Summary

Most insects die soon after birth, and their lives probably contain more suffering than happiness. If you have a lawn that you actively maintain, you should consider converting your grass to hard landscape materials like gravel, or to artificial turf, to reduce plant biomass and therefore insect suffering. This could save you labor in the long run, reduces your probability of tick bites, and may be good for the environment. It may prevent at least hundreds of insects from suffering per dollar and might even save you money. However, if you don't actively manage your lawn, the calculation becomes less clear because eliminating a grass lawn might slightly contribute to climate change.

Note: I assume that in many places, neighborhood rules or building codes might prevent you from replacing a lawn with gravel. Would artificial turf be acceptable? Also, maybe it would be allowed to at least put a strip of rocks around the perimeter of your home, or something like that? Doing so might also slightly reduce the number of insects that come indoors. Finally, when picking out a house, you can consider gravel-friendliness as one factor. Other things being equal, it seems better to choose a house in a more rural area where you can buy a larger amount of more secluded land.

If you can't use gravel, you could consider adding a porch or other structure that would cover some grass, although the amount of land covered per unit cost might be small.

See also "How Much Bug Suffering Would Be Prevented by Converting a Lawn to Gravel?" and "Invertebrates on the Lawn".

Motivation

It's often claimed that there's nothing humans can do to reduce the suffering of insects in the wild. This claim is wrong. One important way to help wild insects is to study the effects of human environmental policies on insect populations and favor those policies that cause fewer insects to come into a nasty, brutish, and short existence. This work is important but involves a high degree of uncertainty in the analysis.

If you actively manage your lawn, then a relatively clear-cut way to reduce insect suffering is to replace your lawn with non-biological materials in order to prevent plants from growing. The reason to focus on eliminating plants is because plants are the "roots" of the food chain. As long as plant energy is created, someone is eventually going to eat it (whether insects, bacteria, or other critters). So just shuffling around population dynamics for heterotrophs isn't solving the fundamental problem that more plant energy means more life. Preventing photosynthesis stops the problem before it gets started.

Material types and costs

Converting grass lawns to gravel, glass, artificial grass, or other materials is a common landscaping practice—one that many people do without giving a thought to insect suffering. You can find tons of articles by searching {gravel landscaping} and similar queries. This page has a slideshow of options.

Another page gives prices for different materials:

Type Price Decomposed granite $35 to $50 per cubic yard Pea gravel at least $33 to $50 per cubic yard River rocks $100 to $700 per ton Crushed granite gravel at least $60 per (cubic?) yard

The cost to install a square foot of pea gravel is $1.1 to $1.8. Another source reports $1-$3 per square foot.

Artificial grass costs $5 to $20 per square foot.

Gravel costs vs. lawn-care savings

The average lawn in the US is about 0.2 acres, or ~8700 square feet. Assuming $1-$3 per square foot, the cost of converting your lawn to gravel would be ~$9K to ~$27K.

Once converted, you can avoid the costs of mowing, watering, etc. This page estimates $800-$1000 to pay for lawn-mowing services per year. If you do the mowing yourself, say it takes 10 hours per year at a cost of $50/hour, or $500. Assume a discount rate of 5% and that the gravel lawn lasts 20 years. Saving $500/year for 20 years has a present value of $500 * (1-1.05‑20)/(.05/1.05) = $6,500. Saving $1000/year has a present value of $13,000. So a gravel lawn might or might not pay for itself. Of course, this calculation also depends on how a non-grass lawn affects your home price relative to a grass lawn, whether gravel is allowed by local regulations, etc.

If converting to gravel saves you money, that's great. Even if it doesn't save you money, it might be worth doing anyway depending on its cost in order to reduce insect suffering. How much insect suffering it reduces is the topic of the next section.

How many insects are helped?

All told, the Earth has about 13.4 billion hectares of land. Your 0.2-acre lawn is about 6*10‑12 of the total. If the world's 1018 insects were distributed evenly across the Earth, your lawn would contain ~6 million insects. This estimate may be too high, because tropical regions of the world house a disproportionate share of insects, although Arctic and Antarctic regions contain disproportionately fewer insects.

This discussion estimates insect densities on farm land as ~10 million per hectare, or ~4 million per acre. That again seems plausibly too high for lawns. Indeed, sometimes I don't see any bugs on lawns, though at other times I see lots of little critters hopping around. But lawns definitely are home to bugs.

A slightly lower estimate of how many insects live on lawns at a given time or are later supported by the biomass produced on lawns (e.g., decomposers eating dead grass clippings) at a given time might be—just making up a number based on personal observation—5 per square foot.a Assuming a lawn of 0.2 acres, that implies ~44,000 insects supported by your lawn's grass at any given time. Grass biomass also supports tons of bacteria, nematodes, etc. (especially when it decomposes). Suppose these collectively matter as much as all the insects. Then your lawn supports a total of 44,000 actual insects + 44,000 equivalent insects = 88,000 "effective insects" at a given time during the non-winter months. (The appendix at the end of this piece estimates a similar, though slightly higher, number of effective insects using an alternate approach.) Making the conservative assumption that these insects go through one generation per year, then you prevented 88,000 effective insect deaths per year. And if the gravel lawn lasts, say, 20 years before needing to be redone, that's 88,000 * 20 = ~1.8 million effective insects saved from a short life and miserable death. And this ignores the fact that many of those insects would give birth to hundreds of baby insects, most of which would die soon after being born. So the actual number of insect deaths prevented may be many times higher.

If converting your lawn to gravel was financially positive from the beginning, then these reductions in insect suffering are just a bonus. If, instead, converting the lawn cost, say, $10K, then you would have prevented ~180 painful insect lives per dollar, which is quite impressive!

Not having a lawn is also good for the grass that doesn't come into existence, assuming you care about plant suffering to a nonzero degree. Michael Pollan: "That the perfume of jasmine or basil, or the scent of freshly mowed grass, so sweet to us, is (as the ecologist Jack Schultz likes to say) the chemical equivalent of a scream?"

Obviously this analysis is simplistic and ignores many relevant factors. For instance, maybe gravel lawns would collect puddles of rainwater, which would allow more mosquitoes to breed than before, thereby increasing mosquito populations. Many more possibilities like this should be explored.

What about climate-change impacts?

Grass sequesters carbon, so non-biological lawns might worsen climate change. This section explores the implications of this point.

Firstly, it's important to note that the net effects of climate change on wild-animal suffering and far-future suffering aren't obvious. But I think it's plausibly somewhat better in expectation to reduce carbon emissions than increase them. How much do lawns help to reduce carbon emissions?

In general, lawns don't sequester as much carbon as trees. Trees sequester carbon because they live for decades before decomposing, trapping carbon year after year as they grow. Grass lives maybe a year before decomposing. Still, grass does sequester some carbon into the soil that isn't fully released back into the atmosphere. Part of the reason for this is that grass has a lot of roots, which can trap carbon in the soil.

If you actively maintain your lawn, having and managing a grass lawn may be a net source of greenhouse gases. "Each hour of lawn mowing generates as much pollution as driving a car nearly 100 miles." (source) One study found that managed lawns in Florida sometimes increased net greenhouse-gas emissions. This blog post rejects claims by the lawn-care industry that managed lawns are net carbon sinks. Of course, if you don't manage your lawns and let them grow wild, they'll be net carbon sinks, but the magnitude of the effect isn't huge (see Table 1 here).

Finally, you should consider using white or light-colored gravel or other materials to increase albedo. In fact, one page actually recommends that you "Replace part of your lawn with a white gravel rock garden. Instead of your grass absorbing heat, the white gravel will reflect radiation back into space."

Calculation if you don't maintain your lawn

Suppose that in the most extreme case, you don't manage your lawn at all but let the plants grow wild. Also ignore the albedo potential of white gravel. How much could the climate-change impact of replacing lawn with gravel be?

This study found that lawns can sequester 46 to 127 g C per m2 per year. Another study found (Table 6) net carbon sequestration rates between 0.32 and 0.74 Mg per hectare per year. Since a hectare is 10,000 m2, this implies 32 to 74 g C per m2 per year.

As above, assume the average lawn is 0.2 acres, or 809 m2. Taking an estimate of, say, 46 g C per m2 per year implies ~37 kg C sequestered per year on the whole lawn. CO 2 has one carbon atom (mass number = 12) and two oxygen atoms (mass number = 16), so 37 kg C implies 37*(16*2+12)/12 = 136 kg CO 2 , or 0.136 metric tons. Using a similar calculation as is done here, and assuming that climate change increases "effective insect" populations by 2% in expectation over uncertainty (e.g., we might be 60% confident that climate change will increase effective insect populations by 10% and 40% confident that it will reduce them by 10%: 0.6*0.1 - 0.4*0.1 = 0.02), we find that our average lawn reduces insect populations by 79,000 per year.

79,000 is awfully close to the 88,000 estimate for how many effective insects were reduced per year by converting grass to gravel. Actually, I rejiggered some of my estimates to make these two come out about the same, in order to demonstrate the degree of uncertainty at play. (Had I let one or the other of these estimates win out, some readers might have falsely concluded that the question was clear one way or the other.) So if you have a lawn that requires no maintainance, then if you also think that climate change will increase wild-organism suffering on balance, then the question of whether to replace your lawn with gravel isn't obvious.

It's important to remember that

Carbon sequestration declines after grass has been growing on an area for 25-30 years because soils become saturated with carbon. (This study discusses lawn sink capacity further.) So if you have an old lawn, it may not be providing much sequestration value after all.

These calculations ignore the albedo potential of white gravel. How much would that consideration affect the results?

I don't have a good way to quantitatively compare the albedo effect versus the carbon-sequestration effect, but albedo could be quite significant, given that deforestation in high latitudes might actually cool the planet more than warm it because deforested land has greater albedo. I also don't know how much more expensive white gravel is.

If you do maintain your lawn

One study for lawn carbon sequestration found net sequestration even for actively managed lawns, especially since irrigation and fertilization increase carbon sequestration. However:

one of the study's authors was working for the Scotts Miracle-Gro Company and so would have been biased to favor lawns and active lawn management

the study apparently didn't consider the impact of nitrous oxide from fertilizer, which "can offset up to about 30 percent of the carbon stored in the lawns"

other studies as cited above have found that lawns can be net sources rather than sinks of carbon (though these studies have been criticized by pro-lawn researchers).

So for actively managed lawns, the jury is still out on their net climate-change impact. Thus, it seems more likely that replacing an actively managed lawn with gravel is a net plus.

Should you manage your lawn less if you don't convert to gravel?

Even if you don't convert your lawn to gravel, it's probably better if you stop using fertilizer, since this presumably increases biomass production and also may contribute to climate change.

You should also probably stop watering your lawn, since water increases biomass production (and may also deplete water resources in your area). Of course, watering somewhat increases carbon sequestration.

The sign of pesticide use is less clear, but because the grass will be eaten by some little critter or other down the road, it's plausible that pesticides increase net suffering by increasing deaths per unit time. If you don't care at all about bacteria, this calculation becomes more subtle, because it's possible that, say, pesticides will mean fewer insects but more bacteria, depending on what kinds of critters eat decomposing grass. Or maybe pesticides don't reduce net insect populations because they just translate to more decomposing insects to offset the fewer insects in live grass. I don't know.

It's unclear to me whether mowing your lawn is net good or bad assuming you still have a grass lawn. Not having a grass lawn avoids the downsides of mowing while capturing the upside.

If gravel is too expensive

If it would cost a lot to gravel over your lawn, you can still make a smaller difference by covering parts of your lawn with various permanent objects, like rocks, big pieces of wood, metal, etc. (I say "permanent" objects because if you move the objects around a lot, they may not inhibit grass growth as much and will crush bugs painfully each time they're moved. For similar reasons, I avoid walking on grass.) Of course, depending on what kind of neighborhood you live in, some of these possibilities may not be allowed. I'm also nervous about covering small regions of soil, since anecdotally if I lift up rocks or pieces of wood on lawns, I find tons of bugs making a home underneath them, which calls into question whether the covering objects actually reduce insect populations at all or instead increase insect populations. In contrast, it seems pretty clear that the gravel on my driveway prevents most surface-dwelling insects from living on that region of land except when passing by.

Weeds in gravel driveways?

A friend told me that weeds used to grow in his gravel driveway, which suggests the same might happen on gravel lawns. This would reduce the benefit of the gravel lawn in preventing insects. I've lived at several houses with gravel driveways and rarely saw weeds, so this problem isn't universal, but a web search shows that many other homeowners do have weeds in their gravel driveways.

The reason I focus this piece on gravel is that alternate materials, like asphalt or artificial turf, are more expensive, and I'd rather not dissuade people from covering their lawns because the material is so expensive. Also, "asphalt lawns" aren't a common landscaping option, so it might be harder to persuade people to go this route, and some neighborhoods might disallow it. Finally, asphalt may be worse for climate change because it's darker than gravel. (White concrete would be better in terms of albedo, but it's quite expensive.)

Buying land to cover it?

This piece has focused on the possibility of covering your own lawn, but in theory, a person could scale this approach by buying land to be covered. If you bought other houses just to cover their lawns, the strategy would be hopelessly expensive, but if you bought land that doesn't have a house on it, the cost would actually be modest: maybe a few thousand dollars per acre. For example, one of the first listings I found on a land-sale website was a sale of 75 acres of hunting land for $89,950. (Covering the land would also prevent hunting on it, of course.) Considering that the cost of covering an acre of land with gravel would be in the tens of thousands of dollars based on the estimates used above, buying land would actually be a small portion of the total cost of covering it. Of course, you'd also have to pay annual property taxes, but even if property taxes were ~2% of the land value (which may be high) paid each year for 20 years, the cost still wouldn't be that big. Also, if you bought huge amounts of gravel, you might get a discount relative to the prices for gravel on just your own yard.

Land in developing nations would be even cheaper. For instance, on an international property-selling site, I randomly found this listing for 75 acres in Belize at a price of $28,307. Presumably insect densities there would be vastly higher than in many parts of the US. That said, there might also be a cost in hiring someone to look after the property if you didn't live there. (Checking up on the property would presumably be less crucial in a developed country where enforcement of property rights could be relied on to a greater degree.)

In general, this approach of buying property to directly address wild-organism suffering on it is interesting. It's sort of the inverse of The Nature Conservancy's work. In terms of direct insect suffering prevented, it might sometimes be quite cost-effective. However, it doesn't have the same far-future impact potential that spreading concern for wild-organism suffering has. It might also make people angry and thereby tarnish the movement to spread empathy for wild organisms. This might be particularly true with respect to buying properties in developing countries, since people there depend more on their land for survival.

Maybe outrage at covering land with gravel would be reduced if the gravel covering was used for a purpose, such as to create (plant-free) rock gardens or trails/mazes based on rock colors. If the area were opened to tourists, doing this might generate some income. However, such a business would require a lot of work in its own right, and it's not clear it would make much money.

The biggest problem with buying property to cover with gravel might be climate change. Because non-home property generally isn't mowed and fertilized, it's probably a net carbon sink, so graveling it over would probably contribute to climate change, which has the potential to offset the benefit of reducing insect populations on the land. This would be less true if you bought farm land, which is "mowed" and fertilized, but presumably by doing that you'd mostly just shift farming to other pieces of property. If more forest or grassland was converted to farm land to compensate for your buying farm land, then this would still reduce carbon sequestration. (If you buy farm land, the price of farm land increases slightly, thereby slightly increasing food prices and hence reducing agricultural production, but this effect is probably small.)

So in practice, I'm doubtful that buying property to reduce wild-organism suffering on it is as promising as it seemed, but we should keep an eye out for other ways in which the strategy could actually work.

Maybe the albedo calculations for white gravel would show that it reduces climate change even relative to unmanaged grass, in which case buying land and converting it to white gravel would be good all around. This possibility seems important to explore further.

Another idea that would reduce concerns about climate change would be to cover land with solar panels rather than gravel.b However, my guess is that solar panels are really expensive, so unless a solar-power business like this would already be close to cost-neutral in its own right, it would probably be an expensive way to reduce insect suffering. And if such a business were profitable in its own right, then your doing it might partially displace solar panels that someone else would have laid down. (Solar panels on roofs don't displace grass, but solar panels on fields do somewhat insofar as they shade the underlying grass.) It would be ideal to lay down solar panels flat on the ground to prevent any grass from growing underneath, but this is less efficient at capturing energy than letting the panels tilt toward the sun.

Promoting gravel lawns

Another approach for making an impact beyond your own lawn could be to create an organization to subsidize home owners for converting their lawns to gravel or other non-biological materials. This should probably only be done for home owners who actively manage and fertilize their lawns. Ignoring the overhead to administer the organization and find interested customers, the cost-effectiveness of this approach would plausibly be similar as for converting your own lawn to gravel (or maybe more so because the organization could focus on those with the highest lawn-maintainence costs first). It would be important to inspect compliance to make sure people follow through on the conversion and don't bring grass back later on. (This inspection process could be compared with verifying a conservation easement. It might get expensive, unfortunately. To reduce inspection costs, maybe only a small fraction of homes could be inspected at random.)

The organization could also run awareness campaigns to inspire people for whom gravel lawns would save money to convert voluntarily. Fortunately, people who spend the most on lawn care are also those for whom converting to gravel is more likely to be altruistically positive because they probably have the most climate-change impact via lawn maintenance. It's unclear what the cost per person convinced would be for an awareness campaign. Behavior change is often hard, although there may be several people who would gladly convert their lawns if they heard of the idea, so there may be some people who are relatively easy to convince. Suppose it costs $900 to convince one person on the margin to switch to gravel lawns, via advertisements, pro-gravel op-eds, etc. As discussed above, typical gravel lawn prevents ~1.8 million effective insects during non-winter months over 20 years. Assuming that bugs and bacteria are active half the year in temperate climates, that's (1.8 million)/2 = 0.9 million effective-insect-years of suffering, and the cost-effectiveness is (0.9 million)/($900) = 1000 effective-insect-years prevented per dollar.

In 2019, someone pointed me to an existing water-conservation program called "Lawn Be Gone!". The website for the program, BAWSCA (n.d.), explains: "Save water by removing your grass! Replace your lawn with a beautiful, drought-tolerant landscape and receive a rebate of up to $4 per sq. ft." Drought-tolerant landscapes often still include plants, but I would guess they're less productive (and therefore support less invertebrate life) than a watered grass lawn.

That said, before invertebrate-suffering reducers invest a lot of resources into promoting incentives like BAWSCA's, they would want to verify that drought-tolerant landscapes do in fact support fewer invertebrates than watered lawns. One concern could be that some drought-tolerant landscapes use mulch rather than gravel, and maybe mulch supports a lot of decomposer animals. Of course, the organic matter in the mulch would already be decomposing somewhere or other, so it's possible that mulching doesn't actually increase the total number of invertebrates in the world. But if the organic matter in the mulch had instead been landfilled or burned, then it would have supported fewer invertebrates than it does when used on a lawn.

Comparison with humane insecticides

Gravel lawns and humane insecticides are both concrete ways to reduce insect suffering. I look more favorably on gravel lawns because

there's less risk of a gravel-lawn campaign being misinterpreted to imply something possibly harmful, whereas a campaign to reduce the pain of insecticides might lead most casual observers to wrongly assume that we're in favor of organic farming

pending more thorough research on how much lawns sequester carbon, promoting gravel lawns seems to have low uncertainty and doesn't rely on assumptions about the run-on ecosystem effects of a different insecticide or whether one type of insecticide is really more humane and equally lethal compared with another

the start-up costs of a gravel-lawn campaign are smaller (all that's required is to promote something that some people already do)

people might look more favorably on gravel lawns, considering that, e.g., they help with California's droughts and might reduce climate change

the case for gravel lawns more prominently brings up the idea that bug lives in aggregate have more bad than good.

The main reason why humane insecticides might have higher payoff is that in the long run, it might be possible to help more insects more cheaply if humane insecticides became widespread, since the dollar cost per hectare affected per year is probably smaller for humane insecticides. But given that most people oppose insecticide use for reasons of human and environmental health, I doubt there could ever be a widespread movement with economies of scale to merely use more humane insecticides without also reducing the quantity of insecticides employed.

Appendix: Estimating effective insects per hectare using net primary productivity

In the main text I estimated ~88,000 effective insects on a lawn of 0.2 acres. That number was derived from guessing what seemed like a reasonable number of insects per square foot based on what I've seen in real life. This section attempts an alternative estimate based on the net primary productivity of lawn grass.

This study reviews the literature to find an average net primary productivity for grasslands of "5.89 to 12.71 Mg dry matter/ha/year". Since I suspect this paper might be biased to overstate carbon sequestration, and in order to keep my estimate conservative, I'll take a smaller figure: say, 2 Mg. The study says (pp. 809-10) that "Each year, ≈10% of the biomass added to the soil may be humified (Duiker and Lal, 2000; Puget et al., 2005; Schimel et al., 1994)." If we conservatively assume that 10% of all the primary production is humified, that leaves 90% to be consumed immediately: 2 * 0.9 = 1.8 Mg.

How much of this mass goes into biomass of insects, bacteria, etc.? And how much of the energy is expended as heat? Following are some estimates of insect ECIs:

One page reports: "Tests on two species of edible grasshoppers, Locusrana migratoria and a species of Melanoplus, fed on several kinds of grasses showed combined ECIs in the range of 10-15% and 8-11%, respectively." (Note that the linked page supports entomophagy, which I oppose.)

This study found ECIs for grasshoppers between 9.5% and 16.5%.

This study found ECIs between ~2% and ~15%.

This page says: "For the German cockroach (Blattella germanica), [the ECI] is well above 40. Silkworms, traditionally cultivated in China and neighboring regions, typically have an ECI in the 20-30 span."

Conversion efficiencies for effective insects should be higher than this, since some of what isn't digested by insects will be eaten by bacteria later. On the other hand, the quoted figures are plausibly high-ish ECIs relative to insects in general because insect farmers will tend to use insects with higher ECIs and harvest the insects once they've grown rather than letting them continue to expend energy after they're already at full size. Assuming these considerations roughly cancel, let's take an ECI of ~10%. I assume this ECI is for growing organisms. ECIs for adult organisms should be roughly 0%, since adults tend not to grow further. So all told, the ECI for organisms that eat grass biomass might be, say, the midpoint of 10% and 0%: 5%. 5% of 1800 kg implies 90 kg of heterotroph-organism biomass by dry weight.

This is primary-consumer production per year, i.e., the increase in biomass of primary consumers per year. In temperate climates, this growth only happens during spring-fall months. If we assume for simplicity (if not accuracy) that primary consumers are all univoltine and live from spring through fall, then the standing biomass of primary consumers by the end of the season would equal the annual production: 90 kg per hectare.

Assume that a mass of bacteria/nematodes/etc. equal to the mass of an insect suffers as much as the insect. Then we can convert 90 kg of dry organism biomass into a number of effective insects by dividing by the dry weight of an insect. This page reports that a fruit fly has a mass of 2–3 * 10−7 kg, while an adult housefly has a mass of 2 * 10−5 kg.

Here's another way to estimate housefly mass. An adult housefly is 6-7 mm long. Say the fly has equal volume as a 4 mm cube. Then it would be 43 = 64 mm3 or 0.064 cm3. Assume it's ~half as dense as water and that its dry weight is ~half its wet weight. Then it would weigh (0.064 g)/(2*2) = 0.016 g, or about 2 * 10-2 g, which is exactly the same as the 2 * 10−5 kg figure from the previous paragraph.

Let's take a housefly as our reference insect. Then the number of effective houseflies is (90 kg)/(2 * 10−5 kg per fly) = 4.5 million flies. This is per hectare per year. It's equivalent to 360,000 effective houseflies supported by a 0.2-acre lawn. This is close to my 88,000 estimate, which is surprising given that I didn't fudge with any of my input assumptions in order to make the number come out right.

If you only care about insects, rather than bacteria/nematodes/etc., then the number of effective houseflies would have to be reduced to account for the fact that a lot of grass isn't eaten by insects. If, say, 10% of digestible grass biomass is eaten by insects, that would imply ~36,000 actual insects supported by the grass of an average lawn, which is about the same as the 44,000 estimate I arrived at by guesstimating insects per square foot.

Estimate using springtails and mites

This article says of springtail bugs: "Ranging from 0.25-10mm in length, there are typically around 10,000 per square metre of soil, rising to as many as 200,000 per square metre in some places." This page says

In sheer numbers, they are reputed to be one of the most abundant of all macroscopic animals, with estimates of 100,000 individuals per square meter of ground,[24] essentially everywhere on Earth where soil and related habitats (moss cushions, fallen wood, grass tufts, ant and termite nests) occur.[25] Only nematodes, crustaceans, and mites are likely to have global populations of similar magnitude [...].

Here are vivid videos showing dense springtails on soil.

A friend of mine (unfortunately!) has a vermiculture bin, and in it, I've seen populations of mites as dense as ~1 per mm2 just on the surface of the bin. That implies ~106 mites per square meter.

Suppose that the decaying grass on your lawn supports 10,000 springtails per m2 = 108 springtails per hectare. Suppose the typical springtail is small—say 0.5 mm. Above I suggested that a typical housefly is 6-7 mm long. So a housefly is about (6.5/0.5)3 = 2 * 103 times bigger than a springtail. Hence, 108 springtails per hectare have about the same size as ~50,000 houseflies per hectare. That's ~4000 houseflies on a 0.2-acre lawn. And if we count mites and other invertebrate fauna as well, this number might be a few times bigger, putting this estimate in a similar ballpark as the 44,000 estimate of bugs on a typical lawn in the main text.

Other density estimates

In the main text, I assumed a very conservative 5 bugs per square foot of lawn. This is at least an order of magnitude lower than the densities of invertebrates per square foot of grassland that are reviewed in this 1937 article: