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Rural Industries Research & Development Corporation
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November 2001
RIRDC Publication No 01/136 RIRDC Project No DEB-1A
Horace writes in the Epistles: “you may drive out nature with a pitchfork, yet she’ll be constantly running back”. The essence of this quote is the key result of this research project. Management of lucerne seed wasp necessitates conceding that this pest can not be eradicated from the Australian lucerne seed production industry. Industry must learn to live with its presence and accept that a degree of damage is a normal aspect of lucerne seed production and that techniques such as sanitation and crop closure timing reduce the pest’s impact.
The research quantifies the economic impact of the wasp as well as providing evidence that sanitation practices and manipulation of crop closure timing reduce the impact of lucerne seed wasps on seed production. Financial losses attributable to seed wasps have been blindly accepted by industry over many years due to a combination of a lack of understanding of the damage caused by this small pest and, more particularly, a belief that spraying regimes have provided adequate control whereas they acted more as a placebo than anything else.
Chemical control of seed wasp is not a viable or sensible option and does not form part of its management strategy. The wasp is exposed for 6 months of the year, through the dryland and irrigated seed production season, to insecticides used in commercial seed production to manage other pests.
Seed wasp resistance to insecticides including malathion, chlorpyrifos, a range of sythetic pyrethroids and endosulfan is commonly reported in the seed producing areas of South Australia and New South Wales. A similar scenario exists in lucerne seed producing areas of North America, where district wide sanitation practices are encouraged.
Producers of irrigated lucerne seed in the south east of South Australia experience financial losses of A$2.0 million per season and the Australian lucerne seed export industry loses A$5.18 million in potential exports. The quantified understanding of damage to seed production by this pest enables the Australian lucerne seed industry to determine whether a concerted effort to manage this pest and reduce its impact on seed production will be developed. The economic impact of this pest should direct the industry to promote the recommendations of this research.
Seed wasp population development is correlated closely with the presence of available host as wasps develop peak populations in response to quantity of available host at the susceptible stage. Whilst seed wasp presence has not been eradicated, sanitation practices such as mowing, grazing and herbicide spraying significantly reduce wasp presence in the seed crops by delaying population development.
Australian lucerne seed producers can individually and in co-operation with adjacent landowners implement simple sanitation practices on a wide scale level to reduce seed wasp populations and hence the damage to seed crops. In combination with alterations in crop closure timing, both sanitation and closing date operate synergistically to diminish seed wasp populations.
The Australian lucerne seed production industry should encourage allies in the industry to assist in the management of seed wasp. Feral lucerne on roadsides and in wastelands such as vacant land blocks or council areas could be strategically managed to reduce wasp population development. Offal dumping in refuse tips should be monitored and managed by council or private owners. Graziers with lucerne for purposes other than seed production should be encouraged to reduce the incidence of lucerne setting seed in pastures in the context of stock management as well as community benefits.
The majority of irrigated seed producers prepare lucerne for seed production in December to have crops flowering in January/February when the warm/hot weather is optimal for pollination. However this management strategy causes maximum wasp damage as the wasps have developed populations in earlier flowering lucerne since hatching in October. Seed wasp population development is correlated closely with the presence of susceptible available host. The wasp develops its population peak in response to quantity of available host at the susceptible stage rather than any environmental condition such as day length or temperature. A key to reducing damage is to have the crop completed flowering before the wasp population reaches its peak in earlier flowering hosts.
Producers need to assess their management concepts, a practice that has slowly commenced in the late 1990’s but chiefly driven by the crops irrigation demands and irrigation restrictions. Closing a crop 2 –4 weeks earlier than normal permits harvest at 100% ripeness as opposed to 80-90% ripeness later in the season prior to autumnal rainfall. Seed wasp damage is reduced by earlier crop closure. Grazing management and weed control strategies are assisted by earlier closure with least impact on lucerne plant health. Aspects such as these need to be addressed by seed producers to compare the economics of management decisions that influence the susceptibility of seed crops to seed wasp damage in conjunction with yield returns.
The discovery of two species of wasps parasitic on the seed wasp requires further research, as both are potential biological control agents. Little is known about the biology of these wasp parasites but industry would need to determine if they considered that the economic impact of the seed wasp warranted further research.
There is not a lot of scope to the management of the lucerne seed wasp. Sanitation and crop closing date are the two fundamental parameters. Potentially the biological control option could be applied if industry funding is made available to commence the required preliminary research. The wasp is a highly evolved parasite with a life history that allows seed producers only the ability to manipulate the size of its population development. Unlike other pests of lucerne such as Helicoverpa puntiger, the wasp does not develop populations in defined intervals such as every 3-4 weeks. The wasp has continuing, overlapping generations, which means that all stages of the life cycle are present at any one time, and a spray to kill the adult will have no effect on the pupae which will imminently hatch after the spray has been applied. Hence manipulation of this pest’s host is currently the primary way to manage its impact on seed production.
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