be used in animal feeds, it has been assumed that the pests on farms may be of foreign origin. Apart from P. tectus, which has been associated with bird nests, which may be a source of infestation, it is not clear whether farms or mills are the source of pest insects.

Establish the most susceptible life stages of 4 species of grain beetles to low oxygen atmospheres:
Complete mortality of insect through the use of controlled atmospheres (CA) requires an oxygen level at or below 1%. If these atmospheres are to be used for long term storage of grain it would be advantageous to be able to use atmospheres with higher O2 contents. These would prevent the growth of the insect population and allow the use of lower application flow rates which would reduce the costs of the operation. The most economical system generates the CA by the combustion of propane in air. AT the 1% O2 level this would also produce a carbon dioxide (CO2) level of 12.5%. If the O2 in the output rises the CO2 is reduced at a fixed rate.

Three different oxygen levels, 4, 5, and 6% and their respective CO2 levels , 8.5, 7.5 and 6.5% were used with a relative humidity of 85% to assess the effect of temperature, at 20 and 25oC, on the effectiveness of these atmospheres for storage life protection. Four species of insect were used, Oryzaephilus surinamensis, Sitophilus granarius, S. oryzae and Tribolium casteneum. As the adult stage is the most likely to invade new grain bulks the work concentrated on their susceptibility and ability to oviposit.

3 replicates of 30 adults were exposed to each of the atmospheres at each set of conditions for a month. Adult survival in all species was not appreciably affected until the O2 level decreased to 4%. The most tolerant was S. granarius with mortalities of 9.9 and 41.9% at 20 and 25oC respectively. This was followed by S. oryzae which produced 1 and 304 while S. granarius had 0 and 21. O. surinamensis was only able to produce offspring at 6% but then only 4 and 8. Each of the species was able to produce progeny after exposure to the atmospheres.

Complete review of Bacillus thuringiensis and parasitoid presence in the UK
The literature is peppered with publications regarding the toxicity of Bt to insects. There appears to be no doubt that Bt can be used to kill arthropods, including pests of stored commodities and primarily Lepidopteran pests such as Ephestia elutella, E. cautella and Plodia spp. However, the vast majority of the published work uses existing isolates and does not outline their origin. There is one major paper on the isolation of 1500 strains of Bt from UK grain stores and feed mills. The strains have not been fully evaluated and the authors will be contacted for further information discussion.

The presence of Bt in samples of Egyptian flour has been demonstrated. The bacterial found numbered 1.9x106/g, of which 10% were spore forming. Microbial contamination of German grain and grain products numbered 3.6x105 - 2.4x107/g grain and 0.6 x 105- 1.1x108/g grain products. Of these, less than 1% were spore-forming products numbered 3.6x105 - 2.4 x107/g grain and 0.6 x 105 - 1.1x108/g grain products. Of these, less than 1% were spore-forming bacteria and were identified as Bacillus thuringiensis and B. cereus. The toxicity of two isolates of Bt from Egyptian stored-grain dust were demonstrated against Ephestia cautella and Tribolium confusum.

A less direct indication of bacterial presence on grain was obtain from samples of settled and air-borne dust from four large grain elevators in Louisiana. Over half (55%) and 16% of the samples of air-borne dust of the settled-dust samples contained Bt. Of the 255 Bacillus colonies studied, 30.9% were Bacillus thuringiensis and of these 94.9% were of the variety aizawai. In amongst the samples was on isolate which proved to be a new serovar. The absence of information on the occurrence of Bt in UK stores serves only to prove that no one has looked for isolates in this. However, the limited information from other countries shows that it is possible to isolate colonies from grain and grain dusts.

The Hymenopterous parasitoids that occur naturally in grain storage are seldom collected for identification and recorded only to taxonomic Order, mainly because identification requires specialist knowledge and nomenclature can be confusing. The presence of parasitoids was completely disregarded in the most recent surveys of pests on farm (1987) and commercial grain stores (1988-9). However, the potential incidence of parasitoids has been assessed to host pest species that are known to occur on storage premises. The published studies in the use of parasitoids to control storage pests have also been consulted, but the number of species attacking Coleopteran pests is limited to the following species listed:

Lariophagus distinguendus Forster (Hym.: Pteromalidae)
Anisopteromalus calandrae Howard (Hym.: Pteromalidae)
Choestosphila elegans Westwood (Hym.: Pteromalidae)
Cephalonomia watersoni Gahan and C. tarsalis Ashmead (Hym.: Bethylidae).
Dibrachys cavus (Walker) (Hym.: Braconidae)

The number of parasitoids known to attack moth pests is more extensive than those of beetle pests. These tend to be either Braconids for Ichneuminids.
Venturia canescences Gravenhorst (Hym.; Ichneumonidae)
Bracon hebator Say (Hym.: Braconidae)
Trichogramma evanescens Westwood (Hym.: Trochogrammatidae)

Other products associated with stored product moths hosts are:
Braconidae: Apanteles carpatus Say, Chremylus elatus Haliday
Ichneuminidae: Hypsicera cuvator (F.) Campoletis viennisis (Gravenhorst),
Acrotomus sccinctus (Gravenhorst), Ascogaster rufidens Wesmael,
Diadegma armillata Gravenhorst

Complete studies comparing productivity and survival of O.surinamenis on grain under variable physical conditions:
Most models use biological data based on artificial diets which probably give an elevated impression of the infestation risk for untreated grain. These studies were intended to give a realistic idea of the risk. It was originally intended to use field strains, collected as part of CE0312 for this study. However, initial samples contained insufficient insect numbers to establish viable cultures. Experiments therefore have commenced using a laboratory study strain of O. srinamensis that had been cultured on wheat for several generations.

The proportion of dockage from farm grain samples was determined and used as the basis for the proportion of damaged kernels (5%) for these experiments. Groups of 50 adult O. surinamensis (equal sex ratio) were placed into 20ml jars containing the appropriate wheat sample. Insects subjected to lower temperatures were acclimated at a rate of 2.5oC per day. Adults remained on the diets for three weeks before removal. Progeny were counted after a further 6 weeks to avoid the F2 generation being included. Longer periods were used for the lower temperatures.
No productivity was observed at 17.5oC, 50, 60 and 70% r.h. on either the wheat or control diet. At 25oC, productivity was substantially lower on wheat than the control diet (991 on the control diet vs. 22 on wheat at 60% r.h.; 1275 on the control vs. 38 on wheat at 70% r.h.).
The delay in the above studies has permitted extra work on parasitoids. Variation in attack rate of different hosts and stages may be owing to, in part, to the inability of the female parasitoid to subdue larger hosts prior to oviposition. Cultures of L. distinguendus appear less productive on a large granary weevil strain (mean weight = 4.17mg) than on a normally sized strain (mean weight = 2.35mg). To assess this hypothesis, different developmental stages of these weevil strains are being compared prior to quantitative experiments using L. distinguendus. Weevil adults (~40 of each sex) are allowed to oviposit for 24h on 5g of at 25oC, 70%r.h., before infested kernels are detected using an acid fuchsin stain and the developmental stage dissected out and weighed. The larval head capsule width was measured using an eyepiece graticule calibrated against a stage micrometer.

The daily productivity of both host strains has also been examined at different densities for subsequent use in parasitoid experiments. At low densities (up to 20 females per 5g wheat) productivity of the normal strain is only about half of that of the large strain. Above this density the difference is almost negligible presumably as a result on disturbance and the limit on the number of kernels available.

Experiments have been set up to determine the attack rate of individual L. distinguendus females on the pupal stage (age 28-29 days) of both weevil host strains. Male parasitoids are much shorter lived than females and all had died within a week. However female longevity was longer than the published data for this species and exceeded the availability of known age hosts. Parasitoid progeny started to emerge after about 14 days with a definite bias in favour of females.

Funding
£293,884

Start date: 01/04/98
Completion date: 21/04/02



Project Documents
• Final Report : Biological studies to optimise non-chemical control of insect pests in stored grain   (1721k)
Time-Scale and Cost
From: 1998

To: 2002

Cost: £305,267
Contractor / Funded Organisations
Central Science Laboratory
Keywords
Grain Pests              
Pest and Weed Control              
Pest Control              
Plants and Animals              
Fields of Study
Arable Crops