Teflon Coating and Anti-Escape Ring Improve Trapping Efficiency of the Longhorn Beetle, Monochamus alternatus

Abstract

The longhorn beetle Monochamus alternatus is an important conifer stem-boring pest and the main vector in Asia of the invasive pine wood nematode Bursaphelenchus xylophilus, which causes a devastating pine disease. Field experiments were carried out to evaluate the effect of coating black panel traps with different concentrations of an anti-adhesive material, Teflon, on Monochamus captures. Tests were also conducted to look at the possible decrease in the Teflon effect with exposure duration as well as the effect of the implementation of an anti-escape ring on the trap. The three tested Teflon concentrations significantly improved the captures of M. alternatus with regard to those with uncoated traps. The beetle captures increased by 108.3% in traps coated with a 4x Teflon dilution, but no significant differences in captures were observed when the traps were coated with either a Teflon stock solution or a 8x dilution. The trapping efficiency gradually decreased with the duration of exposure of the coated traps. No significant difference was observed after one year of exposure, but the trapping efficiency significantly decreased by 36.6% after 2 years and by 72.0% after 3 years. Two-year old, coated traps equipped with an anti-escape ring were nearly three times more efficient than those without an anti-escape ring, but the captures were still lower than those obtained with newly coated traps. The present study provides valuable information for the future development of environmentally friendly prevention methods. Attractants combined with Teflon coating and an anti-escape ring could significantly improve the trapping efficiency of M. alternatus adults, which may allow for the reduction of the population density of M. alternatus and transmission probability of pine wilt disease.

1. Introduction

The longhorn beetle Monochamus alternatus (coleoptera: Cerambycidae) is an important stem-boring pest of conifer. The adults gnaw on bark, causing host decay. Larvae bore the harm of decay, damage the xylem and phloem of trees, seriously affect the growth of pine trees, and even lead to the death of pine trees. It is also the main vector in Asia of the invasive pine wood nematode (PWN), Bursaphelenchus xylophilus (Steiner et Buhrer) Nickle (nematoda: Aphelenchoididae) [1,2].

Originating from North America, B. xylophilus causes a devastating pine disease [3,4]. Since the first outbreak of pine wood nematode disease (PWD) in 1914 on Japan’s Honshu Island, the disease has been found in East Asia (Japan, Korea and China) and Europe (Portugal) [5]. It has now become a worldwide threat to forest ecology and international trade [4]. B. xylophilus has a very short life history and can reproduce rapidly in the host. After the eggs hatch into larvae, they develop into adults in different instars (J2-J4) within a week. B. xylophilus has strong fertility, with each female laying 90–100 eggs on average during the breeding period, and the parasitic nematodes in each infected tree can reach into the millions [6]. A large number of disease-related enzymes, including cellulase and pectinolytic enzyme, are secreted in the host body of B. xylophilus, which mainly cause cell-wall decomposition, and promote the infiltration and migration of pine wood nematode between host cells [7,8,9,10]. B. xylophilus also causes xylem and tracheid dysfunction, hindrances nutrient exchange and water transport, and ultimately causes tree death [11]. Pine wilt disease causes needles to lose water, turn green, red, and yellow, and eventually die; the disease is known as the “cancer” of pines. In China, hundreds of millions of pines have been infected since 1982, when it was discovered in the black pine forest at the Sun Yat-sen Mountain in Nanjing. PWD in Asian pines presents typical pathogene-dominant disease characteristics; that is, the prevalence of the disease does not depend on the growth state of the pines, but on whether B. xylophilus can spread to the region to infect the pines. If a pine tree is infected with pine wood nematode disease, it will die no matter how healthy it is.

In the process of diffusion and infection of B. xylophilus, M. alternatus in Asia as well as other Monochamus spp. in other parts of the world play a key role in carrying, spreading, and assisting the pathogen to invade the host [12]. B. xylophilus cannot complete the infection cycle on its own and needs to be spread via vector insects, which allow it to enter pine trees through wounds caused by M. alternatus feeding or spawning. As an important vector insect of B. xylophilus, the control of M. alternatus has always been targeted as a possible method of preventing and controlling PWD [13]. The longhorn beetle is a kind of insect with a perfect self-protection mechanism. Longhorn beetle larvae grow and develop in tree trunks, are not affected by climate, and have little contact with natural enemies, so the population development is stable. Especially, the adult body wall and elytra are thick and have a strong drug resistance. There are few natural enemies and sufficient food resources. General conventional control methods not only cause serious harm to the forest ecological environment, but also have poor control effects. Since the adults are exposed at emergence, the control at the adult stage may represent a suitable control period. Compared with the cryptic larval stage, the insect traps targeting adults, based on the chemical ecology of M. alternatus, have been widely used in the monitoring, prevention, and control of forest pests as an environmentally friendly control method. At present, this method has become an important part of the integrated control system of M. alternatus [14].

There have been many reports of the successful use of traps to monitor and control pests in the wild. Visual characteristics such as the shape and color of traps play an important role in the host localization of plant-feeding insects, and shape and color are important factors affecting the sexual response of insects. Different species of insects are attracted by specific colors of traps. For instance, longhorn beetles in the subfamily Prionines were more captured by black traps than by green, yellow, or red ones [15]. M. alternatus were shown to prefer brown series [16]. Strom et al. studied the lure effects of different colored traps on Dendroctonus frontalis and found that black, blue, dark brown, purple, green, and red traps attracted more adults than white or yellow funnel-shaped traps [17]. Black traps were superior to white and clear traps for bark and woodboring beetles [18]. In addition, many scholars have carried out research on trap type, location, density, trapping efficiency, and use cost. Higher catches of Monochamus spp. have been reported on panel traps than on multiple-funnel ones [19]. Significantly more M. alternatus were caught using traps placed at the mountainside and the top of the mountain than at the bottom, and more M. alternatus were caught using traps placed in open areas than in closed areas. The density of traps had a significant effect on the trapping effect of traps [20].

The structure design of the trap should not only consider the flight path of the pest, which is conducive to trapping, but also consider the anti-escape function. Studies have demonstrated that treating traps with a surface lubricant to make them “slippery” can increase insect-capture efficiency [21]. Such coating applied to the surface of the trap device and the inside of the trap collection cup can increase the smoothness of the trap panel, which not only prevents insect pests from sticking to the trap panel, but also minimizes the escape of beetles, thus improving the trapping efficiency [14]. Polytetrafluoroethylene (PTFE), also known by its trade name Teflon or Fluon, is a synthetic polymer material that uses fluorine to replace all hydrogen atoms in polyethylene. Teflon is an anti-adhesive and aesthetic material that has excellent chemical inertia as well as good mechanical stability [22]. Teflon has the characteristics of a high temperature resistance and its friction coefficient is very low. Thus, it can be lubricated to become an ideal coating [23]. De Groot and Nott conditioned insect traps with Rain-X (SOPUS Products, Houston, TX) to render their surfaces more slippery, with the goal of increasing the trapping efficacy and retention of insects in traps [24]. The treatment of multi-funnel and plate traps with rain-X and aerosol lubricants has effectively improved the capture efficiency of many species of wood-boring beetles such as Cerambycidae and Curculionidae. For example, four times more individuals of the longhorn beetle Megacyllene Robiniae were captured using traps coated with aerosol lubricants than by using untreated ones [25]. The effects of surface treatment did not differ between multiple-funnel and panel traps [18]. Other anti-escape settings are added to the trapping device. For example, the barrel body is set to a V shape, so that insects crawling here are blocked and unable to escape, so as to prevent the crawling insects from escaping.

However, Teflon is expensive, and its cost could be a prohibitive factor in a large-scale survey program. If the coating is diluted or sprayed before application, it can not only reduce the influence on the surface spectral characteristics of the trap device and the production cost of the trap device, but also minimize the influence of coating on the performance of the trap device [25]. A study also suggested that the trapping efficiency of coated traps did not weaken after one or two seasons of use [26]. Allison et al. demonstrated that the addition of a large lubricant-treated collar to the bottom funnel of a multiple-funnel trap significantly increased the capture of some Lamiinae; a longhorn beetle family to which belongs Monochamus spp. [27]. The premise for further green prevention and the control of M. alternatus is to clarify the influence of various factors on the trapping effect. It can provide further theoretical support for speeding up the research process of new trapping technology.

As of now, there are not many related reports on the evaluation of Teflon coating in the trappings of M. alternatus. The aim of this study was (i) to compare the efficiency of traps coated with different Teflon concentrations with regard to uncoated traps for capturing M. alternatus; (ii) to test if the coating efficiency decreases with the duration of exposure; and (iii) to check if the implementation of an anti-escape ring improve the captures.

2. Materials and Methods

2.1. Trapping Materials

Black cross-vane traps (Model: BF-8, Hangzhou Pheromone Biotechnology Co., Ltd., Hangzhou, China; hereinafter referred to as Hangzhou Pheromone) were used. They consist of a circular top (50 cm in diameter), a cross-shaped panel (66 cm in length and 35 cm in width), a funnel (35.5 cm in upper circle diameter and 5.5 cm in lower circle diameter), and a collection cup (26.5 cm in length and 10.5 cm in diameter), from top to bottom (Figure 1a).

Traps were baited with specific attractants for M. alternatus (Commercial Lures F8 provided using Hangzhou Pheromone) combining the aggregation pheromone Monochamol, (2-undecyloxy-1-ethanol) and host volatiles (α-pinene, ethanol, etc.).

The anti-escape ring (Figure 1b) is constructed in the shape of a funnel with its lower diameter of 5.5 cm, which is stuck in the mouth of the collection cup to prevent the escape of trapped M. alternatus.

The Teflon stock solution (100%) (provided using Hangzhou Pheromone) has been applied on all faces of the trap. Teflon solutions have been prepared using water dilutions either 4 times (25%) or 8 times (12.5%). Teflon solutions have been applied on the traps using the same method as for stock solution.

2.2. Field Tests

In all tests, the traps were suspended from a stick attached horizontally to two adjacent trees. The collection cup was placed approximately 150 cm above ground level, the typical height at which M. alternatus attacks trees [28]. The attractants were hung at the window opening of traps. All lures were replaced every 45 days. The interval of each trap was about 20 m in the same block. Each block was spaced approximately 50 m apart. Beetles were collected from traps once every 7 days, and the position of each lure and trap was changed at the same time to reduce the effects of trap position on beetle capture [29]. All M. alternatus adults captured in each trap on each date were counted, sexed, and recorded while the traps and funnels were cleared of debris.

2.2.1. Teflon Concentration Study

This study was conducted in Jiaxing, Zhejiang province, China (120°95′92″ E, 30°53′42″ N). This location hosts a pure pine forest of Pinus massoniana lamb, with several dead trees due to the PWD. The experiments were carried out between 1 June to 1 July in 2020, which corresponds with the emergence period of M. alternatus adults [30]. The traps shall be checked every 7 days. Four different trap modalities were used, with five replicates per modality: (i) uncoated control trap; (ii) trap coated with Teflon stock solution (100%); (iii) trap coated with a 4x Teflon dilution (25%); and (iv) trap coated with a 8x Teflon dilution (12.5%).

2.2.2. Test of the Possible Decrease in Fluon Trapping Efficiency with Time

The test site was selected at Fuyang, Hangzhou, Zhejiang Province, China (120°10′12″ E, 29° 83′26″ N). Four treatments, with five replicates per treatment, were deployed: (i) traps with newly Teflon coating; (ii) traps with as 1-year-old Teflon coating (which were used in the forest for one year); (iii) traps with a 2-year-old Teflon coating; and (iv) traps with a 3-year-old Teflon coating. The test was conducted from 1 June to 1 July 2021.

2.2.3. Anti-Escape Ring Study

The test site was selected at Anji, Huzhou, Zhejiang Province, China (119°68′22″ E, 30°63′41″ N). Three treatments were used, with five replicates per treatment: (i) traps with newly Teflon coating; (ii) traps with a 3-year-old Teflon coating; and (iii) traps with a 3-year-old Teflon coating equipped with an anti-escape ring. The traps were hung as above. The number of M. alternatus males and females trapped was recorded from 1 June to 1 July 2021.

2.3. Data Analysis

All statistical analyses were conducted using the SPSS 22.0 software, and LSD method was used for multiple comparison of the differences between different treatments and p < 0.05 was considered significant. Use of Origin 2018 software for chart making.

3. Results

3.1. Effect of Teflon Concentration on Trapping Efficiency

All of the three Teflon coated treatments significantly improved the captures of M. alternatus compared to those with the uncoated traps (Figure 2; F = 3.72, df₁ = 3, df₂ = 96, p < 0.05). Although there was no significant difference in the captures using three Teflon concentrations, the traps coated with a 4x Teflon dilution showed the best trapping efficiency with a mean of 3 ± 0.45 beetles/trap, which increased by 108.3% with the captures obtained with the uncoated traps. Compared to the uncoated traps, those coated with the Teflon stock solution and the 8x dilution increased the captures by 100% and 83.33%, respectively, with respective means of 2.88 ± 0.36 beetles/trap and 2.64 ± 0.39 beetles/trap.

3.2. Test of the Possible Decrease in Fluon Trapping Efficiency with Time

Traps with new Teflon coating showed the best trapping efficiency for M. alternatus, with a mean of 3.72 ± 0.54 beetles/trap (Figure 3). This trapping efficiency decreased gradually with the duration of the Teflon-coating exposure. There is no significant difference in captures between newly coated traps and those already exposed for 1 year. The trapping efficiency significantly decreased by 36.56% after 2 years of exposure (of 2.36 ± 0.48 beetles/trap) compared to the new Teflon coating (F = 6.0, df₁ = 3, df₂ = 96, p < 0.05), and even by 72.0% after 3 years of exposure (mean of 1.04 ± 0.32 beetles/trap; p < 0.05). A significant difference was also observed between captures with Teflon-coated traps after 2 years and 3 years of exposure. (p < 0.05).

3.3. Effect of Anti-Escape Ring on Trapping Efficiency

In order to study the trapping efficiency of the anti-escape ring on M. alternatus, we compared the trapping efficiency of old traps with and without the anti-escape ring, respectively. The results showed that the trapping results of each treatment showed a significant difference (Figure 4; F = 13.56, df₁ = 2, df₂ = 72, p < 0.05). The trapping efficiency of the old traps with an anti-escape ring significantly increased by nearly three times as much with the mean of 1.96 ± 0.29 beetles/trap compared to that with three-year-old Teflon coating without an anti-escape ring, with the mean of 0.72 ± 0.14 beetles/trap. The anti-escape ring significantly improved the trapping efficiency of the old traps with the three-year-old Teflon coating on M. alternatus (p < 0.05). However, traps with new coating still showed the highest trapping efficiency of M. alternatus, and there was a significant difference in the trapping efficiency between the traps with a new coating (3.04 ± 0.44 beetles/trap) and the old traps with an anti-escape ring (p < 0.05).

4. Discussion

In recent years, with respect to the construction of a more ecological civilization, the definition of pest control methods has switched from the simple use of chemical pesticides aimed at pest elimination to the deployment of ecological control, biological control, physical control, and other comprehensive means of environmentally friendly pest control [31]. Efficient and environmentally friendly traps and attractants play a very important role in the containment and eradication of insect pests. Techniques such as mating interference, mass capture, and trapping can be directly used to monitor and control insect population dynamics [32,33]. The structure design of the trap should not only consider the flight path of the pests which is conducive to trapping, but also ensure the convenient production, transportation, and installation of the trap. The advantages of intercept traps are their relatively low cost and their easiness to deploy. For coleoptera insects, the commonly used traps mainly include multiple-funnel traps, cross-vane traps, baffle traps, boat traps, etc.

The cross-vane panel trap was initially developed to monitor coleoptera in forests [19,34], as a replacement for or supplement to the multiple-funnel trap [35]. Subsequent work with the cross-vane panel trap demonstrated its utility for trapping cerambycid and buprestid beetles [36,37,38]. The trapping efficacy for beetle species can be further enhanced, sometimes by more than tenfold, by coating trap surfaces with Teflon or other lubricants [23,25,26].

The results of our study showed that Teflon coating could significantly improve the trapping efficiency for M. alternatus. Moreover, beetle captures did not differ significantly when the Teflon solution was diluted by four or eight times. These results are similar to those of earlier studies, where Teflon-treated funnel traps with wet collection cups captured six times more longhorn beetles than the untreated funnel traps with wet collection cups [38]. The captures of the emerald ash borer, Agrilus planipennis (Buprestidae), also increased substantially by coating trap surfaces with Teflon, even when highly diluted [39]. In addition, laboratory studies in insect husbandry have demonstrated that escapes from rearing containers can be reduced by treating container surfaces with Teflon, with this effect not being diminished by dilution [40]. Therefore, the use of a lower concentrated solution of Teflon, thereby reducing the amount to be used per trap, could substantially reduce programmatic costs. Teflon coating applied to the whole trap, and extended, ventilated collection cups resulted in a significant improvement in trap performance. Since a 4x dilution of Teflon coating applied to the black cross-vane traps led to a more-than-100% increase of M. alternatus catches compared to the untreated standard trap designs, such a dilution (or even an 8x one) could be considered a good choice to treat the coating of traps in monitoring and controlling populations of this longhorn beetle.

In the forest, we did not observe any significant difference in the trapping efficiency of M. alternatus adults between newly coated traps and those exposed for one year, which is similar to the findings of Graham et al. [23]. The influence of Teflon on trap efficiency may vary with product formulation and its source as well as with climatic conditions [30]. However, the trapping efficiency of Teflon coating began to decrease significantly after two years, where captures decreased by 36.6% compared to those of newly coated traps. One reason could be that the trap may easily accumulate dust, moss, and other sundries after being placed in the forest for a long time depending on the stand and its canopy density. This accumulation is likely to weaken the smoothness of the trap surface and its anti-adhesive design. Therefore, we suggest that M. alternatus traps should be replaced by newly coated ones after two years. Certainly, it also depends on the costs and purpose of trapping M. alternatus. Depending on the concentration of Teflon coating, the speed at which the trapping efficiency decrease may also be different, which is worthy of further research in the future. Besides, the activity density of M. alternatus adults is also different in different canopy densities. Some studies have shown that the activity density in the canopy of M. alternatus is six times higher than that under it [26]. So, the height at which traps are hung is also a very important factor in the application of the technology behind trapping M. alternatus. In addition, good air circulation conditions between forests are conducive to the full diffusion of the effective components of the attractant in the trap. Therefore, in addition to meeting a certain height, the trap should be considered as being placed far as possible in the mountain foot, mountainside, and mountain top.

We designed an inverted funnel structure and stuffed in the collection cup, which is named an anti-escape ring. Our results show that this ring greatly limited the escape ability of M. alternatus. It significantly improved the trapping efficiency of Teflon-coated traps already exposed for 3 years in forests. However, the captures remained significantly lower than those obtained with a newly coated trap. It maybe due to the fact that the anti-escape resolves the escape problem in the collection cup, but the Teflon coating on other parts of the surface of traps, such as the cross-vane panel, play the important role in trapping M. alternatus adults. Our indoor behavior observation found that some M. alternatus adults can climb in the old collection cup, a several-years-old trap found in the forest, but they will get stuck in the space of the combination between the anti-escape ring and the trap funnel, which will greatly reduce their escape rate. The anti-escape ring can effectively reduce the expense of the trap replacement. However, whether the anti-escape ring improves the trapping efficiency of new traps with a new Teflon coating on M. alternatus needs further test verification.

In recent years, more and more attention has been paid to defining integrated pest control methods which are susceptible to affecting the environment in a limited way. As an efficient and environmentally friendly means of monitoring and controlling of M. alternatus, the study of trap technology can provide effective help in reducing the harm of M. alternatus and preventing the further spread of PWN. The results of this study proved that the attractant combined with the Teflon coating and anti-escape ring could significantly improve the attractant effect, which is of great significance to reduce the population density of M. alternatus and transmission probability of PWD.