Tri-Trophic Level Interactions of T. urticae and Beneficial Inoculants in Three Strawberry (Fragaria ananassa) Varieties

Tri-trophic level interactions between beneficial soil inoculants and plant canopy arthropod pests have been established for some horticultural systems. However,, the live microbial activity of these inoculants as opposed to the combined microbial and nutritive effect has not been investigated for strawberry systems. The potential for microbial activity in sustainable production systems  to impact both plant growth and pest populations could be significant for strawberry growers who are able to manage these interactions to their benefit. The use of live arbusuclar mycorrhizal fungi plus vermicompost (AMF+VERM)  and vermicompost (VERM) singly and their respective steam-sterilized controls were used CRBD to test the microbial activity of these inoculants on strawberry plant growth and the population growth of a key strawberry pest the two spotted spider mite (Tetranychus urticae). These beneficial inoculants were incorporated into the plug media during  strawberry tip establishment of the three strawberry varieties Chandler, Sweet Charlie and Albion. Each variety and soil inoculant combination later received both a spider mite (Mite+) and spider mite free (Mite-) treatment. Both the individual effects of variety and soil inoculant and their combined interaction on plant and arthropod growth were assessed. For plant growth VERM increased leaf area and total final biomass as compared to its control. AMF+VERM was not different from its control in terms of leaf area (P<0.8595) but did have greater whole plant biomass (P<0.0021). Besides an increase in phosphorus uptake by plants planted in AMF+VERM, soil microbes had little effect on plant nutrient uptake, which may indicate observed changes to plant growth are due to other compounds produced as a result of microbial activity. Under the conditions of our study Albion consistently supported the highest (P<.0001) T. urticae populations as estimated by both the calculations of cumulative mite-days and cumulative eggs.  Only for Albion was a soil treatment and variety interaction detected, with AMF+VERM supporting lower cumulative mites-days (P<.0234) and eggs (P<.0061) as compared to its control, while VERM increased cumulative mite-days (P<.0115) and eggs (P<.0060) as compared to its control. For Chandler and Sweet Charlie no significant difference between live soil inoculants and their controls for cumulative mite-days or eggs was detected. For the variety Albion the addition of live AMF and vermicompost appeared to infer a negative effect on T. urticae population growth and fecundity. Further understanding of the interaction of AMF and vermicompost on T. urticae populations is required.