Location: Southern Insect Management ResearchTitle: Characterization of the spatial distribution of alfalfa weevil, Hypera postica, and its natural enemies, using geospatial models
|SHRESTHA, GOVINDA - Oregon State University|
|RIJAL, JHALENDRA - University Of California - Cooperative Extension Service|
|Reddy, Gadi V.P.|
Submitted to: Pest Management Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/18/2020
Publication Date: 9/25/2020
Citation: Shrestha, G., Rijal, J., Reddy, G.V. 2020. Characterization of the spatial distribution of alfalfa weevil, Hypera postica, and its natural enemies, using geospatial models. Pest Management Science. https://doi.org/10.1002/ps.6100.
Interpretive Summary: Alfalfa is the most important forage plant worldwide. Alfalfa is considered a superior feed for livestock as it is quickly digestible, high in protein and cell solutes, and a rich source of minerals and vitamins. In the United States, alfalfa ranks as the fourth most economically important crop, with an estimated annual value of nearly US $8 billion Our study is the first to demonstrate that alfalfa weevil samples are spatially dependent on average distances of 22 m for larvae and 16 m for adults. The integration of range value information into sampling methods may help to improve current alfalfa weevil scouting and monitoring programs. We recommend for the use of a minimum 23 m distance and take 20 sample points in a grid when conducting alfalfa weevil sampling by using either sweep net or by damaged stems counts. This study also indicated that alfalfa weevil larvae and adults are aggregated within the average distance of 22 m, and 16 m, respectively. Thus, it allows the alfalfa producers to know the weevil population hot-spots within the field. In such situation, application of pesticides only on those hot-spots may help to reduce the amount of pesticide use if the sampling warrants to use insecticide only to those high pest density areas. This type of site-specific management is important for reducing the environmental pollution, conserving biological control agents and pollinators, reducing the occurrence of pesticide resistance, and reducing pesticide expenses.
Technical Abstract: Understanding the spatio-temporal dynamics of prey and predator distributions can provide valuable insights into pest management strategies and conservation of natural enemies in agro-ecosystems. The alfalfa weevil, Hypera postica (Gyllenhal), is an economically important pest of alfalfa throughout the western United States. Coccinellidae (Coccinella spp.) and Nabidae (Nabis spp.) are among the most important natural enemies of this species, contributing to the biological control of H. postica in alfalfa fields. The spatio-temporal dynamics of H. postica and these two predator groups were investigated using 81 (= 9 x 9 grid) sample points in each of five alfalfa fields in north-central Montana. The data were analyzed using variogram and Spatial Analysis by Distance Indices (SADIE). Variogram analysis revealed the spatial dependence (aggregation) of H. postica in 17 of 19 sampling times for larvae, and 3 of 12 sampling times for adults. Using SADIE, statistically significant aggregation distribution was evident in 4 of 19 sampling times for larvae, and 5 of 12 sampling times for adults of H. postica. Combined variogram and SADIE showed strong evidence of spatial aggregation of H. postica larval population (~95%) while a moderate level of aggregation in the adult population (~67%) of the sampling times analysed. The average aggregation distances based on the range value of the variogram were ~22 m and 16 m for larvae and adults, respectively. Based on variogram results, natural enemies, Coccinella spp. and Nabis spp. populations were found spatially aggregated in ~ 63% and 5% of the sampling times, respectively. SADIE further supported the variogram results as Coccinella spp. populations (~53% of sampling times) were aggregated in contrast withthe Nabis spp. populations (~6% of sampling times) in alfalfa fields. There was no evidence of significant spatial synchrony between H. postica and its predators, Coccinella spp. and Nabis spp. The possible implications of these findings for Integrated Pest Management (IPM) of alfalfa weevil populations are discussed.