Biological control of fall webworm larva (Hyphantria cunea Drury) and growth promotion of poplar seedlings (Populus × canadensis Moench) with Bacillus licheniformis PR2

Abstract

The production of Populus × canadensis Moench seedling, as an important restoration resource has been constrained by the invasion of Hyphantria cunea Drury, a polyphagous pest that causes leaf feeding damage on nursery seedling production. This study investigated the larvicidal potential of cuticle degrading chitinase and protease enzymes secreted by the Bacillus licheniformis PR2 against H. cunea larvae. In addition, we investigated the growth promotion of P. canadensis seedling through the auxin belonging to plant growth hormones. Treatment of H. cunea larvae with either the bacterial crude enzymes or bacterial culture broth from B. licheniformis PR2 under laboratory conditions resulted in larvae mortality due to the severe degradation of the cuticle characterized by the loss of setae, swelling of sockets, and rupturing of the epicuticle and procuticle, and also caused a repellent effect from leaves, when the larvae suffered minor lacerations only in the epicuticle except the procuticle. Application of B. licheniformis PR2 culture broth on P. canadensis seedlings in the greenhouse experiment caused a mortality rate of 34.4 % and a repellency rate of 75.0 % against H. cunea larvae and reduced the leaf damage rate by 3.7-folds to only 25.9 % compared to 96.3 % in the control group. Accordingly, treatment with B. licheniformis PR2 improved seedling growth by reducing foliar damage caused by H. cunea larvae. In addition, B. licheniformis PR2 produced indole-3-acetic acid (IAA), which improves root hair growth and lateral root development to increase the surface area for nutrient uptake. Thus, inoculation of B. licheniformis PR2 improved the leaf chlorophyll content (photosynthetic rate) of the P. canadensis seedlings by 1.9-folds to 43.26 units compared to 23.26 units in the control group. Consequently, the application of B. licheniformis PR2 also improved the shoot, root, and leaf dry weights by 3.4-, 1.5- and 1.6-folds compared to the control group, respectively. Therefore, this study demonstrates the dual larvicidal and plant growth-promoting potential of B. licheniformis PR2, as an environmentally friendly alternative to chemical pesticides in the control of H. cunea larvae and production of healthy P. canadensis seedlings in the forest nursery.