Management Strategies for Thrips and Tospoviruseson Cut Flower/Specialty Crops, Potted Crops and Bedding Plants
Management Strategies for Thrips
and Tospoviruses
on Cut Flower/Specialty Crops, Potted Crops
and Bedding Plants
Executive Summary
Thrips and the viruses they vector continue to be a major problem for many
segments of the floriculture industry. Research accomplished by the previous
thrips/tospovirus research team focused on the identification, sampling plans,
thresholds, reduced risk pesticides, biological control (predators and fungi)
and post harvest control strategies. We made major progress in all these areas
and much of the work is in various stages of the publication process including
both scientific and trade journals. The research proposed for 2 002-2005
continues the overall effort at developing 1PM strategies for thrips and
tospovirus by building on research accomplishments of the previous
team, addressing new areas and reinforcing ‘traditional’ areas of
importance. Specifically, we will continue to develop information on
the performance of reduced risk pesticides and their compatibility with
natural enemies; evaluate the potential of host plant resistance from a
traditional and ‘induced’ perspective; assess the feasibility of ‘trap’
crops for control of thrips and develop information on
thrips movement into the greenhouse. The goals of
this program are multifaceted and there is an emphasis on developing information
that can be used immediately by growers.
Objectives:
A. Evaluation of insecticides for control of WFT and determine the
compatibility of selected pesticides with natural enemies of thripsB. Examine the potential of induced resistance to manage thrips and
virusC. Assess the feasibility of trap crops and host plant resistance for
managing WFT populationsD. Quantify movement patterns of WFT into and within a greenhouse.
Objective A: Evaluation of insecticides for control of western flower
thrips and determine the compatibility of selected pesticides with natural
enemies of thrips
Introduction, Literature Review & Anticipated Benefits
One of the most serious pests of ornamental crops, especially
greenhouse-grown plants, is the western flower thrips, Frankliniella
occidentalis (Pergande). Originally confined to the western United States
and Canada, the western flower thrips (WFT) has become established in commercial
greenhouses throughout the world. Thrips have a cryptic and thigmotactic nature,
lay eggs inside plant tissues, have a rapid reproductive rate, and have a high
level of insecticide resistance (Robb et al, 1995). These factors combined with
an increase in the volume of ornamental and vegetable plant material shipped
internationally have caused WFT to become a significant pest worldwide. The
physical damage caused by thrips on developing leaves and flowers is often
hidden until plant maturation. Western flower thrips are also capable o.
vectoring plant viruses, including the tospoviruses tomato spotted wilt virus (TSWV)
and impatiens necrotic spot virus (INSV), which can devastate ornamental crops (Daughtrey
et al. 1997). Extensive losses to chrysanthemum, cyclamen, exacum, impatiens,
gloxinia, and Rieger begonias have been recorded in Hawaii, California, and
North Carolina (Parrella, 1995). At this time management of viral diseases in
greenhouses is based upon eliminating all virus sources. Infected plants must be
discarded and the vector must be excluded and/or eradicated from the greenhouse.
In some situations (e.g, when a product is chosen which provides only marginal
control), it is suspected that the use of insecticides may exacerbate the virus
problem by forcing thrips to disperse throughout the greenhouse thus
spreading the virus more completely. However, thrips
movement patterns in the greenhouse are not really understood, and this will be
addressed in Objective D of this proposal. Although progress is being made on
developing resistant floriculture crops using molecular tools against both the
virus (Sherman et al. 1996, 1998) and thrips (Annadana, 2001), the main tools to
manage WFT are the use of insecticides and biological control. While there are a
number of materials for growers to chose from (azadirachtin, neem oil, nicotine,
bendiocarb, fenoxycarb, methiocarb, endosulfan, Beauveria bassiana, abamectin,
acephate, chlorpyrifos, diazinon, dichiorvos, naled, sulfotepp, bifenthrin,
cyfluthrin, fluvalinate, fenpropathrin, lambda-cyhalothrin, resmethrin and
spinosad) these vary considerably in efficacy and not all enjoy registration
across the US. In addition, many are the types of materials that growers should
strive to eliminate from their operations. Only two products (spinosad and Beauveria
bassiana) are what could be considered new, and the performance of B.
bassiana in commercial operations has been disappointing. There is limited
published information on the evaluation of potential new insecticides for thrips
control (Lindquist 1999), There is a need to continue the development of new
insecticides and to try and combine these with other control strategies such as
biological control. Considerable effort has focused on advancing thrips
biological control strategies in floriculture crops, but only two natural
enemies have had any real success: the predatory mites Amblyselus cucumeris and
Hypoaspis miles (Lenteren et al.
1999). We will concentrate evaluating new pesticides in conjunction with these
natural enemies.
Materials and Methods
a) Evaluation of Insecticides
Standardized assays have been developed in the Parrella laboratory for
purpose of conducting standardized bioassays against the WFT (Murphy et al.
1997). Leaves from chrysanthemum
plants will he used in the bioassay. Applications of the target insecticide
are made to plants in the greenhouse after which a leaf is removed and
placed in the test chamber. Adult thrips (males, females and immatures
evaluated separately) are placed on the leaf in the container after the
pesticide has dried. By running the bioassay with leaves
taken from these same chrysanthemum plants, 3 and 7 days after the spray
application, we will be able to gauge the length of
residual control. This data are not available for most of the insecticides
available for thrips control. Dosages tested will include the range of
concentrations that the manufacturer expects to be on the label.
There will be at least five replicates per dosage with a minimum of
20 thrips per leaf. We will control the age of the
thrips use in the hioassays by obtaining them from our laboratory colony. However,
we may have to supplement this with collections of WFT from
field situations: if this is necessary, we will be
unable to control for age. Mortality will he 24h after
thrips are added to the treated leaves. A water only control as well as the
recommended rate of Conserve (as a standard) will be included in the
experiments. Depending on the number of dosages evaluated, we will subject
that data to Probit Analysis or use ANOVA
with Scheffe’s test to separate mean mortality data.
b) Compatibility Studies with Natural Enemies
Compatibility will be evaluated in two
ways: i) assessment of mortality when adult predators are
exposed to residues of di different ages, and ii)
determination of predatory mite impact on WFT populations in large greenhouse
cages where applications of the target pesticide are made.
i) The bioassays described above for WFT (a) will be used to assess the
toxicity of pesticides to adult predatory mites. Identical protocols will be
followed, but we will add immature thrips and pollen (Rijn
et al 1999) to leaves in the chambers to provide food for the
predators. Based on Dr. Heinz’s research from last year’s thrips/tospvirus
management team, we will use cherry pollen (Parrella et
al. 2002). Predatory thrips will be purchased from commercial insectaries.
Although Hvpoaspis is a soil dwelling mite, bioassays on leaf material
should provide the necessary information to evaluate the toxicity of pesticides.
We will not be evaluating the potential impact that these pesticides may have on
predatory mite vigor (e.g. The ability to search leaves and feed on prey). We
will simply be measuring mortality. However, at the time morality is assessed,
observations on the overall vigor (i.e., do they look normal?) will be made.
Ideally, the true impact of the pesticides on the predatory mites and their
ability to regulate WFT populations will be made in the next section.
ii) Trials will be conducted in a 4000 sq. ft. greenhouse located at UC
Davis. The experimental units will be six screened cages (1.3 m high, lx2 m
base), with each enclosing 50 potted chrysanthemum plants (14 pots of 5 plants
each; var. ‘Miramar’). Small fans will be suspended from the center of each
cage and run continuously to maintain a temperature and air flow pattern
similar to that of the surrounding greenhouse. The temperature within two cages
and within the greenhouse will be recorded at 30-min intervals using three
electronic data temperature probes (Heinz et al. 1993). There will be six
treatments and each will be replicated three times over time. Treatments
include: candidate pesticide alone (at label recommended rates), A. cucumeris
(at insectary recommended rates) plus pollen; H. miles (at insectary
recommended rates) plus pollen; both predatory mites plus pollen; pesticide plus
both predators plus pollen; and a thrips only control. In pesticide and predator
treatments, weekly applications of the pesticide and/or predatory will be made.
Predators will be released when the foliage dries although this protocol may be
modified based on the results of the predator residual
bioassays (i, above). Plants will be two weeks old at the
start of the trial and we will evaluate thrips
populations on a weekly basis for 6 weeks. A small yellow sticky card will be
hung in the cage and counted and replaced weekly and we will examine five plants
from each cage (five leaves and one central growth point per plant) and all
thrips and predators counted. At the conclusion of the trial, all flowers from
five Between treatments differences in thrips’ densities will be detected with
ANOVA and means separated using Scheffe’s test.
Objective B: Examine the potential of induced resistance to manage thrips
and virus
Introduction, literature Review and Anticipated Benefits
Induced resistance involves plant-mediated changes associated with
initial attack by herbivores and pathogens that negatively influence
subsequent attackers. The jasmonate pathway (i.e., the octadecanoid pathway) and
the salicylate pathway (conditioning systemic acquired resistance, SAR) are two
of the biochemical response mechanisms that can be triggered by various
attackers. The components of these pathways, jasmonic and salicylic acids (JA
and SA, respectively), act as signals that trigger naturally occurring chemical
responses that protect the plant from insect and pathogen invaders (Thaler et
al. 1999). Under AFE funding, we have been exploring the use of JA
and SA elicitors as tools for controlling thrips and tospoviruses. Our
data shows a wide range of responses, depending on the plant species and use of
wetting and spreading agents. Our findings show that with correct application,
thrips can be dramatically reduced. We do not yet have complete data for
reduction of tospoviruses. We were concerned that possible cross-talk or
negative interactions might occur between these pathways and tested this
possibility. Elicitation of both pathways did not negatively impact thrips
control, although plant defense against some other pests was reduced (Thaler et
a!. 2002). We are asking for funding to build upon these
promising findings. We propose to evaluate two currently registered products
with active ingredients that stimulate the jasmonate and salicylate plant
pathways for defense.
Bion® (marketed in Europe) or Actigard® (marketed in the USA by
Syngenta) has the active ingredient benzo [1 ,2 ,3] thiadizaolc-7-carbothioic
acid-S-methyl ester or benzothiadiazole (BTH). 13TH acts like SA and elicits
systemic acquired resistance (the salicylate pathway). It is best tested for
control of bacteria and fungi. Messenger (Eden Bioscience Corporation) has the
active ingredient harpin (a protein derived from the bacteria Envinia
amylovora). Harpin triggers a cascade of responses that stimulate the
salicylate pathway AND the jasmonate pathway. It has also been shown to
stimulate nutrient uptake and photosynthesis. When it is effective, the response
is initiated within 10 minutes of treatment and depending on the plant species,
may continue for several weeks. We will test these
products alone and in combination as tools for protecting chrysanthemums and
lisianthus against thrips and tospoviruses. There are some other products that
are due to be released that we will watch for and include
if they promise to be readily available.
Thrips and the tospoviruses they transmit remain one of the mnost severe and
intractable control problems that ornamental growers face
across the nation. In California, epidemics have been especially severe in
chrysanthemums and Lisianthus. Plantings of Lisianthus have grown tremendously
in the state and growers have serious problems with tospovirus infection. Thrips
quickly gain resistance to most insecticides and conditions associated with the
Food Quality Protection Act make new strategies for their control even more
important. The only methods for controlling tospoviruses are those
that revolve around prevention. Once a plant has been infected, there is no
cure. New tospoviruses are emerging worldwide, many of them are transmitted by
the western flower thrips and infect ornamentals (Table 1) (Ullman et a!. 2002).
The anticipated benefit of our research is that we will provide a new control
strategy for thrips and the tospoviruses they transmit. The products we
will test are already registered in the USA, are considered “reduced
risk” and are not thought to interfere with other management strategies.
There are two major concerns regarding use of these products that we are
addressing directly with our research. First, responses
have been shown to vary widely with plant species and plant age. Therefore, it
is critical that we determine whether these products work with our crops of
interest (chrysanthemum and Lisianthus) and find out when treatment
is most useful. Second, “yield drag” or subtle, but negative plant
impacts, can occur depending again on plant species and timing of application.
Because the crops we are interested in protecting are
produced for their flowers, it is essential that we fully
investigate effects on growth and yield.
Table 1. Thrips Vector Species and the
Tospoviruses They Transmit
the field experiments will be based on the first 3 months of testing in
the laboratory. A single “best” treatment concentration will be
selected for each crop. Each field experiment will include four chemical
treatments (Actigard, Messenger, Actigard ± Messenger, an
untreated control). For each chemical treatment we will have plots treated at
three different times in plant phenology [chrysanthemum: early (1 week after
planting), at bud set, at the tight bud stage (just prior to the time they would
be cut for sale); lisianthus: early (1 week after planting,) at onset of flower
formation, as flowers begin to open(prior cutting for sale)]. For each treatment
and plant phenology combination there will be 4 replications, the design will be
randomized. Untreated areas will be maintained around each treatment to avoid
interactions. Thrips numbers will be sampled with yellow sticky traps maintained
at plant canopy and collected weekly. Virus incidence will be assessed as in
laboratory experiments (symptoms and ELISA). Tospovirus pressure is consistently
high at both field locations, so we will rely on natural infection as a
challenge in these experiments. To determine the degree of virus pressure we
will use petunia indicator plants as developed under earlier AFE funding.
We will look for any obvious signs of phytotoxicity, e.g.
burning, distortion. To assess possible yield drag and other growth
impacts we will measure: height on a weekly basis,
number of flowers, bud size caliper, and crop uniformity. We will take data on
10 plants at the most central location of the each plot. Data will be analyzed
by ANOVA (when the data meets the criteria for this test,
e.g. for yield) and Chi-square analysis (when non-parametric
statistics are needed, e.g. virus incidence).
Objective C: Evaluation of Trap Crops and host plant resistance for managing
WET populations
Introduction, Literature Review, and Anticipated Benefits
A multi-faceted approach, including incorporation trap crops, may be required
to manage WFT in commercial floriculture operations. The use of trap crops,
which has been successful in agronomic cropping systems (Berlinger et al, 1996;
Theunissen and Schelling, 1998), has not been utilized in greenhouse crop
production systems as a supplemental component of 1PM programs to deal with WFT.
The floriculture industry has been reluctant to incorporate the use of trap
crops due to perceived economic obstacles (Hokkanen, 1991). Trap crops function
by using a pest’s preference for a certain plant species, cultivar, or crop
stage, to prevent or minimize pests from concentrating on the main crop or to
attract pests to a certain area where they can be locally controlled. Trap crops
may comprise up to 10% of the total crop area. This loss of production space
must be offset by gains associated with the trap crop. Insecticide application
cost reduction and increased yields result in net economic gains when trap crops
are used (Hokkanen, 1991). Trap crops may also serve as reservoirs for natural
enemies thereby enhancing biological control (Leius, l967: Powell, 1986). Thrips
have crop preferences (Hoyle and Savnor, 1993), which suggest that the use of
trap crops in greenhouse production facilities may be a feasible option to
manage WET. Identifying main crop/trap crop combinations that are highly
effective in localizing \VFT populations will enhance the
feasibility of spot-spraying insecticides to manage WFT populations. Localized
spray applications will reduce the amount of insecticide applied, minimize
worker exposure and environmental impacts, and reduce labor expenses associated
with applying insecticides to manage WFT. This will allow greenhouse managers
the option of selecting main crop trap
crop combinations that limit WFT populations on floriculture
crops. Host plant resistance, which is used extensively in
agronomic cropping systems, may limit physical
damage caused by thrips feeding. Research on floricultural crops including
chrysanthemum. Dendranthema grandiflora (De Jager
et al. 1995). gladiolus, Gladiolus spp.
(Zeier and Wright, 1995), miniature rose. Rosa chinensis ‘Minima’
(Bergh and Le Blanc, 1997), and cut roses (Guam et a!, 1994) indicate that
certain plant cultivars have various levels of resistance
to thrips feeding. Morphological chemical, or a combination of both may
influence resistance levels. Chemical resistance factors in chrysanthemum
cultivars play a significant role in negatively impacting thrips growth
and survival (De Jager et al. 1995, 1996). For example,
primary and secondary metabolites negatively impact thrips larval performance as
measured by instar length (De Jager et al, 1 996). Despite this, plants with
resistance to pests are not widel grown by greenhouse
managers mainly because any feeding injury that impacts
the aesthetic quality, no matter how slight. may reduce the
marketability and sale of the crop. However,
combining host plant resistance with the use of insecticides may overcome this
obstacle and reduce the number of insecticide applications required. Combining
impatiens cultivars that exhibit resistance to thrips feeding with
effective insecticides ill allow greenhouse managers
to reduce thrips feeding injury and minimize insecticide inputs without lowering
profit margins by reducing the number of insecticide
applications. This will decrease worker exposure to spray
residues, minimize plant injury (phvtotoxicity), and possibly reduce the
potential for thrips populations to develop resistance to currently available
insecticides. In addition, this will allow greenhouse managers to select plant
cultivars and insecticide combinations that will limit.
Trap Crops Experiment 1
Experiments will be conducted to identify thrips trap crop preferences among
different commonly grown ornamental plants. Crops will be scheduled to flower at
approximately the same time because thrips are attracted to plants when they are
in flower (Kirk, 1985). All the crops will be
randomly dispersed in screened cages and uniformly exposed to a predetermined
number of adult WFT. Thrips will be collected from a laboratory-reared colony.
Each cage will contain all the crops being examined in equal
proportions. After a pre-determined time period,
individual open flowers from each plant will be harvested and washed to collect
thrips (Bullock, 1963). Crop attractiveness will be based on the number of thrips
collected from open flowers of each crop. The crops found to be most
attractive to WFT will be tested as a trap crop in subsequent experiments.
Repeated measurements throughout the growing season will determine if thrips
preferences vary by season and to identify trap crops for future
experiments. The trap crops identified from this experiment will be subsequently
compared with two selected main crops, using a randomized complete block design
whereby each trap crop will be paired with each main crop. The main crop is the
group of plants that are to be sold. These main/trap crop combinations will be
replicated over time to include all possible main/trap crop combinations during
a single experimental run.
Experiment 2
Plants used in this experiment will be scheduled to flower at approximately
the same time based on commercial production guidelines for each crop. Plants
will be grown in isolated-screened cages to prevent injury
from natural thrips populations. When plants have open flowers, each main/trap
crop combination will be placed in an isolation cage. The trap crop, comprising
5% of the total growing area, will be equally spaced among the main crop. A
predetermined number of laboratory reared \\‘FT will be evenly introduced
throughout each cage. Thrips will be allowed to disperse throughout the
isolation cage for a predetermined time period based on preliminary studies,
after which WFT numbers on the main and trap crops will be determined as
described in the first experiment. Thrips feeding damage on randomly selected
main crops will be estimated using a visual rating scale and will
be quantified using an image analysis system. Data collected from
each trap/main crop combination will determine if one trap crop is more
attractive to thrips (based on the ratio of thrips numbers collected in
the trap vs. the main crop) than the other trap
crops. Thrips feeding damage will be compared to an non-inoculated control to
determine if trap crops kept thrips feeding damage levels on the main crop near
the levels observed on the control crops. The most preferred trap crop would be
used in subsequent experiments to concentrate thrips into a localized area,
which may lead to reducing the number of broad-spectrum insecticide applications
to the main crop.
Experiment 3
This experiment will be designed to determine if
localized applications of insecticides to a trap crop are as effective in
reducing WFT damage to a main crop as broad-spectrum insecticide applications. A
randomized complete block design replicated over tinie, with predetermined
combinations of crops and insecticide application methods will be examined to
determine the effectiveness of this approach. Plants to be used in this
experiment will be spaced, isolated, and inoculated with adult WFT
as described in experiment one. Inoculations will occur when flower buds
are present on the plants. Approximately 7 to 10 days after inoculation with
WFT, spray treatments will be conducted. Total insecticide volumes
applied to each treatment and the labor required for applications will be
recorded. Plants will be allowed to mature to the market ready stage. After
plants mature, then feeding damage and marketability of the main crop will be
assessed. Plant location in reference to the trap crop will be determined. In
addition, randomly selected flowers from the main and trap crop will be
harvested and immediately immersed in 75% ethyl alcohol. Western flower
thrips numbers on the main and trap crop flowers will also be determined by
dissecting flowers and counting thrips using stereo microscopes. Results
obtained from this experiment will determine if localized insecticide
applications to the trap crop in main/trap crop combination results in similar
thrips feeding injury to the main crop compared to broad-spectrum applications
to the main crop. In addition, the untreated main crop and main/trap
crop combination will be compared to determine if the use of a trap crop reduces
the amount of thrips feeding injury on the
main crop. A comparison of insecticide applications and the labor required will
determine if production costs are decreased with the inclusion of a trap crop.
In addition, determining insecticide volumes and the labor associated with
applications will provide economic costs for greenhouse managers. This will
provide greenhouse managers with a better understanding of
how trap crops may be incorporated into existing IPM
programs to manage WFT.
Host Plant Resistance
Impatiens cultivars that demonstrate varying levels of resistance to WFT
based on previous research conducted at the University of Illinois will be
planted into pots and placed in a randomized complete block design. Resistance
to thrips is known to be variable within impatiens
populations (PanAmerican Seed Company, personal communication). Each plant will
be covered with a screened Plexiglas cage (45.0 cm high x 10-cm
diameter). After a predetermined time period (two or three
weeks), each Plexiglas cage will be removed to infest individual plants with
thrips. Approximately 50 laboratory-reared thrips will be placed on a
predetermined number of plants from each cultivar. Cages will be replaced after
inoculation. Two days after inoculation, each plant will be sprayed with a
selected insecticide. After the inoculation period, each plant will be evaluated
weekly for thrips injury using a visual rating scale based on the number of
leaves exhibiting thrips feeding damage. In addition, thrips feeding damage will
be quantified using an image analysis system. Data obtained from the weekly
evaluations and final damage assessments will be analyzed.
Objective D. Quantify movement patterns of WFT into and within a
greenhouse
Introduction, Literature Review & Anticipated Benefits
A fundamental issue associated with management of
western flower thrips is knowledge of the source of the infestation,
and determination of the most effective methods for eliminating the source. For
many greenhouse operations, it is largely unknown if
western flower thrips infestations continually occur from
resident populations within the range or if they blow in
or fly in from outside sources. Resolution to this problem is important in
determining the values of
plant, and animal tissues, and is rarely found at high concentrations.
Rubidium can be applied as a simple chloride salt (RbCl). In a rubidium-enhanced
environment, insects assimilate rubidium through consumption of plant tissues,
or of other insects. Ingested rubidium acts as an elemental analog to potassium
in biological systems, due to the similarities of the two elements. Because it
can be incorporated into living systems in the place of a common element, a
rubidium marker works its way throughout the food chain.
Herbivores feed on plants that have enhanced levels of Rb, and entomophagous
predators of these herbivores also acquire the mark. The safety of Rb as a
marker is well established, and the application rates used in this study (below
14,000 parts per million) show no detrimental effects to thrips biology.
Detection of the Rb marker will be performed on collected predators prepared
using standardized protocols. After sample preparation, digested arthropods will
have rubidium content analyzed and compared to background levels using a model
220-Z Varian Atomic Absorption Spectrometer (which already resides in the Heinz
lab). Samples found to have Rb levels greater than three standard deviations
above background levels will be scored as positive bearers of the chemical mark.
A maximum of 4 sites will be selected for mark-recapture experiments, at various
locations. These mark-recapture experiments will be conducted for two years,
thus providing a test for individual year effects and for a total of eight
replications in their absence. At various distances from target greenhouses, we
will mark the vegetation with RbCl every four weeks during the suspected thrips
movement periods in the spring-summer. Subsequently, we will collect western
flower thrips from within target greenhouses. Specimens will be placed
individually into small Eppendorf tubes, subsequently dried, and digested prior
to Rb analysis. Because the experimental design is balanced and has two
basic factors, time and habitat type, the most direct analysis would he a
two-factor analysis of variance (ANOVA) with means separation using Tukey’s
procedure with a = 0.05. All analysis
for this objective will be conducted using the SAS analysis package.
b) Movement within a greenhouse: Using the same rubidium marking procedure
outlined above, we will release rubidium marked WFT into
research greenhouses on the Texas A&M campus (following the methods outlined
by Heinz, 1998). Marked thrips released from point sources will be recollected
every four hours after time of release for a period of no more than four days.
These data will allow for use to calculate dispersal and dispersion values for
Western Flower Thrips. Understanding this spread is important to managing
tospoviruses, in applying insecticides effectively, and in selecting and
releasing natural enemies in the most effective manner
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