October 2019 - 8

MSUE VEGETABLE NOTES
Seeking secret ingredient in squash pollination
By Zsofia Szendrei
Michigan State University
It is fall and that means it is winter
squash and pumpkin season to the
delight of many of us who consider
these their favorite group of vegetables.
But did you know that these fruits are
the result of busy insects working in the
summer diligently to carry pollen from
male to female flowers? One of the most
interesting native bees associated with
agriculture is the squash bee.
As its name suggests, it pollinates squash
and pumpkin plants with large flowers
(Cucurbita maxima, C. moschata and C.
pepo). This reliance on their host plants
means that they are efficient pollinators of
these crops. Unlike honeybees, squash bees
don't live in large colonies and therefore
are not suitable for domestication. Squash
bee nests are created by female bees
underground and are stocked with squash
pollen which is the food for the developing
bee larvae. Squash bees in Michigan
usually emerge from overwintering in
early July, when squash flowers become
available. They mate and build nests during
the summer months. The bee larvae
develop underground in the fall, and
overwinter, ready to come out as adults the
following July.
Unlike honeybees that carry pollen in
balls on their hind legs, squash bees carry
pollen loosely on their legs and body,
therefore when arriving at a female flower,
the pollen can easily transfer from the bee
to the flower. Squash bees are constantly
visiting their host plants while honeybees
are collecting from many different plants. If
you want to observe squash bees in action,
you will need to visit the garden in the
morning when bees are active. Since squash
flowers are only open in the morning, these
bees have synchronized their daily activity
window to coincide with the opening of the
squash flower.
During the afternoon, you can still see
male squash bees if you pry the closed
flowers open; they use these as " sleeping
bags " during the day. Squash bee females
spend most of their time carrying pollen to
their underground nests (up to 18 inches
deep) which are near their host plants, and
while they don't share their nests with other
bees, you can sometimes observe many
0.5-inch circular openings in certain areas
of the farm where bees prefer the soil for
nesting. A female bee can make multiple
nests during the summer each with about
five bee eggs. Although it is not entirely
clear how females find the nest entrances, it
is likely that they can recognize land marks
around the entrance hole that are used as
Male squash bees in a flower.
Photo: Zsofia Szendrei
guides. If these are destroyed, she may not
be able to find the nest and will likely start
building a new one instead.
Over the last three years, researchers in
Michigan State University's Entomology
Department have been involving Master
Gardeners in a citizen science project
to learn more about squash bees in the
state. We asked citizen scientists to report
using a survey how the squash crops were
managed as well as the numbers and
types of bees visiting cucurbit flowers.
Squash bees accounted for the greatest
proportion of bees observed at squash
flowers, compared to honeybees, bumble
bees and other bees.
We also learned that in fields that
were fully tilled, fewer squash bees were
observed at flowers compared to gardens or
farms were reduced or no-tillage was used.
Citizen scientists also reported through
the survey that mulching did not have any
negative impacts on squash bees visiting
flowers. Thus, the results of this citizen
science project indicate that in order to
encourage squash bees, tillage should be
kept to a minimum in fields where squash
was planted the previous year until the bees
had a chance to emerge in July. This could
be done by leaving the field fallow until
July and planting a fall crop, or strip-tilling
the field for the following crop in order
to minimize the areas of the field where
the soil is disturbed. On the other hand,
mulching does not seem to negatively affect
squash bees. That is good news for those
that like to use straw mulch or plastic for
growing their plants.
If you want to learn to recognize
squash bees, they are about the size and
shape of a honeybee but have hairy hind
legs, grey-black stripes on their abdomen
and long antennae. They are not known
to sting, so feel free to get up close and
personal with them next summer! VGN
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8 | VegetableGrowersNews.com
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October 2019

Table of Contents for the Digital Edition of October 2019