Grub life cycles

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Many gardeners don't care to look too closely at a grub. We -- and you, too! -- will look because we are curious about everything in the garden. In this case, curiosity pays off in better gardening methods. 

Stages in a grub's life
Yearly cycle
Comparing different grub species' cycles
Identifying individual grubs

Above: Grub. Beetles lay eggs which become these soil-dwelling larvae.
The grubs that are lawn pests eat grass roots.
Below: Pupa: As it reaches maturity the grub pupates. It emerges from its pupa case as a beetle.








  Left: While pupating, our adversary is a legless,
   mouthless  creature.
  For most lawn-dwelling grubs,
  this stage comes in spring. Since it
  doesn't eat and is protected by a relatively hard
  shell during this stage, stomach poisons and
  biological agents are not effective.






Month by month with four kinds of lawn pest grub

Below is a comparison of the life cycles of some of the most commonly seen lawn grubs.


Several important things to notice about these life cycles

  1. Each grub species follows its own schedule. For instance, native North American June beetles require three years to mature. (We do see the species every year when a batch finishes its underground stint and emerges in late May or early June.) In contrast, Japanese beetles mature in just one year, emerging several weeks later than the natives.

  2. Since single-year species such as Japanese beetle and European chafer go from egg to adult three times as fast as a June beetle, each does as much damage in one year as a native June beetle does in three.  Japanese beetles and European chafers also tend to occur in much larger numbers than the native June beetles, so they have the potential to do far more damage.

  3. GrubsLife1YearS.jpgThese grubs feed, rest, mature and emerge at varying times but they have in common a late summer period (chart detail, right) when the grubs are young and feeding. That's when most are vulnerable to pesticides and biological control agents such as predatory nematodes.

  4. There is also a convergence of resting/pupating stages in spring, making that time the least effective time to apply grub killers. (So why are these products marketed so vigorously in spring? Because that's when homeowners are focused on lawn care and willing to spend money.)


Identifying individual grubs

It's helpful to be able to distinguish between grubs and non-grubs, and between grubs of different species. One very important reason to make the effort is to maintain awareness that there are good and bad beetles as well as many other small creatures living below ground. The majority of them are important to soil condition and fertility and so we should not be trying to kill them.

When we can identify a grub as a pest, then we can determine its life cycle and fine tune any control measures.Cutworm4769s.jpg

Grub vs. cutworm

Cutworms are not grubs. Cutworms are rubbery, elusive caterpillars -- moth larvae of various species. There are several especially pesty types such as the black cutworm. They hide out in loose soil by day and move up at night to wrap around and chew on small stems. Since their chewing usually kills the foliage on that stem a heavy population of them can do some serious damage to starter annuals and vegetables.

Right, above: Cutworm moth caterpillar

Right, below: Chafer beetle grubGrubChafrDimen0848S.jpg

Leg number and position is probably the most certain telltale between grubs and cutworms:

Grubs -- beetle larvae -- have 3 pairs of legs, all attached to the body segment right behind the head, a segment called the thorax.
Cutworms -- moth larvae -- appear to have more than 3 sets of legs. In addition to six true legs attached to their thorax they have two or more pairs of "prolegs" attached to the abdomen.



One grub from another

Sometimes we need to know which beetle grub species we're dealing with, at least to narrow it down between troublemaking species and those that are beneficial or benign.
The surest method is to keep suspect grubs in captivity until they emerge as adults.
If you can't do that, look at the grubs' butt ends -- their rasters.

Most grubs are on their way to distinctive looking adult beetles. However, to identify them in their grub state, examine the butt end. What you look for is type and arrangement of hairs, pattern formed by the anal slit and other marks. For the most part, what you have to work with will be descriptions of pest lawn grubs, the object of most turf tenders' interest.

BeetleRastersS.jpg RasterRevChafer7518s.jpg
Above, left: common pest grub rasters. Ugh. Ugly but necessary nitty gritty about grubs.

How good are you at beetle grub I.D.? Look at our sketches of the butt ends of the most common lawn pest grubs, and then at this photo of a chafer's raster.

To learn more about what you've read in this article:

Destructive Turfgrass Insects: Biology, Diagnosis and Control, by Daniel A. Potter
Lawn Care: A Handbook for Professionals, by Henry F. Decker, Jane F. Decker