Imidacloprid, the first neonicotinoid registered for use as an insecticide, is used worldwide against sucking and chewing insect pests in a variety of crops and environments. Some recent studies have suggested that imidacloprid can induce systemic resistance pathways, stimulate faster growth, enhance recovery from abiotic stresses, and increase yield when applied to plants growing under sub-optimal environmental conditions. The limited research has been conducted with agronomic and vegetable crops; few studies have addressed these effects in woody plants.
The central objective of this research was to investigate the physiological responses of woody plants to imidacloprid. Three studies were conducted under controlled-environment conditions in the summers of 2006 and 2007. The first provided evidence that imidacloprid (Merit® 2F) increased the growth of hybrid poplar (Populus nigra clone NC5271) through morphological and physiological effects that are independent of its insecticidal properties. Specifically, the growth enhancing effect of imidacloprid was attributed to increased total leaf area and total number of leaves and was expressed in stressful and optimal nutrient and watering regimes. Both above and below ground growth was increased, suggesting that increased leaf production did not come at the cost of decreased root production.
The second study showed that effects of imidacloprid formulations (Merit® 2F drench and Merit® FXT tablets) differed positively depending on species of plants and nutrient regime. Physiological parameters of hybrid poplar and burning bush (Euonymus alatus) treated with imicaloprid were not significantly different than those of untreated controls. However, the trend towards greater growth and biomass was consistent with that observed in previous experiments. Furthermore, the experiment was conducted over a shorter period of time, which may have precluded significant effects.
Finally, the third study tested the generality of patterns observed with hybrid poplar in the first experiment by extending the evaluation of the effects of Merit® 2F on physiology of burning bush, river birch (Betula nigra), and shrub willow (Salix sericea). Once again Merit® 2F had positive effects on the growth of hybrid poplar grown under optimal environmental conditions as well as the variation of these effects depending on the fertilization rate. The other three woody species also had greater growth when treated with imidacloprid, but the effects were not significant. Laboratory bioassays performed in this study to test the effects of imidacloprid on the performance of twospotted spider mite (Tetranychus urticae), imported willow leaf beetle (Plagiodera versicolora), and fall webworm (Hyphantria cunea) showed that imidacloprid increased the fecundity of twospotted spider mite on burning bush, but only in the low fertility treatment, raising egg production to the same level observed in the high fertility treatment. Imidacloprid decreased larval growth and survival of fall webworm on B. nigra, and P. nigra, and S. sericea and imported willow leaf beetle on P. nigra and S. sericea.