Instrumental economic injury level and economic threshold of spider mite populations on strawberries

Spider mites of the genus Tetranychus are universally one of the significant pests that damage garden strawberries and many other crops. One of the important problems of the pest population management is to be able to identify the thresholds of this pest, which, when feeding, causes a change in the color of plant leaves.

Since it is very difficult to determine visually the level of leaf damage by spider mites, the authors of the study turned to the previously substantiated instrumental method for assessing damage using the relative electron transport rate (rETR) based on chlorophyll fluorescence detection in the leaf. Instrumental thresholds of pest damage were calculated: the economic injury level (EIL) and the economic threshold (ET).

Studies were carried out in the laboratory on the leaves of strawberry grown in conteiners at temperature of 22 ± 2°C, relative humidity of 75 ± 10%, photoperiod (L : D) 16:8 h, as well as on the experimental plot of the strawberry plantation of JSC “Lenin Sovkhoz” (Moscow region); registration of the population dynamics of spider mites on the leaves being made. At the same time, using the JUNIOR-PAM fluorimeter (Heinz Walz, Germany), the dynamics of chlorophyll fluorescence in the leaves of the control (without mites) and experimental (damaged by mites) variants were measured.

As a result, two varieties of industrial strawberries obtained a range of variation of the numerical physiological index, expressed in terms of the ratio of rETRmax between the experimental and control variants, equal to 1.4–2.0 conventional units, which corresponded to the economic injury level (EIL) of strawberry leaves. The latter trait was comparable to the same parameter obtained in the laboratory – 1.6. The economic threshold (ET), which signals the beginning of pesticide treatments, if it is reasonably expected that the number of pests will reach or exceed the value of EIL, also expressed in terms of the ratio of rETRmax between the experimental and control variants, turned out to be equal to 1.1–1.2 conventional units.

The authors recommend using the proposed instrumental criteria for the harmfulness of the Atlantic spider mite on strawberries. We believe that this approach can be applied to other species of spider mites, as well as sucking insects.

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