Effects of biochar on ambrosia beetle attacks on redbud and pecan container trees

Xylosandrus crassiusculus (Motschulsky) and Xylosandrus germanus (Blandford) (Coleoptera: Curculionidae, Scolytinae) are serious pests in ornamental field nurseries and orchards of the eastern USA (Ranger et al. 2016). Females of X. crassiusculus and X. germanus overwinter as mated adults in galleries in infested trees (Addesso et al. 2019; Weber and McPherson 1983). By late winter or early spring, females leave the overwintering hosts seeking new trees for colonization (Ranger et al. 2010, 2016). In Georgia, initial flight activity can be seen as early as mid to late February for X. crassiusculus and early March for X. germanus (Monterrosa et al. 2022). Xylosandrus crassiusculus and X. germanus attack young stressed trees, although some stressed trees may not show any signs of stress instead appearing to be healthy (Ranger et al. 2016). The stressed trees produce volatile stress signals, such as ethanol, and ambrosia beetles are attracted to these signals (Ranger et al. 2010). Some volatile compounds produced by trees are acetaldehyde, acetone, ethane, ethylene, methanol, and ethanol (Ranger et al. 2010). These compounds can be produced when trees are under stress, such as during flood, frost, and drought conditions (Ranger et al. 2016). Under these stressed conditions, affected trees commonly produce acetaldehyde, which is initially converted into ethanol. Ethanol is transported through the xylem vessels of the tree, where it gets converted back to acetaldehyde and acetone (Ranger et al. 2010). Ranger et al. (2010) also showed that among all the volatile compounds produced by a stressed tree, ethanol is the most attractive compound to ambrosia beetles.

Biochar is an organic matter prepared through an oxygen-deprived and intense temperature-controlled pyrolysis process using any organic material (Biederman and Harpole 2013; Bridgewater 2004). Adding biochar to course soils can improve water retention and is a useful tactic for areas prone to drought (Verheijen et al. 2010). It can improve the physical and chemical properties of the soil (Glaser et al. 2002), particularly soil health, improve nutrient uptake (Waqas et al. 2014, 2017), nutrient retention (Gao and DeLuca 2020), and water-holding capacity (Yu et al. 2013). It is unclear if biochar can reduce tree stress by optimizing water retention after adding it as a soil amendment. Similarly, kaolin clay is applied as a particle film on trees to reduce stress, including temperature stress in leaves (Jifon and Syvertsen 2003) and drought conditions (Brillante et al. 2016; Mahmoudian et al. 2021). Moreover, kaolin clay applied on tree trunks moderately reduces ambrosia beetle attacks in nurseries (Werle et al. 2017). It is unclear if repeated applications of kaolin clay further reduce stress from the high temperature on the trunk by enhancing light reflectance and reducing ambrosia beetle attacks.

Preventative insecticide sprays of pyrethroids, such as permethrin or bifenthrin, are recommended before peak adult flights to manage ambrosia beetles in nursery settings (Joseph et al. 2019). Other insecticide products have not been consistently proven efficacious in reducing ambrosia beetle attacks (Reding et al. 2013; VanDerLaan and Ginzel 2013; Joseph 2022a,b). Entomopathogenic and mycoparasitic fungi, such as Beauveria bassiana (Bals.-Criv.) Vuill. (Hypocreales: Cordycipitaceae) and Trichoderma spp. (Hypocreales: Hypocreaceae), respectively, can potentially be used as biological control agents against ambrosia beetles (Castrillo et al. 2016). The efficacy of these fungal products on the reduction of ambrosia beetle attacks has not been extensively evaluated. Thus, our objectives were to determine the effects of stress relievers, such as biochar and kaolin clay, as well as B. bassiana and Trichoderma spp., against ambrosia beetle attacks on redbud and pecan in an ornamental nursery.

The experiment was conducted in a woody ornamental nursery (162,228 m²) in central Georgia from 2020 to 2021 (33.0577778 °N, 84.2430555 °W). In February 2020, bare roots of young redbud (Cercis canadensis L.; Fabaceae) and ‘Nacono’ pecan (Carya illinoinensis [Wangenh.] K. Koch; Juglandaceae) trees were planted using pine bark growing media in 64.4 L black plastic pots. The potted ornamental trees were on drip irrigation, with trees watered for at least 30 min daily. In the spring of 2020, the nursery manager preventatively sprayed permethrin (Perm-UP^® 3.2 EC, [36.8 % active ingredient], FMC Corp., Philadelphia, Pennsylvania) at 567.8 mL per ha (see below for more information) to protect the trees from ambrosia beetle attacks. These potted trees were maintained in the nursery for 16 months, from February 2020 to June 2021. Because the bare root tree may take variable time to establish by producing new roots, not all the treatments were applied in 2020, just a few selected stress treatments (such as kaolin clay and biochar [with or without permethrin]). The potted trees included in the experiment were ∼12–15 m from the wood line and were positioned with 0.6 m between pots. The nursery manager immediately removed and destroyed any tree with ambrosia beetle attacks; thus, the nursery had no resident ambrosia beetle population.

The experiments were separately conducted on redbuds and pecan container trees. The treatments were: (1) biochar, (2) kaolin clay, (3) permethrin, (4) biochar + permethrin, (5) kaolin clay + permethrin, (6) B. bassiana, (7) Trichoderma spp., and (8) nontreated control. Permethrin (Perm-UP^® 3.2 EC, see above) was applied at 567.8 mL per ha. Botanigard^® (B. bassiana strain GHA, BioWorks^®, Inc., Victor, New York) was applied at 1,869.9 mL per ha. Rootshield^® Plus WP (Trichoderma harzianum Rifai, strain T-22, and Trichoderma virens (J.H. Mill., Giddens & A.A. Foster) Arx, strain G-41, BioWorks^®, Inc., Victor, New York) was applied at 222.3 g per ha. To apply these products, the insecticide solution was prepared using a water volume of 373.9 L per ha. The insecticides were trunk-sprayed using a CO₂-powered single boom (one nozzle) handheld sprayer at 206.8 kPa on 25 February and 8 March 2021. Biochar (Aries Clean Energy, Franklin, Tennessee) was applied by adding ∼400 g of biochar to the growing media of each container tree. To deliver biochar to growing media, eight 15 cm deep holes near the root system were prepared using a 1.9 cm diameter tubular soil probe. First, ∼300 g of biochar was added to these eight holes, and second, ∼100 g was spread on the surface of the growing media over the holes. Biochar was added four times from July 2020 to February 2021 at 2-month intervals. Surround^® (Kaolin clay, Helena Agri-Enterprises LLC, Collierville, Tennessee) was applied at 11.3 kg per 278.54 L water. The trees that were designated for Surround^® treatment were individually trunk-sprayed with 6.5 g of Surround^® in 215 mL water using a 1.6 L handheld pneumatic sprayer. The trees were sprayed once a week, beginning 7 August 2020 until the end of the study in June 2021.

The container trees were arranged in a randomized complete block design with 10 replications. To flood stress the trees, the trees were individually bagged into 158.9 L black plastic bags (Husky, Contractor Bags, 0.07619 mm thickness, Poly-America, Grand Prairie, Texas). Each bag received ∼26.5 L of water for 24 h. The bag was zip-tied to the base of the trunk to keep the beetles from landing in the water. After 24 h of flooding, the bags were removed. The flooding procedure was initiated on 9 March 2021 when ambrosia beetle flight activity was suspected. The trees were evaluated on 4, 5, and 8 March 2021 for ambrosia beetle attack holes before trees were flood-stressed on 9 March. The attack holes were counted and marked on the tree using various colored wax pencils. After flood stressing, trees were evaluated for new attack holes at weekly intervals on 17 and 25 March and, 1 and 9 April 2021.

All the data analysis was performed in SAS software (SAS Institute 2016). The number of attack holes data were log-transformed (ln[x +1]) after checking normality before being subjected to one-way ANOVA using a general linear model analysis with the PROC GLM procedure. The treatment was the insecticide or stress relieving products, and the sampling date was included as a repeated measure. The treatment and sampling date were the fixed effects, and the replication was the random effect in the model. The means were separated post-ANOVA using the least significant difference method (α = 0.05).

Before flooding, the densities of entry holes were not significantly different among treatments (F = 2.0; df = 7, 129; P = 0.055), as entry holes were only noticed for Trichoderma spp. and nontreated treatments. Most ambrosia beetle infestations occurred after flooding. For redbud, the numbers of attack holes were significantly lower for the biochar and biochar + permethrin treatments than for the nontreated trees (F = 4.0; df = 7, 317; P < 0.001; Figure 1). There were no significant differences in attack holes among biochar, kaolin clay, biochar + permethrin, kaolin clay + permethrin, B. bassiana, and Trichoderma spp. treatments (Figure 1). Similarly, there was no significant difference in attack holes between permethrin and nontreated tree treatments (Figure 1). For pecan, the number of attack holes was significantly greater for kaolin clay treatment than for the remaining treatments, including nontreated trees (F = 5.6; df = 7, 317; P < 0.001; Figure 2).

Figure 1:

Ambrosia beetle attack holes on redbud tree trunks (mean ± SE). Treatments are denoted on the X-axis. Bars with the same letters are not significantly different (least significant difference test, α = 0.05).

Figure 2:

Ambrosia beetle attack holes on pecan tree trunks (mean ± SE). Treatments are denoted on the X-axis. Bars with the same letters are not significantly different (least significant difference test, α = 0.05).

We sought to determine if biochar could influence ambrosia beetle attacks by reducing stress. The results showed that biochar reduced ambrosia beetle attacks on the trunk of redbud trees. However, this result was not confirmed in pecan as beetles did not attack the nontreated pecan trees. The exact reason for the reduced attacks on pecan is unclear. It is possible that trees were not stressed enough to initiate attacks on the pecan relative to redbud. Pecan trees are vulnerable to ambrosia beetle attacks, especially in the first few years after planting. Although limited studies exist on how the insect communities interact with biochar-mediated soil, studies have shown that soil-applied biochar increased the monoterpene content in the foliage and reduced survival of Douglas-fir tussock moth Orgyia pseudotsugata McDunnough (Lepidoptera: Erebidae) and other herbivores on trees in the forest (Lockner et al. 2019; Raffa and Powell 2004). Similarly, a biochar-incorporated diet has been shown to delay the development and survival of Douglas-fir tussock moth larvae in laboratory conditions (Rice-Marshall et al. 2021). When applied as a soil amendment, biochar can enhance host-plant resistance against insect pests and reduce the pest’s reproductive capacity, as shown against English grain aphid, Sitobion avenae (Fabricius) (Hemiptera: Aphididae) (Chen et al. 2019) and rice brown planthopper Nilaparvata lugens Stål (Hemiptera: Delphacidae) (Hou et al. 2015). These studies suggest that it is likely that biochar may have reduced tree stress, altered the production of ethanol, and reduced ambrosia beetle attack.

In the current study, permethrin did not reduce ambrosia beetle attacks. In previous studies, permethrin reduced ambrosia beetle attacks on ornamental trees in nurseries, but it was not consistent across all studies (Ranger et al. 2016). Flood-stressed trees might have released ethanol at high amounts or at high concentrations. Thus, permethrin applications in this study might have been challenged due to an abnormal, high concentration of ethanol produced by these flooded trees. Other products, such as kaolin clay and entomopathogenic fungi, did not provide adequate control against ambrosia beetles. When kaolin clay was applied to the tree trunk in a different study, it did not reduce entry hole densities or gallery formation (Werle et al. 2017). B. bassiana strain GHA and Trichoderma spp. caused 76–99 % ambrosia beetle adult mortality in a different study (Castrillo et al. 2013).

In summary, biochar applications seem to influence ambrosia beetle attacks on redbud trees in nursery conditions. Biochar was applied to the growing media by creating incisions on the surface of the growing media. Perhaps, it could be incorporated into soil media when the trees are planted so that the developing root system benefits from the stress-relieving properties of biochar. More research is warranted to validate the hypothesis that biochar applied as a soil amendment could reduce tree stress and, thereby, reduce the production and release of ethanol from trees infested with ambrosia beetles. Regardless, biochar can be further developed into a potential tool to reduce ambrosia beetle attacks in nurseries.