Trabala vishnou gigantina Yang (Lepidoptera: Lasiocampidae) is one of the most important defoliators in China. Larvae of T. vishnou gigantina feed on tender leaves primarily causing damage to the crown layer. In the last few years, the use of single species plantations combined with poor management and a lack of adequate monitoring and early warning has allowed populations of this pest to erupt and cause serious damage in Hippophae rhamnoides L. forest stands in Wuqi County, Shaanxi Province, China.
Spatial distribution is one of the most important properties of insect populations. It provides dynamic information about insect populations, describes individual associations, and allows the prediction of variations in spatial distribution patterns. Geostatistical methods provide very powerful tools for explaining the spatial distribution of many species. These methods allow both the direct measurement of spatial distribution of organisms and tracking of changes in distribution over time because the exact location of the species has been documented at a specific point in time.
To further investigate spatial distribution and effectively control this pest species, spatial distribution patterns of T. vishnou gigantina pupae were analyzed using geostatistical methods in pure H. rhamnoides forests with different slopes and densities. Results show high levels of pupating occurred mainly in mid-August. Also, males pupated 3.7 days earlier than females. Pupae were also widely distributed on southeastern facing shady slopes, and on west and north facing sunny slopes. Pupae gathered mainly at heights of 30-75 cm on trees. Analysis of isotropic variograms indicated T. vishnou gigantina pupae spaced themselves 95.609, 47.379, 79.238, and 42.415 m apart in densely forested shady slopes, open forests on shady slopes, densely forested sunny slopes, and open forests on sunny slopes, respectively, while the intensity of local spatial continuity was 0.760, 0.561, 0.649, and 0.407, respectively, in the above stands. Also, semivariogram analysis revealed the Gaussian model had the best fit to T. vishnou gigantina pupae distributional patterns in dense forests, implying an intense spatial aggregation pattern, while the linear model fitted best with the open forest, implying a random distribution. As the maps with Kriging interpolation described, T. vishnou gigantina pupae exhibited an edge effect and an obvious directional distribution; the pupae had a clustered distribution at the edge of the plantation and occurred in point locations scattered inside the plantation. Numbers of clustered plaque gradually declined toward the center of the forested area. This was reflected in the tendency of the pupae to cluster together on a spatial scale, and in the direction of their proliferation.
Currently, T. vishnou gigantina infestations are most commonly controlled using chemical pesticides. However, H. rhamnoides is an economically and ecologically important plant, so residues need to be considered when spraying pesticides. Based on the spatial distribution characters, we suggest picking the pupa manually on the sunny slopes and especially on the edges of seabuckthorn plantations at a height of 30-75 cm to limit reproduction of this pest species. Research results demonstrate the spatial distribution patterns of T. vishnou gigantina pupae in pure H. rhamnoides forests, providing a theoretical basis and practical guidance for controlling populations of this insect pest. This research will provide a meaningful foundation for integrated pest management.