1. Introduction
As the world population and demand for food and fiber increases, the ability to sustainably grow plants is expected to be impeded by climate change, the loss of arable land, and increasing challenges from insects, weeds, and diseases [
1,
2]. Although integrated pest management (IPM) has contributed to the sustainability of our agricultural (Ag) systems, IPM is still highly dependent on chemical pesticides and transgenic crops engineered with bacterial protein toxins and herbicide resistance [
3,
4,
5]. Resistance to pesticides is a growing challenge [
6,
7,
8,
9,
10]. Most recently, insect resistance to genetically modified organisms (GMOs) has become a world-wide concern [
10,
11,
12,
13].
Public preference is clearly for insect control and food grown without chemical pesticides, and plants that are not genetically engineered [
14]. One approach of interest to our group has been taking advantage of applications in the material sciences for pest management. To illustrate the power of this approach in arthropod vector biology and human disease prevention, we developed the non-insecticidal, T (trapping) bed net that was 13 times more efficacious in Africa for killing mosquitoes than a long lasting, insecticide-treated bed net [
15]. In another example, non-insecticidal, mosquito bite resistant-textiles and garments were developed that looked like and felt like everyday clothing but prevented blood feeding using only textile structure; prior to this research, insecticides bound to clothing were mostly used to prevent mosquito biting [
16].
The use of textiles and physical methods to prevent insect infestations in crops was used before for kiwi [
17], apples [
18,
19], peppers [
20], tomatoes [
21], cucumbers [
22], cabbage [
23,
24], and spinach [
25] and for other applications including greenhouses [
26]. The technology was mostly based on size exclusion, similar to how window screening prevents insects from entering a home. A challenge with this approach is the size of the pore needed to exclude small insects like thrips or caterpillar neonates. Small pore sizes reduce sunlight, air and water penetration. However, research studying mosquito biting across clothing [
16] found that blocking insect penetration was not limited to just size exclusion. The use of a tortuous path allowed for larger openings across the textile while at the same time excluding insect penetration. We have applied this new knowledge in this paper to examine the use of spacer fabrics as an insect barrier for crop production.
3. Results
To determine the feasibility of using a knitted 3-D fabric to exclude insects from plants, two spacer fabrics (Plant Armor Gen 1 and 2;
Figure 2A,C–E) were made and compared to a commercially available, single layer (knitted textile) crop cover, Proteknet (
Figure 2B). The three layers of the spacer (3-D) fabric and the general method of weaving that produces the structure are illustrated in
Figure 2A. The size of the holes on the outside layers of Plant Armor Gen 1 were different. The pores in the top layer were 349.13 ± 18.15 µm (mean ± SD) in diameter (
Figure 2C) while the pores in the bottom layer facing the plant were 99.24 ± 12.90 µm (
Figure 2D). For Plant Armor Gen 2, the pore sizes were the same on both sides, 15.58 ± 2.97 µm (
Figure 2E). The average hole diameter in Proteknet was 46.51 ± 4.91 µm (
Figure 2B). The average fabric thickness for Proteknet, Gen 1, and Gen 2 were 0.25 ± 0.01, 8.9 ± 0.06, and 1.07 ± 0.01 mm.
The weight per unit area of Proteknet was 3.23 ± 0.04 g/m
2, Plant Armor Gen 1 was 49.21 ± 1.63 g/m
2, and Plant Armor Gen 2 was 14.59 ± 0.10 g/m
2 (
Figure 3A;
F = 2192.8, df = 2,
p < 0.0001). The percentage sunlight penetration was 100, 80, and 75%, respectively (
Figure 3B; no variation in replicates for each treatment). Proteknet had no significant impact on reducing light (<1%, the precision limit of our measurement), while Gen 1 and Gen 2 were similar. The rate of simulated raindrops and their penetration in grams across the textile over time was almost identical between the three textiles once they reached water saturation (
Figure 3C). The water first penetrated Proteknet at 15 s, Plant Armor Gen 1 at 30 s and Gen 2 at 60 s. The differences between textiles resulted from the water filling the internal empty space of the 3-D fabrics; once filled, water dripping.
Below the fabric started water filling was minimal for Proteknet since it was a single layer of cloth. Once the threshold for filling was reached for the 3-D fabrics, the slope of water weight increase with time was similar for all textiles tested (
Figure 3C). The insert in
Figure 3C shows a diagram of the experimental device used for water penetration (explained in detail in the Materials and Methods). There were large differences in the air permeability of the fabrics (
Figure 3D). Proteknet had the highest air permeability at 5500.7 ± 39.2 m
3/m
2/min followed by Plant Armor Gen 1 at 3309.7 ± 22.5 m
3/m
2/min and Plant Armor Gen 2 at 1621.4 ± 37.1 m
3/m
2/min (
F = 1921.60, df = 2,
p < 0.0001). Water vapor transmission rate in the control was significantly higher in the control (
F = 15.15, df = 3,
p < 0.05) with no differences in transmission rates between textiles (
Figure 3E; Tukey’s test,
α = 0.05).
The durability of the textiles was investigated using a Martindale abrasion test (
Figure 4). The assay uses equal pressure and simultaneous circular rubbing of the textile surface, looking for the first visible change in textile structure. Observations were made without magnification, and the end-point is the first appearance of a hole in the textile. The number of cycles of circular motion to the endpoint is a measure of durability. The endpoints for Plant Armor Gen 1 and 2 were similar in appearance (
Figure 4C), represented by the first appearance of a small hole (= disruption of the planar surface structure of the fabric). Proteknet degraded much faster because of its significantly lower mass per unit area (
Figure 3A) and single layer (
Figure 2B). A hole never appeared, but instead, there was a total disruption of the planar structure of the fabric (
Figure 4C, left picture). Based on cycles alone, Plant Armor Gen 1 performed the best reaching the endpoint at 60,000 and 58,000 revolutions for the top and bottom of the cloth, respectively, followed by Plant Armor Gen 2 at 18,000 cycles, and then Proteknet at 10,000 (
Figure 3B;
F = 160.15, df = 3,
p < 0.0001). The Proteknet end-point at 10,000 revolutions was a total breakdown of the planar structure of the fabric, suggesting the durability was much lower than what the quantitative Martindale abrasion test results suggested.
The Petri dish assay measured penetration from the top chamber that lacks a food source and hydration across the textile to the bottom chamber that contains a cabbage leaf (
Figure 1A). The insects placed in the top chamber must pass through the test textile to gain access to the bottom chamber. The number of insects in the bottom chamber was a measure of the textile resistance to penetration. The adult thrips penetration rate was higher for Proteknet followed by Plant Armor Gen 1 and then Gen 2 (
Figure 5A–C, respectively). The time to 50% penetration (
Figure 5D) was significantly different between the textiles—12.0, 29.7, and 174.7 min, respectively (95% confidence intervals did not overlap determined by probit analysis). Adult thrips were used in these studies because of their uniform size and minimal developmental differences in physiology and size compared to different instars and age within a stadium. Age within a stadium greatly affects the physiology of an insect, its attraction to food, and mobility. Larvae were also much more difficult to sort without causing harm.
When this experiment was repeated with unfed bollworm,
H. zea, caterpillar neonates, the Plant Armor Gen 2 resistance was ≥90% at 0–11 h and 89% at 12 h (
Figure 6), after which the experiment was ended because of reduced neonate viability. They had no access to water or food during the experiment but were still mobile at 12 h. Proteknet and Plant Armor Gen 1 penetration by
H. zea neonates were similar with no significant differences in the time to 50% penetration—2.35 and 3.11 h, respectively (confidence intervals overlapped determined by probit analysis), and clearly differed from Gen 2 (
Figure 6; 95% confidence intervals did not include Gen 2).
Small cage studies were conducted with whole plants (
Figure 1B) to further examine whether Plant Armor Gen 2 would be 100% resistant to unfed
H. zea neonate caterpillars. The results were unequivocal. Cabbage plants not covered and exposed to the insects were infested with
H. zea larvae while plants covered with Plant Armor Gen 2 fabric were not infested (
Figure 7A). Not a single neonate was found on the covered plants after 10 days shown as zero in
Figure 7A. Plants that were covered showed no insect damage visually while the plants not covered had major insect damage (
Figure 7B).
A three-month, small field plot study was conducted during summer months to assess plant (cabbage) agronomics when covered with Plant Armor Gen 1 and Gen 2 compared to the commercially available, plant cover, Proteknet. The planting layout and fabric coverage is presented in
Figure 1D and the actual test plots shown in
Figure 1C before the right half of each plot was covered with fabric. No pesticide treatments were made to protect the plants before or during the experiment. The plants were exposed to ambient natural environmental conditions and insect stresses. Weeds were controlled for the uncovered plants by pulling them from the soil by hand. Plants that were covered did not have weeds. The raised beds were 61 cm apart end-to-end (
Figure 1C). The uncovered and covered plants received the same amount of watering. The textile cover was not removed and the water was sprayed onto the cover with the same spray parameters used for the uncovered plants. The assignment of a textile type to a particular plot was random, and the same was the case for each plant shown (
Figure 1C). Cabbage was used for these studies to be consistent with the small cage study (discussed earlier).
There were significant differences between the treatments both for the wet weight (
F = 12.58, df = 5,
p < 0.0001) as well as the dry weight (
F = 18.01, df = 5,
p < 0.0001). In the Proteknet test, there were minimal visual differences between the uncovered and covered plants (
Figure 8A) which was also reflected in no statistically significant differences in the plant wet weight (
Figure 8D) or dry weight (
Figure 8E) (Tukey’s test,
α = 0.05). In contrast, the Gen 1 covered plants looked larger (
Figure 8B) and had an average 1.96-fold higher wet weight (
Figure 8D) and 2.13-fold higher dry weight (
Figure 8E) (Tukey’s test,
α = 0.05). For Gen 2, the covered plants also looked larger (
Figure 8C) reflected by an average 2.93-fold higher wet weight (
Figure 8D) and 1.78-fold higher dry weight (
Figure 8E) (Tukey’s test,
α = 0.05). The expectation was that the size of the uncovered plants would be similar between field plots, but this was not the case. This is best illustrated in the wet and dry weight determinations (
Figure 8D,E, respectively). The uncovered plants were significantly smaller in the Proteknet test, Gen 2 were larger, and Gen 3 the largest by weight, regardless if they were covered or uncovered.
There were no temperature (1200:
F = 0.58, df = 2,
p = 0.568; 2400:
F = 0.20, df = 2,
p = 0.979) and humidity (1200:
F = 1.29, df = 2,
p = 0.299; 2400:
F = 0.40, df = 2,
p = 0.679) differences outside versus under Proteknet, Plant Armor Gen 1, and Plant Armor Gen 2 over 7 d in the small plot study (
Figure 9A,B and C,D, respectively). These measurements were taken daily at 1200 and 2400.
4. Discussion
Organic farming that minimizes any negative environmental impact and uses recycled materials is not only highly valued and “sought after” by the public but could have significant economic benefits to the farmer. Any new technology should consider challenges to the sustainability of agriculture in the face of reduced arable land because of population growth and global warming. Considered in our current research was the use of physical methods, i.e., textile barriers, to reduce insect access to plants that at the same time might reduce the need for pesticides and water and enhance plant growth.
Insect barriers like window screening on homes and crop covers use the simple principle of size exclusion. The holes in the barrier are smaller than the size of the insect body, and the insect cannot fly or crawl through the holes. The challenge with this approach, exclusion of small insects like thrips or newly hatched caterpillar larvae require holes so small, they block light, air and rain and trap heat and humidity under the textile reducing or preventing plant growth. Our group has been studying mosquito penetration of fabrics for blood feeding on humans and found critical head morphometrics that governed their ability to penetrate a textile [
16]. For mosquitoes, we found that textile pore size, thickness and the tortuosity of the path from outside to inside of a cloth worked together to prevent blood feeding. Based on this research, we constructed 3-D spacer knits that were 100% mosquito bite proof. When made into garments, they looked and felt like everyday clothing and had a high degree of thermal comfort with high air and water vapor transmission. This research showed that an insect barrier is not limited by size exclusion. A tortuous path allows for larger holes than would be expected based on size exclusion only. The objective of this paper was to apply this knowledge and demonstrate proof of concept of using 3-D fabrics as a crop cover to prevent herbivory and improve plant growth.
We report here the first successful use of spacer fabrics to reduce insect penetration and improve plant growth. The pores of Plant Armor Gen 1 on the inside layer facing the plant were 99 µm and on the outside 349 µm. Proteknet, a single layered crop cover used in agriculture had pores 57 µm in diameter. The TP
50 against tobacco thrips adults in a worst-case Petri dish penetration assay we developed, was 30 min for Gen 1 (with the larger holes) compared to Proteknet (with smaller holes) where the TP
50 was 12 min. An even smaller holes and more tortuous path in Gen 2 increased the TP
50 to 175 min for thrips. Gen 2 fabrics demonstrated greater than 90% bite resistance from 0–11 h and 89% at 12 h for neonate caterpillars while neonates were able to penetrate Gen 1 and Proteknet (at equal rates). The Petri dish assay used in these studies measured textile resistance based on the rate insects moved from a top chamber across the test textile to a lower chamber containing a plant leaf. This movement could be a result of either positive gravitational tropism, innate dispersal behavior, random locomotor activity, attraction to the increased humidity, color and odorants, or in combination, from the cabbage leaf. Regardless of the reason for movement into the bottom chamber, insects were found in the bottom chamber over time, and the percentage of insects in the bottom chamber was used as a measure of insect resistance across the fabric. Because this was a simple two choice assay with equal volume limits for the top and bottom chambers, insect crowding in the top chamber at the assay start could affect the movement rate to the bottom chamber, and short travel distances existed between the top and bottom chambers; this test was considered a worst-case scenario bioassay. Our hypothesis is that resistance would be higher under field conditions where the space outside of the cover is essentially infinite (and insects have other choices for feeding and moisture) and insects must travel larger distances in many cases to find the covered plant. Therefore, the level of bite resistance in the Petri dish assay should not be automatically considered the same as that in a real world, field application for any of the textiles studied. More detailed research will be needed to understand the important interactions between pore size, thickness, and the tortuous path of a textile and the many different insect body sizes and behavior, crop plants, and environmental conditions possible to prevent plant herbivory. In the case of mosquitoes, real-world percentage penetration could be mathematically defined with a high level of accuracy [
17].
Exclusion of insects from plants cannot come at the cost of adversely affecting plant agronomics. Since Plant Armor Gen 2 had high H. zea resistance to penetration in both Petri dish assays and longer-term small cage, laboratory studies and Gen 1 and 2 demonstrated higher resistance to tobacco thrips adults, small field plot trials were conducted to compare their impact on plant growth compared to a Proteknet positive control. The physical characterization of each textile showed that we had similar water and moisture vapor transmission but light penetration was reduced by 20 and 25% and air permeability by 40 and 71% for Gen 1 and Gen 2, respectively, compared to Proteknet. Over a 3-month small field plot study with covered versus uncovered cabbage, plant size based on plant total wet and dry weight was about twice that of uncovered plants for Gen 1 and 2. This difference did not occur for Proteknet. The increased plant size above ground of the covered plants was clearly visible before the plants with their roots were removed from the soil. No differences in temperature or humidity under the plant covers compared to outside of the cover for all three textiles was found. The field plots were in direct sunlight and not shaded. There was a difference in the size of the plants overall between the different raised beds; the smallest plants were in the Proteknet plot, followed by Gen 2 and then Gen 1. The reason for these differences between plots is unknown, but since comparisons were made between uncovered and covered plants in the same plot, the study conclusions are reasonable (but require further study).
The effect of shade nets on plant agronomics was described before [
20,
28,
29]. Shade from netting provided a favorable plant respiration rate [
30] and lowered surface temperature [
21,
30,
31], provided a more amenable microclimate under the cloth [
23,
24,
32], and improved the richness of leaves [
33]. Shade cloth color also affected plant growth [
34]. Gen 1 and 2 provided greater shading than Proteknet as measured by our light penetration experiments; the visual color of the cloths tested were similar. Other possible reasons that the Gen 1 and 2 covered plants were larger compared to uncovered plants may be due to better soil moisture retention because of shading, differences in either the plant, soil, or both microbiomes, lower insect and animal plant damage, reduced competition with weeds, and the growth of larger leaves to compensate for less light.