December 2021 - 9

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Tom Bilbo, postdoctoral research scholar in North Carolina State University's
Department of Entomology and Plant Pathology, scouts for twospotted spider mites.
Photo: Dean Peterson
Beneficials bring
twospotted spider
mite control on farms
By Dean Peterson
VGN Correspondent
The predatory mite Phytoseiulus
persimilis can feast on twospotted
spider mites and is widely released
as a beneficial in greenhouses and
some very high value field crops, like
strawberries.
It can be effectively released into
tomatoes grown in the field, but
there are challenges.
P. persimilis must be introduced
early. It must suppress twospotted
spider mites before their population
explodes.
However, P. persimilis is a
specialized predator and feeds almost
exclusively on twospotted spider
mites. If P. persimilis is released too
early, before spider mites are present,
the predator will disperse out of
the field in search of spider mites
elsewhere.
" P. persimilis is a highly specialized
feeder, " said Jim Walgenbach,
professor and Extension entomology
specialist for fruits and vegetables
in North Carolina State University's
(NCSU) Department of Entomology
and Plant Pathology.
" They move quite rapidly since they
almost exclusively feed on twospotted
spider mites, " Walgenbach said.
" They're going to go where the spider
mite populations are the highest. " If
that results in a rapid dispersal out
of the field, there probably won't be
enough P. persimilis left to suppress
the spider mites in the tomatoes.
The NCSU release threshold for
P. persimilis in field tomatoes is when
scouting finds 0.5 spider mites per
leaflet, which occurs shortly after
spider mites are first detected in the
field.
" In the absence of a control, mite
populations can increase very rapidly
so it's important to put the predators
out early, " Walgenbach said.
" As with most predator-prey
dynamics, there is a lag time between
the buildup of spider mites and
P. persimilis, so 'jump starting' the
P. persimilis population with releases
helps to shorten that lag time, " said
Tom Bilbo, postdoctoral research
scholar in NCSU's Department of
Entomology and Plant Pathology.
Meticulous scouting urged
Scouting for twospotted spider
mites is challenging because they are
so small, but they do prefer to feed
on the upper parts of the tomato
plants. This means the most consistent
result when scouting is to check the
leaflet from the first or second fully
expanded leaf on the top of the plant.
" We look at a leaflet on 10 plants,
and we do that in about five to
eight locations to get an idea on
population density across the field, "
Walgenbach said.
Keeping the cost down is another
challenge when introducing
P. persimilis into tomato fields. The
typical release rate of about 20,000 P.
persimilis per acre used in strawberries
is simply not economically viable in
tomatoes.
Rather than releasing in whole fields,
NCSU researchers have had good
results releasing at a rate of 20,000
beneficials per acre but only in a small
portion of a field - maybe only a tenth
See BENEFICIALS, page 8
VGN | DECEMBER 2021 | 7
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December 2021

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