On-farm companion planting and bioinoculants for enhancing biological control of twospotted spider mites in strawberries

Project Overview

Project Type: On-Farm Research
Funds awarded in 2024: $29,268.00
Projected End Date: 03/31/2026
Grant Recipient: University of Florida
Region: Southern
State: Florida
Principal Investigator:
Dr. Oscar Liburd
University of Florida


No commodities identified


No practices identified

Proposal abstract:

One way to enhance the capabilities of biological control agents
such as N. californicus is to provide additional
resources to improve their survival within the field.
Neoseiulus californicus preferentially feeds on spider
mites, but they will also feed on other arthropods or even pollen
when the spider mite population is too low for survival (McMurtry
et al. 2013). Releases of N. californicus could be done
early in the season as a preventative treatment, and companion
plants could be used to provide resources such as food and
shelter for the predatory mites when spider mite populations are
low. This could, in turn, prevent the need for multiple releases
of predatory mites. The companion plants could also provide
resources for other native predators and, thus, increase their
numbers in the strawberry planting too.

Sweet alyssum, Lobularia maritima (L.), is a common
companion plant in greenhouses and agricultural fields in areas
with warmer climates. Sweet alyssum is inexpensive and easy to
grow, making it an economical option for small organic growers.
It flowers continuously and the pollen and nectar provide food
for various beneficial insects (Amorós-Jiménez et al. 2014,
Pumariño and Alomar 2004), including N. californicus
(Ragusa et al. 2009). Ragusa et al. (2009) found that N.
survived on sweet alyssum pollen for 10 days
but could not reproduce on it.

A previous study by Hyte et al. (2023) found that the addition of
sweet alyssum did not improve the efficacy of N.
released against spider mites in strawberries.
However, sweet alyssum was planted only at the ends and in the
middle of each treatment plot with 12 feet between each clump of
sweet alyssum. Only predatory mites on the strawberry plants
closest to the sweet alyssum clumps were able to utilize the
resources provided by the sweet alyssum. Also, Neoseiulus
was only released in the strawberries and not
on sweet alyssum. In addition, samples were not collected from
the sweet alyssum flowers to determine if N.
mites were present. Lastly, the experiment was
conducted on a research farm where numbers of other native
predators were too low to collect data on the effect sweet
alyssum has on their populations.

 When plants are damaged by the feeding activities of
herbivores, they may release volatiles in response to attract
beneficial arthropods. Strawberry plants infested with spider
mites are more attractive to N. californicus than those
without spider mties (Rezaie et al. 2018). Other factors can also
impact volatile release rates in plants, such as interactions
with soil microbes. Soil inoculations using
beneficial bacteria has been shown to increase the attractiveness
of certain plant species to parasitoid (beneficial)
(Pangesti et al. 2015). In strawberries,
inoculation of the soil with bacteria Bacillus subtilis
may increase the attractiveness of the plants to predators such
as N. californicus. Our preliminary work has
found that soil inoculations using B. subtilis led to
significantly reduced mite populations. It was also found that
the addition of B. subtilis led to the attraction of the
predatory mite, N. californicus, while none were found
in untreated control plots. These findings indicate that soil
bioinoculants may be used to increase the recruitment of
beneficial arthropods in the field.

Our initial objective is to investigate different
tactics of deploying sweet alyssum in the strawberry production
system to determine the best method of enhancing biological
control services
. This is based on the
hypothesis that the establishment of sweet alyssum will improve
biological control services of N. californicus and other endemic
predators in the system.
Two layouts for
interplanting sweet alyssum with strawberries will be compared to
the layout from Hyte et al. (2023) and a control treatment where
N. californicus mites are released but no sweet alyssum
is planted. The first layout involves planting sweet alyssum
along one side of each strawberry bed. This should avoid
competition between the sweet alyssum and the strawberry plants
while providing a continuous stand of sweet alyssum for predators
to utilize. Potential drawbacks to this arrangement are that the
sweet alyssum could be too far away from the strawberry plants,
or it could slow harvest since only one side of each bed will be
accessible to pickers. The second layout involves planting the
sweet alyssum on the beds in between the two rows of a typical
strawberry bed. This keeps the sweet alyssum out of the
harvesters’ way and places it close to the strawberry plants.

A secondary objective is to investigate the use of
soil bioinoculants for enhancing the biological control of N.
californicus. The hypothesis is that the inoculation of the soil
with the bacteria, Bacillus subtilis may increase the
attractiveness of the strawberry plants to predators such as N.
californicus and effectively reduce the population of twospotted
spider mites.
We will first investigate how
inoculation of the soil with B. subtilis may impact
spider mite populations alone. Soil inoculation may recruit
natural enemies (beneficial arthropods) to provide control of
spider mites, as indicated by our preliminary study. Soil
inoculants are also known to provide additional benefits to
plants such as improved yield and resistance to pests (Esitken et
al. 2010, Pappas et al. 2021). However, it is possible that
inundative releases of N. californicus are needed to
maximize the benefits of soil inoculation. Therefore, we will
also investigate the combination of B. subtilis soil
inoculation and N. californicus releases. Pairing B.
and N. californicus should enhance
biological control of spider mites and lead to a greater
reduction in spider mites than either B. subtilis or
N. californicus individually.  The additional
benefits of soil inoculation, improved yield, and resistance to
pests should also be seen in this system. While this system has
the potential to reduce the grower’s costs by reducing the number
of predatory mite releases needed in a season, it is also
possible that B. subtilis is effective enough on its

Project objectives from proposal:

Both field studies will be conducted at Frog Song Organics (John
Bitter) farm in Hawthorne, FL, during the Florida strawberry
growing season, which is from mid-October to mid-March. Each
study will be replicated twice, once in the 2024/2025 growing
season and again in the 2025/2026 growing season. Strawberry
plants will be transplanted into the field as plugs at the start
of the season. The strawberry variety used will be selected by
the grower. All plants will be grown on double-row, raised beds
covered in black plastic mulch and be irrigated using a drip
irrigation system. Strawberry plants will be allowed to grow for
approximately four weeks following transplanting before
treatments and sampling begins.

 For objective 1 to
investigate different tactics of deploying sweet alyssum in the
strawberry production system to determine the best method of
enhancing biological control services
. The sweet
alyssum/strawberry plots will be 7.6 m x 2.1 m, consisting of two
double-row beds with plants spaced 30 cm. apart (100 plants per
plot). The sweet alyssum study will have five total treatments,
1) an untreated control (without predatory mites and sweet
alyssum), 2) predatory mite N. californicus, 3) sweet
alyssum grown in the beginning, middle, and ends of each plot, 4)
sweet alyssum grown continuously along the sides of each bed, and
5) sweet alyssum grown continuously along the center of each bed.
Neoseiulus californicus will be released once against
twospotted spider mites in the field. The predatory mites will be
released at the preventative rate of 25 per m2 after
arrival to the laboratory. A small sample of N.
, ~ 5 mL of bran, will be examined under a
dissecting microscope after arrival and before release to ensure
that the mite predators are in good condition. Sweet alyssum will
be planted in a greenhouse at the University of Florida in
Gainesville, FL. The sweet alyssum will be allowed to grow three
weeks before being transplanted into the field at the same time
the strawberry plants are transplanted.

For objective 2 to
investigate the use of soil bioinoculants for
enhancing the biological control of N.
The study plots will be 73.2 x 73.2
cm., consisting of three double-row beds with plants spaced 30 cm
apart (32 plants per plot). For both studies, plots will be
spaced 7.6 m apart to provide a buffer zone between individual
plots. Treatments in both studies will be arranged in a
randomized complete block design for each study, with four
replicates per treatment. The bioinoculant study will have a
total of four treatments, 1) an untreated control (without soil
inoculant and predatory mites), 2) predatory mite N.
, 3) the soil bioinoculant, and 4) predatory
mites and soil bioinoculant combined. The same protocol for
N. californicus used in the sweet alyssum study will be
used for the treatments containing N. californicus in
the bioinoculant study. For the treatments with the soil
bioinoculant, Fulzyme™ SP (Bacillus subtilis) will be
applied to the soil around every strawberry plant at the
recommended rate (2.24 kg/ha) in 20 mL of water. The soil
bioinoculant will be applied once every two weeks.

In both experiments, leaf samples will be collected every week
from both plots to monitor populations of twospotted spidermite,
N. californicus, and other relevant arthropods. Six
trifoliates will be randomly collected from each plot and stored
in a press-and-seal bag. Leaves will be brought back to the Small
Fruit and Vegetable IPM laboratory in Gainesville, Florida, for
observation under a dissecting microscope with at least 10x
magnification. The number of twospotted spidermite and N.
motiles (all non-egg stages) and eggs will be
counted and recorded. Other relevant arthropods will also be
recorded. While not being observed, leaves will be kept
refrigerated (<4.4 °C). All leaf samples will be processed
within one week to minimize the effects of leaf decay and mite
reproduction on sample quality.

In the sweet alyssum study (objective 1), flower samples from
sweet alyssum and strawberry plants will be collected to
determine if predators are using the pollen as a food resource.
Twenty-five sweet alyssum flowers will be randomly sampled from
each plot by placing them in 50 ml centrifuge tubes containing 10
ml of 70% ethanol. Once strawberry flowers begin to appear in
November, 5 strawberry flowers per plot will be collected using
the same method as for sweet alyssum flowers. Samples will be
processed using the shake-and-rinse method developed by Arévalo
and Liburd (2007). Briefly, the tubes will be shaken for 1 min
and then the contents dumped onto a mesh screen placed over a
white 300 ml polyethylene container. The flowers will then be
rinsed with DI water and then discarded. The rinsate will then be
poured into a 9-cm petri dish and examined under a dissecting
microscope. Numbers of predatory mites, other predators, and any
pests, such as flower thrips, will be counted and recorded.

Fruits will be harvested from every plot and weighed for both
experiments. Harvesting will occur as necessary, up to three
times per week, starting from when the first ripe fruit is
detected in the field. Collected fruits will be rated as either
fresh marketable, processing marketable, or unmarketable.
Unmarketable fruits include those that have been severely damaged
by pests or fungal pathogens. Processing marketable fruits are
those that are not suitable for fresh market sales due to
aesthetic injuries, but still useable for processed products such
as jam. The remaining healthy fruits are categorized as fresh

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.