_Applied_  
_sciences_  
_Article_  
**The First Evidence of the Insecticidal Potential of Plant Powders from Invasive Alien Plants against Rice Weevil under Laboratory Conditions**  
**[PERSON] 1,*, [PERSON] 2, [PERSON] 3, [PERSON] 3, [PERSON] 3 and [PERSON] 1**  
1 Department of Agronomy, Biotechnical Faculty, University of Ljubljana, Jamnikarjeva 101, SI-1000 Ljubljana, Slovenia, [EMAIL_ADDRESS] Scientific Research Centre of the Slovenian Academy of Sciences and Arts, Ivan Rakovec Palentological Institute, Novic tip 2, SI-1000 Ljubljana, Slovenia; [EMAIL_ADDRESS] Slovenian Institute of Hop Research and Brewing, Chemical Analysis and Brewing, Cesta Zalskega tabora 2, SI-3310 Zalec, Slovenia; [EMAIL_ADDRESS] (M.O. ); [EMAIL_ADDRESS] (I.J.K. ); [EMAIL_ADDRESS] (K.R.) * Correspondence: [EMAIL_ADDRESS]  
Received: 7 October 2020; Accepted: 2 November 2020; Published: 4 November 2020  
**Abstract:** In a laboratory experiment, we studied the insecticidal effects of invasive alien plants on the rice weevil. The research was carried out in two parts. In the first part, we studied the insecticidal properties of seven different plant species, namely, Bohemian knotweed (_Fallopia \(\times\) bohemica_), Japanese knotweed (_Fallopia japonica_), false indigo-bush (_Amorpha fruticosa_), tree of heaven (_Ailanthus altissima_), staghorn sumac (_Rhus typhina_), Canada goldenrod (_Solidago canadensis_), and giant goldenrod (_Solidago gigantea_). Mixtures of powders and wheat were prepared in two different concentrations, namely, 2.5 w% and 1.25 w%. The experiment was performed at temperatures 20 \({}^{\circ}\)C and 25 \({}^{\circ}\)C and at two humidity levels, 55% R.h. and 75% R.h. Very low mortality (below 8%) was found when using combinations with the higher relative humidity. No significant differences were observed between the effects of these concentrations. In the second part of the experiment, Norway spruce wood ash and diatomaceous earth (product SilicoSec(r)) were added to the powder obtained by milling leaves of four different invasive plant species (Canada goldenrod, staghorn sumac, tree of heaven, false indigo). In the independent application, wheat was added to the powder at a concentration 2.5 w%. In the treatments that involved mixtures of powder and wood ash/diatomaceous earth, we applied 1.25 w% plant powder and 1.25 w% wood ash or 1.25 w% plant powder and 450 ppm of a SilicoSec(r)preparation. The positive control was carried out as two separate treatments with 2.5 w% wood ash of Norway spruce and 900 ppm of the SilicoSec(r)product, while untreated wheat represented the negative control. The experiment was performed at two temperatures (20 \({}^{\circ}\)C and 25 \({}^{\circ}\)C) and two R.h. values (55 and 75% R.h.). The mortality of beetles was recorded on the 7 th, 14 th, and 21 st day after the start of the experiment. Higher mortality rates of rice weevil adults were found at the higher relative humidity, and an important factor of mortality was also the day of exposure, as a higher mortality was found when the exposure of individuals to the tested substances was for a longer time period. After 21 days at 25 \({}^{\circ}\)C and 55% R.h., the combinations in which the lower concentration of Norway spruce wood ash was added to the powder of invasive alien plants achieved more than 90% mortality of beetles. By adding the plant powder of invasive alien plants to wood ash, we achieved a greater insecticidal efficacy of invasive plants and lower concentrations of wood ash. Nevertheless, the results of our research do not indicate any great usefulness of the plant powder of invasive plants in suppressing the rice weevil. Additional studies should primarily focus on the insecticidal efficacy of powder from the genus _Solidago_, which in our study, displayed the greatest insecticidal potential among the tested invasive plants.