Web Extra: Shetlar's insecticide research

While not the main focus of my research, I believe that many turfgrass managers do not have a true appreciation for the diversity and abundance of organisms that inhabit turfgrass systems. Unfortunately, some environmentalists have labeled managed turf as being an “urban desert with a monoculture of grass.” This short-sighted view glosses over even the botanical biodiversity found in most lawns, grounds and golf courses.

Remember that a cultivar of corn or wheat is pretty much a single genetic strain of corn or wheat, but a cultivar of most turfgrass is actually a group of plants that exhibit desired traits. This means that the group has much greater genetic diversity than the corn or wheat and this diversity will likely change over time, depending on environmental conditions and the management level. This would be like calling a forest made only of maples, a monoculture, when, in fact, there may be four to five species of maples in the forest. In most lawns, there are two to three species of grass cultivars present which also contributes to greater biodiversity. Even blends of Kentucky bluegrass, perennial ryegrass or tall fescue consist of two to three cultivars selected from very different lines of breeding in order to achieve diversity for withstanding environmental, disease and insect attack. But, I digress. Let’s look at the arthropod diversity within a typical Ohio lawn.

The research plots used in my studies are sodded Kentucky bluegrass maintained at a standard lawn height with medium to high maintenance schedules (e.g., fertilized with ~4.0 lb. N/1000 square foot per year, mowed once or twice a week at 3.0-inch, irrigated sufficiently to keep green during drought, and treated with broadleaf herbicides when needed). Sub plots were treated with various insecticides registered or being developed for turfgrass usage. Each sub plot was sampled prior to insecticide treatment and weekly thereafter. To assess the arthropod populations, 4.25-inch diameter cup cutter samples (consisting of the turf, thatch and approximately 2-inches of underlying soil) were pulled and subjected to Berlese funnel extraction (Picture 1). Most of the arthropods contained within these cores (as well as earthworms and slugs) move downward (the funnels have 40W light bulbs held over the cores), drop from the soil, slide down the funnel sides and fall into a jar of alcohol where they are preserved for later inspection.

The number of organisms per square foot was found to be an average of 3,480 in the pretreatment samples extracted in mid-July and analyzed (Table 1). However, the population variance was astoundingly high, meaning that there was not a uniform distribution of arthropods in each plot or sample examined. The number of organisms ranged from a low of 859 to a high of 6,100 per square foot. Ground nesting ants are a major source of this variation as single samples often had hundreds or no ants. This is simply due to having taken a sample through an ant colony or a sample that had no ant colony. For this reason alone ants were discounted when running the statistical analysis.

Even then, the variance was still large. Throughout the study, there were relatively steady average numbers of total organisms in the control plots per square feet. Therefore, we want to know more. What populations (at the identification level of family) are changing to maintain this relatively steady number of total animals throughout the year?

After completing the assessment of the non-target effects of insecticide applications to lawn turf, and since sifting through these Berlese funnel samples is so much fun (just kidding), I decided that a closer look at the population changes of specific groups was warranted. I also decided to take more samples per plot in an attempt to reduce the variance. After examining the pretreatment samples or two additional sites where the complete set of insecticides were applied, I found an average of 3,397 organisms per square foot with a low of 1,056 and a high of 8,648 arthropods per square foot in the control plots. Back to the drawing board? No.

The project has now taken on another angle, ecological. We are noticing that there is nearly 1.5 to 2 times the number of oribatid (scavenger) mites than non-oribatid (predatory) mites in each sample. We are also seeing that there is a trend for combined mite and combined Collembola (springtail) abundance to be closely proportional in terms of distribution in and around each other. In short, when there is an increase in mites, there is a near proportional decrease in Collembola abundance, and vise versa. In my current studies an agricultural pest, Symphyla (garden centipedes), has been noticed in some samples at numbers that would be considered damaging levels in an agricultural crop. Though there was no evident damage seen in the turf, this is something that should be pursued further. Symphyla feed on root hairs and can cause stunting of plants. Is turf susceptible to similar damage or does the feeding of symphylans encourage attacks by disease?

As far as the non-target impact study goes, we noticed that there was very little to no impact of applications of either registered or yet-to-be registered insecticides on non-target turfgrass-inhabiting arthropods. In some cases, abundance of some arthropod groups increased the following spring from their autumn populations. However, what we are noticing with this follow-up study is that this increase in numbers may be due to chance or other environmental factors. It appears that weather conditions (cold, heat, rainfall, etc.) are an overriding force regulating arthropod populations to a much greater extent than the temporary effects cause by insecticide applications.

What does all this mean? It is obvious that our average lawn habitat is not a “desert” as claimed by some environmental groups. However, to get a good grasp of what is really happening beneath our feet, a multi-faceted study needs to be conducted examining not only the arthropod make-up but also: turf cultivars and turfgrass population genetics, fungi, weeds, micro-nutrients, organic matter cycling, weather conditions, and etc. This vast “desert” of an ecosystem in all of our backyards is more of a tropical jungle, but on a Lilliputian scale, with its own set of checks and balances with resource competition and biological/biomass carrying capacity.

Table 1. Typical Arthropods Recovered From July Pre-treatment Plots (numbers represent average number of critters in control plots per square foot).

Diplopoda: millipedes (3.35)

Chilopoda: centipedes (0.30)

Symphyla: garden centipedes (205.84)

Arachnida: spiders (7.41)

Acarina: mites (1969.1)
 
Oribatidae: scavengers and mycophagous - feed on organic matter (1093.97)
 
Non-Oribatidae: (3 other families) predatory mites (875.13)

Hexapoda: insects (1293.01)
 
Diplura: primitive insects (10.35)
  
Campodeidae (6.19), Japygidae (4.16)
 
Collembola: springtails (1212.01)
  
Entomobryidae (46.08), Isotomidae (764.30), Sminthuridae (221.98),    Onychiuridae (179.66)
 
Coleoptera: beetles (58.77)
  
Staphylinidae (8.42), Carabidae (0.30), Elateridae (1.02), 
  
Other families (4.67), beetle larvae (44.36)
 
Diptera: fly adults and larvae (5.08)
 
Hymenoptera: parasitic wasps and an abundance of ants (5.08)
 
Thysanoptera: thrips (1.02)
 
Sternorrhynca: aphids (0.71)