Documenting Diversity at the Katharine Ordway Natural History Study Area
Tenebrio molitor, Yellow Mealworm Beetle
Common Names: Yellow Mealworm, Yellow Mealworm Beetle, Flour Beetle, Golden Grub, Darkling Beetle 2, 8
The Yellow Mealworm Beetle resembles ground beetles
according to its similar size, shape, and dark color. The Yellow
Mealworm will generally grow to be slightly larger than .5 inches long
and at its adult stage will become a dark brown or black color3.
However, a few key characteristics differentiate the Yellow Mealworm
Beetle from other beetles. The Yellow Mealworm Beetle has several
distinct features in regards to its abdomen shape and design. Like many
other beetles the Yellow Mealworm has its abdomen evenly split into two
segments with its wings hidden safely underneath. The Mealworm Beetle’s
distinct feature is the evenly divided, linear grooves running along
the entire length of its abdomen. A less noticeable yet dissimilar
trait of the Tenebrio molitor is that it has only four tarsal segments
on its hind legs, as opposed to the five that most ground beetles
possess. This characteristic results in the Mealworms inability to move
around as quickly as other ground beetles4. The Black Mealworm Beetle
is a very similar beetle to its close relative the Yellow Mealworm
Beetle. The key variations between these two beetles are in regards to
the size and shape of their thorax and abdomen. The Yellow Mealworm has
a more rectangular shaped abdomen rounding off at its end, as well as a
wider thorax equal to the width of its abdomen. The Black Mealworm has
a more spherical shaped abdomen that forms a sharper tip at its end,
along with a noticeably thinner thorax when compared to the width of
its abdomen4, 5. Whether obvious or difficult to perceive, the Yellow
Mealworm Beetle has numerous features that make it unique and
distinguishable to itself.
The Katherine Ordway Natural History Study Area is
home to an immense amount of diversity. Its river, prairies, forests,
and ponds house many different species. The forest’s leaf-litter is
home to a tremendous amount of varying invertebrates, along with some
small vertebrates, and it provides the soil and surrounding plants with
necessary nutrients. The leaf-litter is one of the main habitats in
which an adult Yellow Mealworm Beetle will choose to reside. Yellow
Mealworm Beetles prefer the darkness and being in contact with other
objects. Due to the leaf-litters abundant covering and high resource
for nutrients, they will often be found there as well as underneath
rocks and logs5. Yellow Mealworm larvae will also be found around
locations of human food storage7.
These beetles are notorious for being scavengers and
decomposer of the earth’s floor. They will eat decaying leaves, sticks,
grasses, plants, as well as insects and animals3, 5. This can be good
for the environment because they will eat organic matter that is not
usually eaten by other organisms. They are mostly nocturnal but will
also be seen active in dark environments during the day3, 5. However,
they can also be pests for grasses, their seedlings, and grains5. These
beetles do not have much contact with other live organisms. Due to
this, they may display a very unique behavior when disturbed. When they
feel endangered they flip onto their head, taking a defensive position.
The beetles then spray chemicals from their rear scent gland towards
the danger. The Yellow Mealworm Beetle’s combination of positive and
negative contributions to the environment along with its lack of
contact with other organisms makes this beetle even more unique.
Yellow Mealworm Beetle’s have very distinguishing
life cycles. They undergo a process referred to as complete
metamorphosis. This life cycle is comparable with butterflies and
consists of four distinct life stages6. The first stage is as an egg.
The female beetle will lay approximately 500 eggs. The eggs are white,
bean shaped, and about 1.25mm in length. They are laid in the soft
ground and will hatch into larvae after 1-2 weeks2. When the larvae
exit the eggs, they are referred to as Yellow Mealworm since they have
still not become a beetle yet are an active organism. These larvae are
notorious for being pests in barns, mills, and other areas of human
food storage. They undergo between 9-20 molts, shedding of their outer
shell, and grow slightly more with each molt. The Yellow Mealworm will
reach a length of approximately 25mm7. When the larvae are ready to
pupate they spend several days as prepupae near the surface of the food
they are infesting7. While in the pupae stage they are between .5-.75
inches in length and change from white to black as the beetle is about
to emerge5. Unlike a butterfly, the pupae have no outer covering.
Depending upon the temperature the pupae will remain for 3-30 days. The
pupae will appear motionless though inside the mealworm will be turning
into a beetle2, 6. When the beetle emerges as an adult, they will
survive for about an additional 30 days. During this time they will
mate, lay their eggs, and will repeat the process. Accumulating all
four cycles, their total life expectancy is 3-5 months5.
During their short lives they encounter many dangers. While in the
wild, birds are their most prominent predator. However, as of recently,
humans have become even greater predators to the Yellow Mealworm.
Humans breed and sell them for bird food and fish bait while they are
in the larvae stage of their life cycle. Thus, the life of a Tenebrio
molitor can vary greatly between each individual in their species.
Scientists believe that Mealworms evolved from
Eurasia and have spread throughout the world because of human
activity8. Currently, Yellow Mealworm Beetle’s reside throughout the
northern United States, Western Europe, and parts of Australia8, 9.
Minnesota and the rest of upper U.S. region make a great location for
them to thrive because of the forests and the many farms which they
invade to infest on the feed.
The Yellow Mealworm Beetle sample was collected at
the top of a hillside in a woodland area at Katherine Ordway Natural
History Study Area in Inver Grove Heights, MN. The specimen was
underneath the leaf-litter which was very moist at the time.
1. Bug Guide. 2007. http://bugguide.net/node/view/101010
Australian Wildlife. 2010.
3. First Class Pest Solutions. 2002.
4. Stored Product Insects. 2003. http://www.the-piedpiper.co.uk/th7h.htm
5. The University of Arizona: Center for Insect Science Education Outreach. 1997. http://insected.arizona.edu/mealinfo.htm
6. Foss Web. 2009. http://lhsfoss.org/fossweb/teachers/materials/plantanimal/tenebriobeetles.html
7. Hort Fact. 1998. http://www.hortnet.co.nz/publications/hortfacts/hf401013.htm
8. Immuno CAP. 2008. http://www.immunocapinvitrosight.com/dia_templates/ImmunoCAP/Allergen____28207.aspx
9. Encyclopedia of Life. 2010. http://www.eol.org/pages/1041700
Compiled by David Melms,
Biodiversity & Evolution (BIOL 270), Professor Sarah Boyer. Spring 2010.
Specimen collected at Macalester College's Katharine Ordway Natural History Study Area on April 15, 2010.