Effective Laser Fly Control with Modulated UV-A Light Trapping for Mushroom Fungus Gnats (Diptera: Sciaridae)

Abstract

Fungus gnats (Sciaridae) are one of the most devastating pests on mushroom farms. Generally, they are controlled using relatively inefficient physical means, like sticky or pheromone traps, or with chemical pesticides. Here, we have proposed an integrated fungus gnat control system combining a UV-A LED source at 365 nm and a high-power laser diode at 445 nm. The 365 nm UV-A LED serves a light trap, since previous studies have concluded that fungus gnats show maximum attraction in the range of 365–390 nm. The UV-A LED is also modulated at different frequencies, and the response of the gnats corresponding to these different frequencies was observed. We utilized an Arduino Uno microcontroller to run the integrated device, and a BASLER USB camera was used to capture the images. Our experiments indicated that a frequency of 40 Hz is the optimal choice for attracting the gnats most rapidly. Within 20 s of exposure, the UV-A LED operated at 40 Hz was found to trap approximately 80% of the gnats. In a restricted trapping zone measuring 2.5 × 2.5 × 3 cm³, our integrated module, which includes a 40 Hz modulated UV-A LED and a laser, resulted in a survival rate of only 50% for the total number of gnats. This outcome was accomplished through periodic 200 ms long exposures, amounting to a total duration of 2 min for a group of 100 gnats.

1. Introduction

Fungus gnats (Insecta: Diptera: Sciaridae) are an omnipresent pest species in a wide variety of plants around the world. They can be found in small gardens and crop plantations. Although generally considered to be not much of a nuisance as a pest, some species can attack and feed on important cash crops like potatoes, mushrooms, ornamental plants (such as begonias and young orchids), and a handful of others crops as well [1]. While the adult fly does not cause any damage to the plants and is generally a hindrance to the workers, the larval stage causes the most significant damage of all growth stages. During this stage, the larvae can aggressively feed on the roots and the absorbent hairs of the roots, thereby stunting the growth of the plant by cutting off or reducing the nutrition intake from the soil or substrate [1,2]. If seedlings, especially delicate ones, such as legume seedlings, are affected by fungus gnat larvae, it leads to wilting, thus reducing the efficiency and quantity of produce [1,3]. Also, this rampant feeding can cause injuries to plants, which serves as a gateway for soil borne bacteria and other disease-causing agents to enter the plants, thereby shortening the life span and the possible utility of the plants [3]. Even when not feeding on the plants directly, they can consume the organic matter in the compost and the growing substrate, leaving fewer nutrients for the plants themselves [4]. The larvae also feed invasively on the mycelium of mushrooms, thereby causing significant damage to mushroom plantations [2,3,4,5,6]. Even when not feeding on the plants directly, they can consume the organic matter in the compost and the growing substrate, leaving fewer nutrients for the plants themselves [4]. The environmental conditions used for mushroom cultivation are perfect for their rampant reproduction [7]. This unhindered attack on the mycelium layers negatively affects the quality and quantity of the mushrooms, thereby rendering them worthless for trade and human consumption. If left uncontrolled, these gnats can wreak havoc, leading to decreased yield and significant losses to the growers [8,9]. Current control methods utilize physical, chemical and biological applications such as sticky traps, pesticides, and insecticides, or the introduction of microbial agents to control the reproduction of the fungus gnat itself [10,11,12,13]. But these methods have their own problems or shortcomings. Physical methods can tend to be tedious and time consuming, ending up being rather ineffective when taking into account the effort involved and the resulting effectivity of the method. Chemical methods, such as pesticides and insecticides, have come under the scrutiny of the scientific community in recent times, with the possible ill effects on health of the consumers resulting from improper application of the same. There have been studies observing the residues of the common pesticides such as benomyl, parathion, malathion, beta-cypermethrin, diflubenzuron, and pyriproxyfen, among others [14,15,16]. These studies have indicated that residues can be found on mushrooms for up to 7–10 days after spraying them with the aforementioned insecticides. These chemical residues can accumulate in the body over time, and there has been research which indicates that these pesticides, especially parathion and malathion, can lead to carcinogenesis of the breast tissue [17]. There is also a likelihood of the gnats developing resistance through generations to these pesticides, necessitating increased dosage and frequent application, thus increasing the associated risks [18,19]. Use of microbial or viral agents or other biological means, such as nematodes or killer mites, for the control of fungus gnats can be effective, but generally the effectiveness depends on the time of application, with earlier application being better [20]. These bacterial or viral agents can perhaps hinder the other beneficial organisms in the substrate, affecting the balance of the symbiotic system.

Recently, there have been a few innovative ways utilized to trap the fungus gnats by using LEDs as attraction traps. As an example, broad spectrum LEDs with UV peak emission, or fluorescent black light sources with UV and blue emission have been used as appropriate sources for light trap in mushroom plantations and farms [21,22]. In other studies, focusing on the response of fungus gnats to different wavelength stimuli, it has been concluded that wavelengths in the broad band UV range and green range (365–390 nm and 525 nm) elicit the highest attraction response from the gnats [23,24,25,26]. There have been studies where LEDs traps have been used in congruence with the existing physical methods to improve the efficiency of these physical methods of fly trapping. Yellow glue traps along with 530 nm peak emission LEDs have been utilized resulting in double trapping efficiency when compared to just the glue trap for multiple greenhouse pests, such as dark winged fungus gnats (Sciaridae), sweet potato whiteflies (Aleyrodidae), western flower thrips (Thripidae), and leaf hoppers (Cicadellidae) [20]. Use of LEDs seems like the right direction for application as standalone traps or for enhancing the efficacy of existing traps since there has been evidence of color preferences and color-based flight behavior in a variety of flies [27]. In general, these light traps, when operated alone, leave the flies to die after trapping them, with no active mechanism involved in the eradication of the pests. This can result in the escape of a significant percentage of the trapped flies, thereby reducing the applicability. Use of lasers as a means of instant elimination of these trapped flies is an interesting approach. There have been a few studies with respect to the use of lasers as a means to effective pest control systems, but none have been used synergistically along with another light-based trapping mechanism to our knowledge. The effects of 532 nm and 1064 nm laser exposure on Drosophila melanogaster larvae have been studied thoroughly; 532 nm was found to be the better than 1,064 nm at eradicating the larvae [28]. Experiments on adult D. melanogaster samples and found that exposure of 650 nm semiconductor laser beam for 1282 s at 60 mW power or more resulted in a 99% mortality rate [29]. In another study, it was demonstrated that using a 532 nm, 500 mW laser for 2.5 s caused lasting damage to aphids within 3.5 s of exposure, achieving a 100% kill rate. [30]. Also, the effect of different frequencies on the behavior of various animals and insects have been a topic of interest. These studies have concluded from the experiments that there is a strong positive correlation between body size and metabolic rate on temporal perception, as measured by critical flicker fusion frequency, which is a lowest flashing frequency at which the flickering is perceived as constant [31].

In the study, we have developed an all-optical system, including a light trap, which employs a modulated UV-A lamp with 365 nm to attract the fungus gnats to an assigned small area, and a cheap engraving laser at 445 nm, which works as the elimination laser. Initially, we discuss the experimental procedure used to study the response of the gnats to different modulated frequencies and the resulting observations. Forty Hz frequency from the UV-A lamp was observed to elicit the maximum response of the gnats, suggesting it to be the optimum modulation correlating to the most efficient attraction. The aforementioned trapping area was then made subject to a few-watt blue laser irradiation of millisecond time duration, thereby incapacitating the gnats in the trap effectively. These experiments and their corresponding results are discussed in the latter part of the article.

2. Materials and Methods

2.1. Fungus Gnat Rearing

To facilitate our research, we sourced adult fungus gnats from a cooperative mushroom farm plantation (Lohas Biotech, Tainan, Taiwan). The specific genus that we have used for our study is the dark winged fungus gnat (Diptera: Sciaridae: Bradysia sp.). Here, we utilized a laboratory rearing method to study the behavior and life cycle of these pests in a controlled environment, based on this group’s approach [32]. Peat moss was procured from the same local horticultural supplier to the mushroom farm and was chosen as the substrate for the rearing process. In the laboratory, we used plastic containers measuring 15 × 10 × 3 cm³ to house the fungus gnats. Each container was filled with 100 g of peat moss, which was moistened with 30 milliliters of water. To provide nutrition for the larvae, raw potato shavings and 2 × 2 cm² potato disks were added to the substrate. This ensured that the newly hatched larvae had a suitable food source to support their growth and development. To maintain optimal conditions for the fungus gnats, the containers were kept under controlled environmental conditions. To mimic natural conditions found on mushroom farms and plantations, a 16 h daylight cycle, a constant temperature of 26 °C, and a humidity level of 85–95% were maintained. Refreshing the substrate every three days involved adding 30 milliliters of water to maintain the desired moisture level and replacing the potato shavings and disks to ensure a constant supply of food for the larvae. As the potato disks began to sprout in the growing medium, the effects of the larvae’s aggressive feeding on the plant roots were quite clear. When compared to new potato disc growth in substrate without fungus gnats, the affected plants were visibly stunted and were observed to wilt within 7–10 days of sprouting. Based on the previous literature [33,34] and our observations throughout the fungus gnat rearing, this seems to be a telltale sign of damage to the absorbent hairs and, eventually, its roots. Thus, there seems to be an urgent need for effective fly control measures.

2.2. Modulated Frequency Testing

To investigate the behavior of fungus gnats in response to different frequencies of LED light exposure, we constructed a transparent arena using glass slides and acrylic sheet materials. The dimensions of the arena were 150 × 25 × 3 mm³. By utilizing a Basler USB camera controlled by pylon viewer software and an Arduino Uno microcontroller, we achieved precise monitoring of the gnats’ movement within the arena. The setup involved modulating LED lamps across frequencies ranging from 10 Hz to 100 Hz for a duration of 40 s. Figure 1A depicts the experimental setup for modulated frequency testing. To optimize the LED output, a buck–boost circuit was employed, regulating the voltage and current supplied to the LEDs. This ensured consistent and efficient operation throughout the experiments. The LED lamps were powered by a 12V-3A external power supply, while the Basler camera and Arduino Uno microcontroller received power from their respective notebook ports. The LED illumination and image acquisition processes were synchronized using an external trigger generated by the Arduino Uno microcontroller. A simple code was written in the Arduino IDE environment to achieve the synchronization. This seamless coordination minimized noise in the data and ensured accurate tracking of the gnats’ behavior. Images were captured at 200 ms intervals, resulting in a total of 200 images for post-processing and analysis. Figure 1B shows the acquired image of the transparent arena with the gnats inside.

2.3. Image Processing for Fly Counting

To analyze the acquired images, we employed MATLAB software. The acquired images underwent background subtraction to eliminate static elements and focus on the gnats’ movement within the arena. This process involved utilizing an averaged background image derived from a set of 200 background images. After background subtraction, the resulting images were cropped to isolate the arena area. The left half of the images, which indicated the gnats’ movement towards the light when viewed sequentially, was extracted. Figure 2A portrays the schematic of the image processing pipeline, while Figure 2B shows the result of the background subtraction that isolates the arena for further fly counting. Various image processing techniques, including distance transform and watershed functions, were applied to segment and track the gnats’ movement and count them accurately. Video S1 in the supplementary materials shows the result of this processing in real time. The processed images revealed the trajectories and positions of the gnats over time, providing valuable data for analysis. By counting the number of gnats at different time instances during the LED light exposure for each modulated frequency, we obtained quantitative information about their response to the light stimuli. Origin software was used to plot and visualize the fungus gnat count data, facilitating further analysis and interpretation of the results.

Thus, the experimental setup for modulated frequency testing and image processing techniques allowed us to gain insights into the behavior of fungus gnats to different frequency of light stimuli and develop an effective fly control solution using LED light exposure. At the same time, the experimental setup and image analysis enabled precise tracking and counting of the same.

3. Experimental Results

3.1. Light Power and Wavelength

The impact of varying intensity on the response of fungus gnats has been a relatively unexplored area of study, prompting us to investigate this aspect in our research. We considered three intensities based on three possible outputs of the UV-A LED, which were 0.020 W, 0.190 W, and 0.380 W. There was a positive response when higher intensities were applied at elevated powers, consistent with the findings of a previous study conducted by Cloyd, R. A., and his group [1]. As a control, the attraction of fungus gnats to normal ambient fluorescent lights were studied. There was no correlation, as shown in Figure 3D. Based on our observations and the aforementioned work, the maximum output of the UV-A LED was chosen for further experiments. Regarding the wavelength parameter, we found that the 365 nm UV-A LED light demonstrated the most effective response from the gnats. This aligns with previous observations and emphasizes the preference of fungus gnats for this specific wavelength. The attraction response to the UV-A LED light further supports the importance of considering wavelength as a crucial parameter in designing effective light traps for attracting and controlling fungus gnats. By utilizing the optimal wavelength, our laser-based fly control device can precisely target and lure these gnats, enhancing the overall efficiency of the system.

3.2. Optimal Modulated Frequency

In addition to investigating the impact of intensity and wavelength, we conducted a comprehensive frequency study to determine the response of fungus gnats to different frequencies of LED light exposure. This study aimed to identify the optimal modulation frequency that elicits the highest and quickest attraction response from the gnats. To cover a wide spectrum of frequencies, experiments were conducted using frequencies ranging from 10 Hz to 100 Hz. Remarkably, our results revealed that the gnats exhibited the highest attraction response at a frequency modulation of 40 Hz, followed by 50 Hz and then 100 Hz. These frequencies consistently elicited rapid and significant attraction responses from the fungus gnats. These results were subject to Pearson’s correlation coefficient to statistically analyze the presence of any discernable correlation between exposure time and increase in the number of fungus gnats being attracted to the illumination spot. For each of the frequencies as well as the control light, observations were recorded at 200 milliseconds intervals for a time duration of 40 s, resulting in a total of 200 observations for each frequency as well as control light. It was observed that for 40 Hz, 50 Hz, and 100 Hz, the Pearson’s correlation coefficients were 0.53, 0.76, and 0.64, respectively, indicating moderate to strong positive correlation. When compared to control, the Pearson correlation coefficient was 0.38, indicating weak correlation. The Pearson correlation for each of the frequencies and control light were statistically significant at p value < 0.001 (two-tailed test). Thus, the UV-light trap works positively to attract the fungus gnats, compared to ambient fluorescent light. At the 40 Hz modulation frequency, approximately 33 out of 40 gnats in the test set were attracted to the light source within a remarkably short duration of 14.4 s. Similarly, at 50 Hz and 100 Hz, 34 gnats were attracted in 21.6 s and 35 gnats were attracted in 22.8 s, respectively. These observations were subject to sigmoidal fitting to see if there was any appreciable statistical significance. Figure 3A–C represents the results of sigmoidal fitting. Figure 3D represents the graph of fungus gnat response to ambient fluorescent light. It can be seen that 40 Hz shows the fastest attraction compared to the other frequencies, statistically as well. These findings highlight the significance of frequency modulation in influencing the response behavior of fungus gnats towards the LED light source. The observed average attraction rate of 82% across all tested frequencies underscores the effectiveness of the LED light trap in attracting the gnats. The optimal frequency modulation of the LED light trap creates an effective trap for the gnats, ensuring a rapid and efficient trapping process.

3.3. Integrated Laser Fly Control with UV Light Trap

Building upon the insights gained from the frequency study, we proceeded to integrate the UV-A LED light trap with a 445 nm engraving blue laser, creating a novel laser fly control system with an integrated LED light trap. This integration aimed to enhance the efficiency and effectiveness of fungus gnat control by combining the attractive properties of the LED light trap with the lethal capability of the laser. For the study, a conical trap area with a smaller triangular volume to trap the gnats was made out of acrylic sheet. The gnats were lured into a trapping area measuring 2.5 × 2.5 × 3 cm³, and the light trap operated at a 40 Hz modulation frequency based on the optimal frequency determined from our previous experiments. Figure 4A depicts the overall system schematic mentioned above. The introduction of 100 gnats into the laser trap and the activation of the laser beam every 10 s for a duration of 200 milliseconds allowed for sufficient time to capture the gnats within the trap. By limiting the exposure duration to just 200 milliseconds, we ensured that the structural integrity of the trap, made of acrylic material, would not be compromised by potential melting due to high local temperatures. This approach minimized the risk of damaging the trap while effectively eliminating the gnats. In total, 100 gnats were included in the experiment, and after the laser exposure, only 50 gnats remained alive, resulting in an elimination rate of approximately 50% within just 2 min. These promising results demonstrate the effectiveness of the integrated laser and UV light trap system in significantly reducing the population of fungus gnats in our experiment. The presence of deceased gnats within the arena, as shown in Figure 4B compared to Figure 4C, depicts the results of the integrated system in achieving targeted fungus gnat control. While successful within lab tests, work is ongoing to further study and understand the real-time applicability on mushroom farms.

4. Discussions

The presence of fungus gnats can have detrimental effects on mushroom plantations, as these pests proliferate rapidly and can disrupt the delicate balance that is required for profitable cultivation. Also, by rearing the fungus gnats in the laboratory, we gained valuable insights on the larvae’s most damaging phase for agricultural crops. Although there has been an increase in the use of chemical means such insecticides in the last four decades, the absolute economical value of crop losses due to pests and the proportion of crop losses on the whole has increased [35]. Moreover, traditional approaches, such as chemical-based pesticide application, present additional challenges and risks, including environmental concerns, rising resistance to the pesticide, and potential health hazards. Considering other conventional control methods that often rely on the application of sticky traps or pheromone traps, our laser-based system can alleviate the effort required on the part of mushroom cultivators by providing them with a prototype plug-and-play system, lightening their burden of having to place physical traps. Our approach can be considered as a step towards incorporating automated systems in mushroom plantations that require minimum supervision while assisting the conventional methods, which can lead to high quality produce without the need for overuse of insecticides.

By studying the response of fungus gnats to different frequency modulations of a light trap within the UV-A range (approximately 365 nm), 40 Hz frequency was found to elicit the fastest response from the gnats. With this knowledge, we engineered a UV-light-trap-based laser fly control system that provides a possible substitute to the conventional methods. This targeted approach can enhance the sustainability and ecological balance of mushroom farms, allowing for the coexistence of fungus gnat control measures and natural biological processes. Also, 365 nm is attractive to fungus gnats the most, while not eliciting response from beneficial insects like bees, thereby minimizing the impact on beneficial insects and possibly extending its application to other fruit or vegetable orchards as well. For instance, there have been studies conducted on naive honey bee foragers (Hymenoptera: Apidae) and bumble bees (Hymenoptera: Apidae) which indicate that these insects show maximum attraction to 410 nm and 430–480 nm, respectively [36,37,38].

Thus, our approach utilizing the precise frequency of 40 Hz of 365 nm UV-A light creates an irresistible lure for the gnats, effectively containing them in the trap. Furthermore, utilizing the 445 nm integrated laser to specifically target the gnats trapped in the container enhances the efficiency of our system by instantly killing the gnats, providing an additional mechanism to terminate captured gnats swiftly and effectively and thereby effectively contributing to reducing or controlling the fungus gnats in a mushroom plantation.

5. Conclusions

Our research and development efforts have demonstrated the potential of laser-based fly control as a promising choice for managing fungus gnats on mushroom farms. By leveraging the advantages of optimal frequency modulation and UV light attraction, our system offers a reliable and efficient means of controlling these gnats. The approach of integrating a laser with a UV light trap presents a breakthrough in the field of fly control, providing a comprehensive solution that combines attraction, trapping, and termination of fungus gnats. We believe that our findings contribute to the advancement of sustainable and environmentally friendly practices within the agricultural industry. The successful development of our laser-based fly control device opens up new possibilities for pest management in various farming systems. The application of light-based control methods can be extended beyond mushroom farms, potentially benefiting other agricultural sectors facing similar pest challenges. The optimal frequency modulation and targeted attraction approach can be adapted to target different fly species, offering customized solutions for specific farming needs. The combination of optimal frequency modulation, UV light attraction, and integrated laser termination offers an efficient, environmentally friendly, and economically viable solution. By employing our laser-based fly control device, farmers can effectively manage fungus gnat populations, ensuring healthier crops and reducing potential yield losses. This innovative technology paves the way for a more sustainable and ecologically conscious approach to pest management in agriculture, benefiting both farmers and the environment alike.