Abstract
The efficacy of two formulations (Foray® 76B AVIO and Rapax® AS AIR) containing different Bacillus thuringiensis kurstaki (Btk) strains (ABTS-351 and EG-2348, respectively) was evaluated against Lymantria dispar larval populations in cork oak forests in Sardinia (Italy), in 2018 and 2019. The experimental design involved the following treatments: (I) untreated control; (II) Foray® 76B at the dose of 2.0 L/ha; (III) Foray® 76B at the dose of 2.5 L/ha; (IV) Rapax® AS AIR at the dose of 2.0 L/ha. Aerial applications were carried out using a helicopter equipped with four electronic rotary atomizers adjusted to sprinkle 160 micron-sized drops. Btk efficacy was evaluated by assessing the larval density reduction 7, 14, and 21 days after the application in each experimental plot in comparison with an untreated check. In addition to field surveys, the mortality of second and third instar larval samples, randomly collected from each plot after treatment and fed with foliage from the same plot, was determined in the laboratory. All Btk treatments were similarly effective, and no differences in larval density reduction among Btk strains and doses were found in either year. Twenty-one days after application, the average larval density reduction in the field was approximately 70% in all treated plots in 2018, whereas in 2019 it reached 80% only in areas treated with Foray 76B at 2.5 L/ha. Laboratory observations showed that all Btk-based products were effective against gypsy moth larvae, with significant differences in mortality between untreated control and the different Btk treatments. Our results shed light on the possibility of alternating different Btk strains for resistance management purposes and of applying lower doses than labeled, in order to achieve cost savings for product shipment and distribution and to reduce the environmental impact.
Highlights
Cork oak (Quercus suber L.) is an evergreen species typical of Mediterranean pure and mixed forests or included in extensive agroforestry systems [1]
Two aqueous suspension concentrate (SC) Bacillus thuringiensis kurstaki (Btk) formulations were used in this study: (1) Foray® 76B AVIO (Sumitomo Chemical Agro Europe S.A.S., Machelen, Belgium), containing 20 billion international units (BIU)/L of Btk strain ABTS-351 expressing the insecticidal crystal toxins Cry1A(a), Cry1A(b), Cry1A(c), and Cry2A and (2) Rapax® AS AIR
All Btk applications in this study successfully contributed to the reduction in L. dispar larval populations in the forest
Summary
Cork oak (Quercus suber L.) is an evergreen species typical of Mediterranean pure and mixed forests or included in extensive agroforestry systems [1]. A major threat to Mediterranean cork oak is the gypsy moth, Lymantria dispar (L.). Gypsy moth is a univoltine species whose larvae can feed and develop on more than 300 different plant species [7,8,9], even though Quercus species are the main hosts in natural environments [10,11]. Mostly targeting young oak shoots, causes a significant loss of the photosynthesizing surface, which leads to a tree growth decrease and consequent cork production reduction of up to 60% in totally defoliated trees and 40% in partially defoliated trees [12]. The ordinary development of infested trees might be affected in the following year too, resulting in delayed bud burst and additional cork growth reduction [13]
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