Red Wiggler Calculator and More

Worm Reproduction for Vermicomposting

Like many others who've picked up Vermicomposting as a hobby, I started with a lot of questions. One of the more complicated ones was "how many worms do I need to start vermicomposting?". I say "complicated" because not only does it depend on what your immediate (and end) goals are, but if part of your goal is to have the worms reproduce, it's helpful to have an idea as to how fast they'll reproduce. And "how fast will red wigglers reproduce" is a search term that will get you hundreds of entirely different, and often conflicting... answers.

Long story short, I wanted to take some of the research and common "rules of thumb" out there and create a flexible online simulator that would let me simulate reproduction by easily changing variables. Let's face it - pen-and-paper for each set of variables simply takes way too long!

Below, you'll find the simulator, which is essentially a calculator that outputs to a fancy chart. Below that, you'll find details about the options, and some further discussion. Reading the details/discussion below the calculator is important! By default, the simulator assumes perfect lab conditions with infinite food/space for the worms. Our rubbermaid tubs filled with cardboard and trash aren't exactly the same thing.

Calculator and Chart

...because sliders are more fun than pen and paper!
Loading worm calculator...
Please be patient!

Bugs

There are still a few lingering bugs I haven't fixed. Couple of the more prominent ones:
  1. Initial times to hatch/mature can be off by a day or so.
  2. Once things are rolling (after initial hatches and matures), hatching happens 1 day late while maturing happens 1 day early.

Deciphering the Options

Before going into more interesting questions/answers, here's a brief overview of what all the options above do.

  • THINGS TO AVOID: Using large numbers. Casting too far into the future (high start days or # of days). Reason is that the calculator remembers each and every worm and has to do math on all of them, each day. I optimized the calculator so that it can now keep track of trillions of worms if necessary, but you don't want to start testing the limit because your browser might slow down during calculations or even freeze.
  • The Basics: Enter the worms you're starting with. Adult means fully mature (developed clitellum so they can reproduce). Young covers from the moment a worm has hatched (a tiny thread) up until the day they become an Adult. You'll find Young worms referred to as wormlings, juvenilles, babies, immature worms, etc. Cocoon means the tiny little yellow balls which are essentially the "worm eggs". You can manually enter larger numbers than the sliders allow, but I don't suggest going too crazy here.
  • Display Options: These options all affect what you see on the visual graph. Just play with them as you watch the chart to get a feel for what they do. Again, avoid really high numbers.
  • Advanced Settings - Tweak Timeline: Enabled by default, which spreads your starting worms/cocoons across various ages, slowly ramps up the cocoon-laying time over the first interval, and is a little more realistic. This has the side effect of creating a smoother graph. If disabled, your starting adults worms will all lay cocoons on exactly the same day (no delay or ramp-up time), all your wormlings will be considered "freshly hatched" (will all have to go through the full mature time and will all mature on the same day), and all starting cacoons will be considered "freshly laid" (will all hatch at the same time after the full incubation duration) - you'll end up with a much more "spikey" graph.
  • Advanced Settings - Weekly Death Rate: Sometimes bad things happen to good worms. Or maybe you harvest the population on a regular basis. Heck, maybe you're just curious whether your worms could outproduce a high rate of loss and want to see what that would look like in a chart. In any case, if you want to simulate worm death, you can set the weekly rate. It's pretty flawed and mainly designed for fun/interest: It's not quite exact, has a double-impact on Day 1, is simply divided by 7 to get the daily rate (not a correct way of doing things), and also starts turning worms into "partial" worms behind the scenes, so don't place too much weight into the results.
  • Reproduction Rate - V&R (1987): Uses values from The life-cycle of the compost worm Eisenia fetida (Oligochaeta) (J.M.Venter and A.J.Reinecke). This was a study conducted across 600 days, where 10 original worms were used, and reproduction/growth was noted throughout. Cattle manure was used, and humidity was kept between 75-80% (with a couple exceptions where it dipped). Worth mentioning that if you turn on "weight" in the display options, it uses their data for calculating it.
  • Reproduction Rate - D&E (1997): Uses values from Vermiculture Technology - Chapter 3: Biology and Ecology of Earthworm Species used for Vermicomposting (Jorge Dominguez and Clive A. Edwards). The day ranges used in the book were simply averaged to get the values for the calculator.
  • Reproduction Rate - 60 day doubling: On the web it's extremely common to find 60-90 days listed as the time for red wigglers to double in number (and/or weight). This option was added just to give a baseline where you can see how that might look. It is not based on any actual data, and instead uses values that happen to work out.

Questions and Answers:

Q: How accurate is the calculator? Will I see identical growth rates in my bin?

A: Umm... don't use the results you get from the calculator for anything incredibly important.

The V&R and D&E studies were done under carefully controlled environments. Yes, the results are obtainable. No, you probably won't get the same production levels in a typical bin. The best you can usually do is to mirror as many of the "lab conditions" as possible - ~80% humidity, temps of ~25C, a quality food source, and do your best to keep overpopulation from inhibiting your growth too early on. In a typical "worm bin", you'll have other challenges to overcome too - avoiding aneorobic conditions, trying to keep the moisture level consistent (almost impossible - an area of your bin will probably always be either too wet or too dry), etc.

On top of all that, the calculator's imperfect. Even if there aren't any remaining bugs in the basic math, it makes some assumptions. For example, it assumes that your Original Adult Worms all matured on "Day 0" and then determines their weight based on the age where that took place (dicated by the setting for # of days for a wormling to reach adult) - this will usually make those original worms "lighter" than an actual random sample of mature worms (though they will increase in weight as days go by in the calculator). In fact, weights are extrapoloated from 1 specific study, so don't put too much weight into the weight regardless. The graph can also be +/- 1 day when it comes to figuring out when a cacoon hatches, wormling becomes adult, etc.

The calculator never slows down the growth rate either (whereas in reality, reproduction will decrease as population density increases). You'll get exponential growth in the calculator as time goes on, to the point where you're got millions/billions/trillions rather quickly whereas in reality your worms will usually have overpopulated their environment and slowed reproduction well before they hit those numbers.

Short version: Use the calculator as a simulator for "what might be possible", but don't expect that reality will necessarily match the simulation.

Q: Can I use the simulator for other types of worms besides red wigglers?

If you manually input the appropriate values in the "Advanced Settings", sure (presets are all for red wigglers).

Note that weights will not be correct - the values for red wigglers are hardcoded into the calculator, so don't enable "show weight" if using another type of worm. Obviously you'll have to make sure your temperatures, humidity, food source, environment, etc are all appropriate for the type of worm you're simulating.

Q: How do I determine when my growth rate will actually slow down in my bin?

A: You'll want to start by determining your bins square footage. For simplicity, let's assume a 0.9 x 1.2 foot bin (around 1 square foot).

Next we want to figure out when growth will slow down. Here are some stocking densities from Manual of On-Farm Vermicomposting and Vermiculture (Glenn Munroe):

  • 0.5 pounds per square foot (2.5kg/m2): considered minimum stocking level
  • 1 pound per square foot (5kg/m2): low end of common stocking density - slower reproductive urge
  • 2 pounds per square foot (10kg/m2): high end of common stocking density
  • 4 pounds per square foot (20kg/m2): possible to get density this high
Using our small bin example, (1 square foot), growth will slow once we reach 1 lb/ft2.

1 lb of worms is generally considered to be ~1000 worms on average. Use this as a "rough rule of thumb" - it's not exact, and depends on the weights/ages of the worms. 1000 worms that are all a year old and grew up in perfect conditions might be a lot more than 1 pound. On the other hand, 1000 baby worms will weigh considerably less.

The result: In our small bin, growth will really slow down once we hit about 1000 worms if we use the "rough rule of thumb". On the other hand, if we use the calculator's weight (by enabling "show weight), we might hit a pound once we have around 500 adults and 1500 wormlings (or some other combination depending on our starting values). In reality, the true number will probably lay somewhere in between.

Coming up with values for other sized bins follows the same process. Basically:
  1. Find the square footage of your bin.
  2. Use 1 pound per square foot model to determine at what worm-weight reproduction will slow. So if your bin is 1 ft2, then it should slow at 1 pound. 2 ft2 at 2 lbs, 5 ft2 at 5 lbs, etc.
  3. Plug your starting worms into the calculator. Select "Show Weight".
  4. Scrub the timeline until you reach the worm-weight from #2.
In reality, worms probably don't have a sudden population density "cliff" where reproduction suddenly drops off (it's likely more gradual). Unfortuantely I don't have a way to quantify that right now - if at some point I come across enough data for me to slap together a rough density-vs-rate-of-decline formula, I may add it to the calculator.

Behind the Scenes - The Algorithm:

Briefly, this is the way the algorithm works, using the 4 reproduction values when V&R (1987) is selected as an example:
  1. Initial worms are generated and given "ages" that include cacoon age (before hatching). Including cacoon age is done because the algorithm needs to keep track of how old cacoons are (so it knows when to hatch them). When the algorithm needs the true worm age (after hatching) it just subtracts the days-before-a-fresh-cacoon-hatches number from the age for the necessary math.
    • Adults are created and given an age of Days-for-cacoon-to-hatch + days-for-wormling-to-reach-adult + 1 day. Example: 23 + 50 + 1 = 74 days old.
    • Wormlings are created and given an age of Days-for-cacoon-to-hatch. Example: 23 days old. If tweak-timeline is selected, the ages are evenly distributed along the possible ages for a wormling. Example: 23 to 73 days old.
    • Cacoons are created and given an age of 0 days old. If tweak-timeline is selected, the ages are distributed along the possible ages for cacoons. Example: 0 to 22 days old
  2. Each day, the following takes place:
    • New cacoons are laid (based on days-for-each-adult-to-lay-cacoon setting). Each adult worm keeps track of when it last laid a cacoon.
    • Worm deaths are calculated based on weekly-death-rate. This is done in a pretty broken fashion so don't rely on it's results too heavily (divides weekly by 7 to get daily, reduces worms in some cases and creates partial worms in others, plus other issues...).
    • Weights of Adults and Wormlings are calculated, based on 0.0025g/day for the first 60 days, 0.0160g/day for the following 30 days, and 0.0016/day thereafter. Those values are mean rates based on the V&R study. Days are counted *after* hatching. Note that cacoons themselves aren't given any weight.
    • Worms increase in age by 1 day.
    • Worms are counted and tallied into Adult/Young/Cacoon categories based on age. During this process, if a cacoon "age" has reached days-before-a-fresh-cacoon-hatches, it is hatched, and baby-worms-per-cacoon are born. When baby-worms-per-cacoon is a decimal number, the remainder is carried over until the next hatching. Example: 2.7 means that 2 will hatch, and 0.7 will be carried over - if another hatching takes place it will become 3.4 (2.7 + 0.7) meaning 3 are hatched with 0.4 left over, and so on.
    ...this repeats for every day selected. Note that it always starts from Day 0. If you set the chart to display from Day 20 to Day 50, it actually calculates from Day 0 to Day 50 (the chart simply doesn't display the first 20 days).
On the surface, it's pretty straightforward. There are a number of little quirks in the math, but it should be fairly accurate for short and medium-term time periods.