Recently a grower from the UK sent us a photo of damage to a pumpkin
plant and wanted to know what it was and whether it could be controlled.
This pest had apparently been a problem for a long time, to the extent
that it was questionable whether it was worthwhile planting pumpkins or
squashes again.
Like all vegetables, pumpkins and squashes are no strangers to diseases and bugs. Squash Vine Borer (
Melittia cucurbitae),
which is what was affecting our grower's pumpkins, is only one of the
range of pests that affects members of the cucurbit family (members of
which include pumpkins, squashes, zucchinis, cucumbers, melons etc).
Besides the UK Squash vine borer lives in most temperate areas of North
America, although not on the Pacific Coast. This pest is renowned for
the fact that you cannot see it until it is too late (when the plant is
wilted and does not recover when watered). Close examination will
reveal that the stem has been severed just above ground level.
Symptoms
When
you examine the base of affected plants, you will notice a small hole
and some frass (sawdust-like droppings). The hole is a sign of the
presence of the Squash Vine Borer. If there are multiple numbers of
borers, these pests can actually cause the stem to break - especially if
it is windy - or to rot, depending on the weather. The injury will
also prevent the plant from taking up water and nutrients, which will
ultimately cause it to die.

The Squash Vine Borer is the larvae of
a species of black moth that attacks both wild and cultivated varieties
of squash. The moth looks like a bee or wasp because of its movements,
and the bright orange scales on its hind legs. It has a wing span or
around 25 to 37mm (1 - 11/2 in). The females usually lay reddish-brown
eggs on the leaves, and when they hatch the larvae bore their way into
the base of the stem; the larvae develop and feed inside the stalk,
eventually killing the leaf. From the leaf, the caterpillars migrate to
the main stem, and with enough feeding damage to the stem, the entire
plant may die.
Squash Vine Borers can affect hubbard squash,
pumpkins, zucchinis, gourds and other squashes. It is not common to
find them on melons or cucumbers. Control Gardeners find Squash Vine
Borer a difficult pest to combat. Some try to avoid it by timing the
production season to harvest before the pests can build up, or after
they have peaked. However, there are cultural control measures that can
be implemented.
As Squash Vine Borers overwinter in the soil as
larvae in cocoons, removing and destroying old vines after harvesting
will help. Tilling the soil will also help to destroy any cocoons that
may be there. Birds will also assist you with this job once the soil is
tilled.
Once your crop is planted, if you are vigilant, you may
be able to remove eggs when they appear. As the larvae usually bore
into the stem close to ground level, a paper or other wrapping at the
base of the stem may assist as a barrier. If you notice the plant
wilting or a hole at the base of the stem, it is possible (if you are
very careful) to slit open the stem and remove the larvae, and then
cover the stem with moist soil so that it can grow more roots. In fact
if you notice a hole, you can even just pile some moist soil over the
affected area to try and promote new root growth so that the plant will not die.
An
old trick for growing pumpkins and squash is to cover the vine with
earth at various points along its length, inducing rooting at several
points, thereby continuing to feed the developing fruit despite the loss
of the original stem. After the vine has taken root at multiple points,
the infected portion of the plant can be cut off, along with another
inch where the larvae is eating into healthy tissue, without significant
damage to the plant. Row covers are another option that can be used up
until bloom.
Some gardeners use pesticides, but these are
ineffective after the larvae are inside the plant. It is also not
recommended as these pesticides used to kill the adult moth may
contaminate the flowers and thus kill pollinators through poisoned
nectar or pollen.
Lucia Grimmer is the main author of best selling books
How to Grow Juicy Tasty Tomatoes and
How to Grow Great Potatoes.
Lucia
Grimmer is a world renowned expert in plant disease and nutrition who
trains horticulturists, agronomists and professional growers. She has
won awards for her technical papers and regularly conducts trials on a
range of crops with the objective of improving yield and reducing
disease.
Co-author Annette Welsford also has a horticultural
background, however her true skills like with taking complex technical
information and representing it so that even novice gardeners find it
easy to understand.
Their books are considered to be the authoritative "bibles" on
and growing tomatoes
and great potatoes have sold to thousands of novice and professional gardeners in 85
countries, and have been featured on TV, radio and leading gardening
publications in 4 continents.