Figure 1. Graphic diagram of (A) choice, (B) no-choice study. The clay models were placed at various turfgrass canopy levels (6). Images by Fawad Khan
The aesthetic and commercial value of turfgrass can be jeopardized by feeding or the mere presence of insect pests (7). If we take a vertical section of turfgrass, it can be broadly subdivided into three zones — above ground, thatch and below ground
(8). Many pest insects occupy and utilize these zones as part of their life cycle, herbivory or predation. Some insects remain in one zone entirely or move out after spending some or most of their life. However, it is unclear how predators interact
with the prey within the narrow canopy of turfgrass. The prey could be an insect pest, and its biology, density, behavior and movement within zones may influence how predators interact. Many insect pests are reported within the turfgrass systems,
such as armyworms, cutworms, mole crickets, ants, etc.
Turfgrass is maintained at various heights. The abundance of predators and prey organisms tends to increase with the increase in turfgrass height. This hypothesis was proven for rove beetles and spiders in cool-season turfgrass, as more rove beetles were
found in taller turfgrass (2). Regardless of the turfgrass height, it is important to understand where predatory activity or predation is concentrated in the turfgrass canopy. This information will help refine management tactics with minimal disruption
to beneficial organisms.
Figure 2. The wooden stake deployed within the turfgrass canopy.
Materials and methods
Field experiments were conducted at the University of Georgia, Griffin. The turfgrass field was planted with Tifway bermudagrass and maintained at 3.15 inches (8 centimeters) high. The grass was mowed weekly for eight weeks.
Clay models were used to determine insect interactions within the turfgrass canopy. The models were prepared using green, nontoxic and nonhardening clay that simulated the caterpillar pests. A previous study demonstrated that predators actively interacted
with these models and created distinct impressions or interaction marks (5). The clay models were prepared and glued to wooden stakes.
Choice and no-choice studies were conducted. For the choice study, three clay models were attached to a wooden stake at 1 inch (2.54 centimeters), 2 inches (5.08 centimeters) and 3 inches (7.62 centimeters) from the thatch surface and were referred to
as lower, intermediate and upper canopy levels, respectively (Figures 1A and 2). When deployed, the lower-canopy-level clay model sat on the thatch surface. For the no-choice study, a clay model was attached to one level on the stake at 1 inch, 2
inches or 3 inches (Figures 1B and 2). The models (treatments) were replicated 30 and 10 times for the choice and no-choice experiments, respectively. Because fall armyworms were active from July to September in Georgia, the experiments were conducted
during this period.
Many types of impressions were recorded after 24-hour exposures on the clay models. The major impression types recorded were paired mark, scratch, deep cut mark, prick, dent and U-shaped mark (Figure 3).
The percentage of damaged clay model surface area was calculated for some impression types, such as deep distortion, stacked surface impression, scooped mark and granulation. The affected surface area on the model was estimated using a scale system (0
= 0% and 10 = 91%–100% impressions on the model surface).
Figure 3. The impression types observed on the clay models.
About 92% of 720 clay models had at least a predator interaction or impression. All choice and no-choice experiments showed that impressions were more severe on the clay models placed at the lower canopy level than intermediate and upper canopy levels
In July, the numbers of scratch, deep distortion, scooped and granulation impressions were greater on the lower canopy level than on the upper canopy level. The densities of paired marks were more abundant on the upper and lower canopy levels (Figure
5). The prick impression was greater on the upper than the intermediate canopy level. Other impressions recorded in the July experiment were similar across canopy levels. In September, paired marks were more abundant on clay models at the lower canopy
level than those at the intermediate and upper canopy levels (Figure 6). The numbers of scratch marks on the clay models at the lower and upper canopy levels were greater than those at the intermediate canopy level. Other impressions were similar
in densities among placement heights.
In July, greater densities of paired marks were found on clay models at the lower canopy level than those at the intermediate and upper canopy levels (Figure 7). More scratches were observed on clay models at the lower canopy level than those at the intermediate
canopy level. However, more densities of pricks were found on the clay models at the upper canopy level than those at the intermediate canopy level. Other impressions were similar across the canopy levels. In September, more paired marks were observed
on clay models placed at the lower canopy level than those at the upper canopy level (Figure 8). Other impressions were similar among canopy levels.
Figure 4. Mean (±SE) severity rating observed on clay models in choice experiments during (A) July and (B) September 2020 as well as no-choice experiments during (C) July and (D) September 2020. The same letters above the bars denote no significant difference (6).
Discussion and conclusion
It appears that active populations of insect predators are more abundant on the soil or thatch surface than on the canopy above the soil surface in turfgrass. Larval stages of armyworms and cutworms are active within the canopy of turfgrass. Predators
likely eat them, especially those on the soil or thatch surface. Moreover, the predators are likely to affect the behaviors of pest insects, such as mating, egg laying and movement (4). Moreover, paired marks near the soil layer were the most abundant
common impression. Previous research showed that ground beetles, ants and other predators generated the paired marks on the clay models (5). In turfgrass, the most abundant predator is ants (1). Ants consume the eggs and larvae of armyworms and other
Some unintended factors could have influenced the data. It is unclear how many of these impression types were created by birds. The vertical placement of clay models could have reduced or overestimated impression types and their densities. Because the
clay models were positioned to one side of the wooden stake, it could have limited the exposure to predators and reduced the attack rates. Many other factors could reduce the abundance of beneficial insects, such as height, temperature, etc.
In conclusion, greater interactions with foraging predators on clay models were observed along the surface of soil or thatch. High densities of rove beetles and spiders were more prevalent in taller rather than in the shorter turfgrass (3). Thus, more
research is warranted investigating the role of turfgrass height and disruptive cultural practices on predators in various turfgrass genotypes.
Figure 5. Mean (±SE) impressions (A) paired marks, (B) scratches, (C) deep cut marks, (D) deep distortions, (E) pricks, (F) dents, (G) stacked surface marks, (H) scooped marks and (I) granulations observed on clay models in choice experiments during July 2020. The same letters above the bars denote no significant difference (6).
Figure 6. Mean (±SE) impressions (A) paired marks, (B) scratches, (C) deep cut marks, (D) deep distortions, (E) pricks, (F) dents, (G) stacked surface marks, (H) scooped marks and (I) granulations observed on clay models in choice experiments during September 2020. The same letters above the bars denote no significant difference (6).
Figure 7. Mean (±SE) impressions (A) paired marks, (B) scratches, (C) deep cut marks, (D) deep distortions, (E) pricks, (F) dents, (G) stacked surface marks, (H) scooped marks and (I) granulations observed on clay models in no-choice experiments during July 2020. The same letters above the bars denote no significant difference (6).
Figure 8. Mean (±SE) impressions (A) paired marks, (B) scratches, (C) deep cut marks, (D) deep distortions, (E) pricks and (F) dents observed on clay models in no-choice experiments during September 2020. The same letters above the bars denote no significant difference (6).
The research says...
- Impressions were more abundant and severe on clay models placed at the lower canopy level of turfgrass than on those placed at the intermediate and upper canopy levels.
- It seems predators are active on soil and thatch and likely to encounter prey at the soil level.
- Administration of cultural practices, such as mowing, and management tactics, such as insecticide use, warrant consideration for the conservation of beneficial insects.
Experiments outlined in this article were supported by the Hatch project, the University of Georgia and the Fulbright Foreign Student Program.
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Fawad Z.A. Khan (email@example.com) was a Fulbright doctorate scholar in Shimat V. Joseph’s laboratory at the Department of Entomology, University of Georgia, Griffin Campus, and is currently an assistant professor at the Institute of Plant
Protection, MNS University of Agriculture, Multan, Pakistan. Joseph (firstname.lastname@example.org) is an associate professor of turf and ornamentals entomology at the Department of Entomology, University of Georgia, Griffin Campus.