BIOLOGICAL CONTROL OF SPIDER MITES IN …walnutresearch.ucdavis.edu/2011/2011_247.pdfBIOLOGICAL CONTROL OF SPIDER MITES IN WALNUTS . ... interior portions of the tree canopy. ... populations

BIOLOGICAL CONTROL OF SPIDER MITES IN WALNUTS

Stephen C. Welter, Walt Bentley, and Frances Cave

ABSTRACT

Early season predator-prey ratios for phytoseiids to spider mites failed to successfully predict

effective biological control in a walnut orchard in King’s county. One of the more surprising

findings was the absence of the predatory mite, G. occidentalis, which has been considered the

dominant mite predator in walnuts for more than 35 years. What is not clear is how widespread

is this unexpected absence of this predator in other walnut orchards in King’s county or other

growing regions of CA. Similarly, the reason for its absence is also not known at this time. The

dominant predator in the early portion of the growing season was Amblyseius similoides with

Euseius spp increasing as the season progressed. Preliminary efforts were started in 2011 to

determine if mid-season releases of G. occidentalis could re-establish this species of predators in

an unsprayed walnut orchard. The effects of late leafing cultivars of walnuts on mite

colonization did not appear to be a significant obstacle for either the spider mites or phytoseiids

nor did it appear to differ significantly from other deciduous tree crop systems. These data

suggest that spider mite management in walnuts needs to be carefully re-evaluated relative to its

assumptions and possible changes in its current management programs, e.g. changes in

insecticide use patterns. To realize the full potential benefits of pheromone mating disruption for

codling moth, we need to be able to rely on the effective biological control of other pest species,

e.g. spider mites, that have traditionally been observed in non-disrupted orchards.

INTRODUCTION

The recent development of pheromone mating disruption for codling moth held promise for less

disruption of secondary pests, such as spider mites, for walnut growers. While this has proven

true in some cases, it has not been universal even in walnut growers who were not treating with

insecticides that may be eliminating the predatory mites in the family Phytoseiidae. The lack of

biological control of spider mites has been reported as more common in the central and more

southern portions of the Central Valley, but the data are currently just anecdotal. Biological

control of the spider mites, Pacific spider mite and the two spotted mite, has traditionally relied

on the suppressive effects of the western orchard predatory mite, Galendromus (=Metaseiulus)

occidentalis. In earlier research programs, G. occidentalis was always reported as the dominant

species driving biological control of spider mites in many deciduous tree fruit systems including

walnuts.

California Walnut Board 247 Walnut Research Reports 2011

Two areas were identified as potential reasons why biological control of spider mites in recent

years was less reliable in non-disrupted orchards than expected, especially compared to other

deciduous crops such as apples. One difference between pome fruit and walnut systems is the

timing for leaf development early in the season compared to the time of emergence by spider

mites from their overwintering state. It had been proposed that the early emergence of spider

mite populations in the spring was largely unsuccessful due to the lack of foliage. A similar

emergence by the predatory mites would also be unsuccessful if its host spider mite populations

failed to survive. The alternative means for colonization in walnuts by spider mites might also

include spider mite ballooning in on silk threads from adjacent crops such as almonds which leaf

out earlier than walnuts.

The most common pattern of colonization in crops such as almonds or pears is the early spider

mites emerge from under the bark of the tree or from the ground and move into the central

interior portions of the tree canopy. Over time, the population increases in number and colonizes

higher into the tree canopy as well as into the more exterior portions of the canopy.

Our goals for 2011 were to 1) document the spatial pattern of emergence for the overwintering

populations 2) determine the species of pest and predator that were emerging over time and 3)

an unexpected goal to see if populations of G. occidentalis could be established later in the

season using artificial inoculative releases of predatory mites. For objective 1, if no colonization

of the central portion of the trees was observed early in the season combined with later

infestation on the outside portions of the tree canopy, then we would have assumed that

populations of spider mites from outside of the walnut orchard were important contributors to

pest outbreaks. Conversely, if the more traditional pattern of colonization was observed, then

our efforts to managing internal populations would need to remain the focus.

OBJECTIVES

1. Evaluate dynamics of spider mite and predator mite populations in walnuts

a. Early season migration from ground cover into trees (movement on tree trunk)

b. Population distribution and development of spider mites and their associated

predators in canopy

2. Identify predator mite species complex in walnuts

3. Monitor results of late season G. occidentalis releases for re-establishing G. occidentalis

populations in walnuts


PROCEDURES

All studies were conducted in a mature walnut orchard located in eastern Kings County, CA

(36°25'1.10"N 119°32'47.87"W) and focused on two blocks consisting of Vina and Chandler

cultivars. Trees in the Vina block were approximately 30 to 35 feet tall, those in the Chandler

block were younger, smaller trees ranging from 20 to 28 feet. Our sample areas received no

insecticide or miticide treatments during this study. While setting up two areas for sampling

doubled our original number of replicates, we felt that the inconsistency of areas even within an

orchard for developing spider mite populations warranted spreading our risk across two

locations.

Evaluations of spider mite and predator mite populations.

Movement between ground and trees. A total of 20 trees were selected to monitor for

movement of mites between ground and tree canopy. Ten trees in each of Vina and Chandler

plantings were marked with half the tress located north and half in south areas of each cultivar.

Mite movement from was monitored by creating an intercept trap with double-sided sticky tape

around the circumference of the tree trunk. A first layer of duct tape (3M Corporation,

Minneapolis, MN) was wrapped around the tree trunk between two and three feet from the

ground to provide a relatively broad and uniform surface on which to place the actual trapping

tape. A single strip of 3/4 inch wide Scotch ® Permanent Double Sided Tape (3M) was centered

on the duct tape and wrapped around the tree circumference. Tape was replaced on a two week

schedule beginning 4/6/11 and ending 8/3/11. Tape collection was performed by removing the

tape in approximate 6 inch sections, placing it on engineer’s graph paper and covering the page

with a piece of transparency paper (3M). Samples were returned to the lab where the tape was

examined under a dissecting microscope and the number of spider mites and predator mites was

recorded. The transparency film could not be removed from the tape without damaging the mite

sample. Though we were able to identify and count the broad categories of “spider mites” and

“predator mites” through the transparency, we could not remove specimens from the tape to

prepare slide mounts and make species identifications. The entire length of each tape sample

was examined on the first collection. Subsequently, randomly selected one-inch sections totaling

1/3 of the length of each sample tape were counted. Mite numbers are reported as average

number per inch.

Tree canopy samples. Leaf samples were collected from a tree immediately adjacent each tape

sample tree. This was done to avoid any potential impact on migration from the ground cover up

the tree trunk that might have been caused by the barrier of the tape samples. Leaf samples were

taken from six areas of each tree: low canopy and high canopy at center, low south, high south,

low north and high north sides of each tree. A long reach extending bypass pruner was used to

cut five leaves from each of the high sample areas. Samples in the Vinas were collected with the


added height provided by a pruning tower. Samples from the Chandlers, were all taken from the

ground because the tree spacing, canopy, and irrigation prevented reasonable access with the

tower. With the extending pruners, high samples in the Chandlers could be collected from about

16 to 18 feet. Low samples were generally collected by hand. One penultimate leaflet was

removed from each leaf and the five leaflets of each sample were placed in a labeled paper bag.

Bags were returned to the lab in an ice chest and refrigerated until processed. Leaf samples were

collected six times from both Vina and Chandler blocks at biweekly intervals from 5/4/11 to

7/19/11. Samples from 8/3 to 8/31 were continued only in the Vina block.

Leaf samples were processed in the lab by examining both sides of each leaflet under a

microscope. All motile spider mites and predator mites were counted, and all predator mites

were preserved on slide mounts prepared with Hoyer’s media. Predator mites often hid in the

hairs where leaf veins joined the midrib. In order to find and isolate these predators, each vein

had to be cut at this junction to expose the cavity under or behind the hairs. Slides were later

examined and predator mites identified using the keys developed by Grafton-Cardwell, et al. (in

prep).

G. occidentalis release and recovery.

As the season progressed, we noted that G. occidentalis was rarely if ever found in our early

season samples. Obviously, these findings suggested that the populations of G. occidentalis had

been eliminated, but did not suggest the limiting factor. Since this area of the orchard would not

be receiving any insecticide or miticide treatments, we decided to try and re-establish a

population of G. occidentalis. Galendromas occidentalis (Nesbitt) was obtained from Biobest,

USA (McFarland, CA) for release into single tree replicates in the Vina block. Release trees

were located two rows (50 feet) north of each of the ten trees used for the ongoing canopy

samples. A release was made into five trees of the south set on 7/28/11 and 8/10/11, and a single

release into five trees for the north set on 8/10/11. Predator availability was very limited at the

earlier date, thus, we opted to release into fewer trees at that time. The predators were supplied

on cut bean plants and released by letting the predators crawl off the plant cuttings that we

placed in the central part of the tree. The number of predators released was estimated at 20 per

tree for five trees on 7/28/11 and 180 per tree for all 10 trees on 8/10/11.

We continued to monitor the original 10 trees as our “non-release” replicates. Original leaf

sample protocols were continued for this set. The 10 newly designated predator release trees

were sampled from two canopy areas only, center high canopy and south low canopy, by

collecting five leaflets from each area. Leaf samples were collected 8/3/11, 8/17/11, and

8/31/11. All spider mites and predator mites were counted and predator mites preserved as

described above.


A final leaf sample was taken 10/12/11 from the 10 non-release and 10 release trees to determine

if predator species composition had changed and if G. occidentalis remained in the system. Five

leaflets were taken from each of three sample areas (high canopy center, low canopy north and

south) of each of the 20 trees. Leaves were processed to count, preserve and identify predator

mites only using the techniques described earlier.

RESULTS

Evaluations of predator mite movement and within canopy distribution

Movement on tree trunk. Mite movement was detected on sticky tape barriers placed on tree

trunks with the first samples we collected in early April. Both varieties of walnut showed a

similar pattern with both spider and predator mite capture rising throughout April and peaking

around the 1st of May (Figure 1). Capture patterns were similar in both cultivars. While it

appears that the Chandler block had greater numbers of both spider mites and predator mites

early in the season, these differences were not significant for the spider mites (F=1.05, P=0.3),

though were significant for the predator mites (F=7.5, P=0.01). Movement continued to be

detected at reduced levels, particularly for the predator mites, into August. Mite counts for these

surveys were standardized to number of mites per linear inch of sticky trap. Circumference of

the trees varied, with the Vinas averaging 25 inches the Chandlers averaging 21 inches. Early

season ratios (Figure 1C) suggested that adequate predatory mites were not emerging from the

overwintering sites in early to mid May at a rate that would be predicted to control spider mite.

Ratios of spider mites to predators peaked at ca. 175:1 in late May. However, the absolute

counts are difficult to interpret relative to risk of outbreaks in the canopy given the level of

foliage in the tree canopy versus the surface area of the sticky tapes.

Leaf samples. Canopy leaf samples were started in early May when trees had leafed enough to

be sampled. Leaf counts for the Chandler block are shown in Figure 2. Spider mites counts on

the leaves were low throughout the entire sample period. From 5/4 through 7/19/11, cumulative

number counted totaled only 7 spider mites. In the same time period, phytoseiid mite counts

totaled 212. Leaf samples were terminated after 7/19/11 in the Chandlers as there were not

enough spider mites to warrant additional samples. We cannot determine the reason there was

no spider mite outbreak in the Chandlers, given the high ratio of spider mjtes to predator mites in

the tape samples and the lack of evidence that the phytoseiid mites recovered are effective

predators of web-spinning mites. While we cannot attribute this to biocontrol, the reasons

remain unclear.

Leaf samples from Vina block did indicate a low level of spider mite infestation, but even this

looked very similar to the Chandler samples (Figure 3). A total of 60 spider mites and 314

phytoseiid mites were counted through 7/19/11. A small increase in distribution of spider mites

(percent of leaflets with mites) was noted by 7/19 (Figure 4) and was of concern as phytoseiids,


which were broadly present through the canopy were not reducing the spider mite population.

Subsequent samples in August clearly indicated the phytoseiids present did not have a

controlling impact (Figure 4). The higher densities of predators compared to spider mites in

mid-June should have foretold of a very successful biological control for the year for the Vina

block. As subsequent data revealed, spider mites easily outstripped the population size of the

predators resulting in levels requiring intervention with miticides for the rest of the orchard.

What is clear from these data is that the prescence of predatory mites without any regard to

species identity is not sufficient for pest management practioners to make adequate early season

decisions. As revealed in the following sections, the early season species of predators were

primarily from the genus Amblyseius, which is a facultative predator capable of surviving on

pollen. Early season production of high levels of pollen by the walnut catkins is expected to

provide an ample supply of resources (pollen) for these predators to reach high numbers, but they

may in fact prove to be ineffective predators for spider mites in walnuts.

Distribution of mites through the canopy is indicated in Figures 5 – 7. As soon as the phytoseiid

population in the canopy reached consistent detection levels, they were widespread through all

sample areas of the tree – high and low canopy, center and periphery of the tree. Spider mites

were present in low numbers across all areas of the tree canopy until August when the population

increased dramatically. Spider mite infestation is typically clumped as a single female will feed

and oviposit in a small area. In sampling, this leads to highly variable counts on leaf samples.

While the data analysis for spider mites failed to distinguish any statistical difference in their

distribution in the canopy, there is a clear spike in spider mite numbers at the center of the tree

(Figure 7). Similarly, detection of spider mites or predators from the early portion of the season

through May was restricted to the low portions of the canopy. The pattern of infestation did not

appear to be dramatically different from colonization patterns observed in other systems. The

slow increase in percent infested leaves by spider mites starting in around mid-late June

foreshadowed the dramatic increase in numbers that would follow in August.

Phytoseiid mite identification.

Taxonomic keys for the Phytoseiidae are based on adult females. Thus, the identification of our

samples is based on a fraction of the total predaceous mites collected during this study. None the

less, our estimates of relative proportions should roughly correspond to the actual field

population structure. All slide mounted specimens were obtained from leaf samples, as mites

could not be removed intact from the tape intercept barrier. The vast majority of phytoseiids we

have collected and identified to date are either Amblyseius similoides Buchelos & Pritchard or

Euseius spp. Two species of the genus Euseius appear in our samples, E. tularensis Congdon

and E. quetzali McMurtry. As these species are difficult to separate, further review of the

samples should be conducted to verify the identifications, and thus, we will refer to these

collectively as Euseius spp. for this report. Only one specimen of G. occidentalis has been

positively identified from samples collected prior to 8/3/11.


The population structure in our samples differs significantly from that described by McMurtry

and Flaherty (1977) in walnut orchards in the adjacent Tulare County. They found four

phytoseiid species in their samples; Typhlodromus occidentalis Nesbitt, Typlodromus citri

Garman and McGregor, Amblyseius hibisci (Chant) and a relatively less frequent component,

Typhlodromus caudiglans Schuster. (Note that taxomonic revisions have resulted in many

species being assigned to new or different genera. Thus, T. occidentalis is currently named

Galendromus occidentalis (Nesbitt).)

Little is known about the phytoseiid mites dominating these orchard samples. Species of

Euseius are generalist feeders consuming pollen, non-webbing mites, small insects and plant sap

and demonstrating no association with Tetranychus mites. Amblyseius similoides is also a

generalist feeder with no know associations with spider mites. Potential food resources we

observed on our leaf samples included pollen, tydeiid mites and thrips. With very favorable

ratios of phytoseiid mites to spider mites, it appears these species are not capable of impacting

the spider mite population.

G. occidentalis release and recovery

In July it appeared that the spider mite population in the Vina block was becoming detectable at

a sustained low level density. Our leaf samples indicated only 0.07 spider mites per leaflet, but

this had doubled from our previous sample. Furthermore, when we looked at the distribution of

mites in the Vina leaf samples, by late July spider mites were being found in an increasing

percent of leaf samples collected, even though predator mites were detected in more than 70% of

the leaf samples from late June onward (Figure 4). As G. occidentalis did not appear to have

much presence in the orchard, but is known as an effective predator of web-spinning mites, we

decided to release and attempt establishment. G. occidentalis was released into a total of 10

trees, each located 50 feet from an ongoing leaf sample tree. Releases were made on two dates,

the first into only five trees because of very limited availability of the predators, and the second

release two weeks later into all 10 designated release trees.

The spider mite population in the entire Vina block was increasing rapidly by late July, and the

grower applied a miticide in early August to the rest of the block, leaving our study areas

untreated.

Leaf samples taken three times following the start of releases, did not indicate any decline in the

spider mite population growth (Figures 8 and 10A). From August 3 to August 31, the spider

mite counts on non-release tree samples increased 45-fold while those on release tree samples

increased 6-fold. The number of predators on the same leaf samples decreased by about 30

percent in the non-release trees and increased by 50% in the release trees (Figures 8 and 10B).

The ratio of spider mites to predator mites was less than 5 to 1 on the release trees and only 0.6


to 1 in the non-release trees on August 3. This ratio increased each sample period regardless of

treatment, though the increase was less pronounced in the release trees. On August 31, those

ratios were 16:1 in the release trees and 42:1 in non-release trees (Figure 10C). General

guidelines for G. occidentalis indicate spider mites will be controlled by this predator if there is a

ratio of 10 spider mites to 1 G. occidentalis. The time between release and our continuing

samples was short and perhaps inadequate for the released predators to propagate and distribute

enough to be detected in our samples. Clearly, control was not obtained in the three weeks that

we continued sampling. Sampling was terminated as the orchard approached harvest as access

with a pruning tower was no longer feasible.

We collected a final leaf sample on October 12th

, specifically to identify the phytoseiid species

complex that was present and compare it to mid season samples. Figure 9 shows the species

composition as a percentage of the total identifiable mites (adult females) collected. Of the 398

predators counted on leaf samples from 10/12/11, 197 were females that could be identified. Of

these, 76 were from the ongoing non-release trees and 121 were from the release trees. Samples

taken from the non-release trees on 7/3 and 7/19 were based on females of 82 and 96 specimens,

respectively. Two points are noted by this chart; 1) there was only a single specimen of G.

occidentalis found in the sample from 10/12, and this was recovered from a non-release tree; 2)

the relative proportion of the major species switches between the mid- and late season samples.

Thus, A. similoides represented 94% of the total phytoseiid sample in early July, but in October

represented only 21% of the sample. Conversely, Euseius spp. represented only 6% of the

phytoseiid mites in early July, but had grown to 78 % of the sample collected in October.

Theproportion of A. similoides to Euseius spp. was essentially the same in release and non-

release trees on 10/12/2011.

The spider mite and predator mite dynamic is a complex issue and one that has clearly changed

over the past four decades. G. occidentalis was an important component at one time because of

the selective advantage it had with resistance to organophosphates (OPs) at a time when the OPs

were a major part of a growers pest management program. It is possible that development of

new insecticides has changed this advantage, perhaps allowing additional species to establish

even though they may not be species relevant to spider mite control. Furthermore, some of the

newer chemistries may impact the predator mites and alter the dynamics that affect the biological

control of spider mites.

SUMMARY

Based on data from just one year, it does not appear that the colonization events of mites in

walnuts are dramatically different from those observed in other tree crops. The result obtained in

this study that was relatively unexpected was the near absence of G. occidentalis given the

widespread belief that effective spider mite control was achieved in walnuts from this predator.


What is not clear is if this lack of G. occidentalis is a wide-spread phenomenon and/or if this

absence of the previously dominant predator is also regionally defined. Our study was

conducted in Kings County because of reports of poor spider mite control. It is possible that

other areas such as the N. Sacramento region have greater populations of G. occidentalis, which

may explain the perception that spider mites are less of a problem. Conversely, more effective

spider mite control may result from different climatic conditions that may reduce the growth

rates of spider mite populations. Similarly, it may also be that other orchards in King’s County

may have higher levels of G. occidentalis such that effective biological controlis possible.

As a final note, this apparent reduction in G. occidentalis has been noted in other orchard

systems as well, e.g. pears. A statewide review of the distribution of phytoseiids in multiple

crops has already been initiated by Dr. Beth Grafton-Cardwell.

Several questions seem to emerge from our study that are both measurable and potentially of

importance for predicting effective spider mite suppression as follows:

1) How much variation exists between orchards within a local area (e.g. King’s county)

relative to the species of predatory mites?

2) How much regional variation exists relative to the species of predatory mites (S. Central

Valley versus N.Sacramento Valley)?

3) What is the predicted efficacy of the Euseius and A. similoides spp for regulating spider

mites?

4) Can inoculative releases of G. occidentalis re-establish this predator as an effective

biological control agent of spider mites in walnuts?

5) Are the newer insecticides responsible for the selective loss of G. occidentalis from

walnuts?

REFERENCES

Grafton-Cardwell, E. E., J. McMurtry, D. Haviland and Y. Ouyang. A Key to the Phytoseiids of

Agricultural Crops of California. UC ANR Publications (in preparation)

McMurtry, J. A. and D. L. Flaherty. 1977. An ecological study of phytoseiid and tetranychid

mites on walnut in Tulare County, California. Environmental Entomology 6: 287-292.


Figure 1. Mites captured on tape intercept traps placed around trunks of two walnut cultivars. A)Spider

mites, B) Predator mites, C) Ratio of spider mites:predator mites.

0

5

10

15

20

25

4/1 4/15 4/29 5/13 5/27 6/10 6/24 7/8 7/22 8/5 Ave

rage

Nu

mb

er

of

Mit

es

/ In

ch

Date

2011 Walnuts: Mite Movement on Tree Trunk A ) Spider Mites Captured on Tape Intercept

Vina

Chandler

0.00

0.20

0.40

0.60

0.80

1.00

4/1 4/15 4/29 5/13 5/27 6/10 6/24 7/8 7/22 8/5

Ave

rage

Nu

mb

er

of

Mit

es

/ In

ch

Date

B) Predator Mites Captured on Tape Intercept

Vina

Chandler

0

50

100

150

200

250

300

4/1 4/15 4/29 5/13 5/27 6/10 6/24 7/8 7/22 8/5

Rat

io S

pid

er

Mit

es

:

Pre

dat

or

Mit

es

Date

C) Ratio of Spider Mites : Predator Mites

Vina

Chandler


Figure 2. Average number of spider mites and predator mites per leaflet for ten trees in the Chandler

block. Averages shown include all sample areas of the tree.

Figure 3. Average number of spider mites and predator mites per leaflet for ten trees in the Vina block.

Averages shown include all sample areas of the tree.

0.0

0.2

0.4

0.6

0.8

1.0

5/1 5/15 5/29 6/12 6/26 7/10 7/24 Ave

rage

Nu

mb

er

of

mit

es

pe

r le

afle

t

Date (2011)

2011: Chandler Block Leaf Samples

Spider Mite

Predator Mite

0.0

0.2

0.4

0.6

0.8

1.0

5/1 5/15 5/29 6/12 6/26 7/10 7/24 Ave

rage

Nu

mb

er

of

mit

es

pe

r le

afle

t

Date (2011)

2011: Vina Block Leaf Samples

Spider Mite

Predator Mite


Figure 4. Percent of leaf samples with mites present for ten Vina trees samples throughout the study. A

leaf sample consists of the five leaflets collected from one area of one tree. Percentages are based on

sixty samples per time interval (six sample areas per tree from each of 10 trees).

Figure 5. Distribution of predator mites in the tree canopy samples for the Chandler block. Counts

include all motile phytoseiid mites.

0%

20%

40%

60%

80%

100%

5/4 5/18 6/1 6/15 6/29 7/13 7/27 8/10 8/24 9/7

Pe

rce

nt

leaf

sam

ple

s w

ith

mit

es

Date

2011 Walnuts: Percent of Leaf Samples with Mites Vina (non-release trees)

spider mites

predator mites

0.0

0.2

0.4

0.6

0.8

1.0

High North High South High Center Low North Low South Low Center

Av

era

ge N

um

ber

of

P

red

ato

r M

ites / L

eafl

et

Location of Sample from Tree Canopy

2011 Walnuts: Predator Mite Distribution in Tree Canopy Chandler Block

5/4/11 5/18/11 6/1/11 6/21/11 7/6/11 7/19/11


Figure 6. Distribution of predator mites in the tree canopy samples for the Vina block. Counts include all

motile phytoseiid mites.

Figure 7. Distribution of spider mites in the tree canopy samples for the Vina block. Rapid increase in the

spider mite population occurred in August. Counts include all motile spider mites.

0.0

0.2

0.4

0.6

0.8

1.0


Av

era

ge N

um

ber

of

P

red

ato

r M

ites / L

eafl

et

Location of Sample from Tree Canopy

2011 Walnuts: Predator Mite Distribution in Tree Canopy Vina Block

5/4/11 5/18/11 6/1/11 6/21/11 7/6/11 7/19/11 8/3/11 8/17/11 8/31/11

0

5

10

15

20

25

30

35


Av

era

ge N

um

ber

of

S

pid

er

Mit

es / L

eafl

et

Location of Sample in Tree Canopy

2011 Walnuts: Spider Mite Distribution in Tree Canopy Vina Block

5/4/11 5/18/11 6/1/11 6/21/11 7/6/11 7/19/11 8/3/11 8/17/11 8/31/11


Figure 8. Percent of leaf samples with mites present for ten predator release trees and 10 non-release

trees. A leaf sample consists of the five leaflets collected from one area of one tree. Percentages are

based on twenty samples per time interval (two sample areas per tree from each of 10 trees).

Figure 9. Species composition as percentage of adult female Phytoseiids collected from walnuts in July

and October 2011, Kings County, CA. Samples were obtained from the non-release trees of the Vina

block on all dates, and from trees with G. occidentalis release on 10/12.

0%

20%

40%

60%

80%

100%

8/1 8/15 8/29

Pe

rce

nt

of

sam

ple

s w

ith

mit

es

Date

2011 Walnuts: Percent of Leaf Samples with Mites Vina (release and non-release trees)

Release trees

Non-Release trees

Release trees

Non-Release trees

Predator mites:

Spider mites:

0%

20%

40%

60%

80%

100%

7/6 7/19 10/12 10/12 Release Trees

Perc

en

t o

f S

am

ple

Sample Date

2011 Walnuts: Predator Mite Species as Percent of Sample

A. similoides Euseius spp. G. occidentalis


Figure 10. Counts of A) Spider mites, B) Predator mites, and C) ratio of spider mites : predator mites on

three sample dates following releases of G. occidentalis on 7/28 and 8/10/2011.

0

5

10

15

20

8/3 8/17 8/31

Ave

rage

Nu

mb

er

of

Sp

ide

r M

ite

s /

Leaf

let

Sample Date

A) Spider Mite Counts

Non-Release

Release

0.0

0.2

0.4

0.6

0.8

1.0

8/3 8/17 8/31

Ave

rage

Nu

mb

er

of

P

red

ato

r M

ite

s /

Leaf

let

Sample Date

B) Predator Mite Counts

Non-Release Release

0

10

20

30

40

50

8/3 8/17 8/31 Rat

io S

pid

er

Mit

es

: P

red

ato

r M

ite

s

Sample Date

C) Ratio of Spider Mites : Predator Mites

Non-Release

Release


Documents

BIOLOGICAL CONTROL OF SPIDER MITES IN …walnutresearch.ucdavis.edu/2011/2011_247.pdfBIOLOGICAL CONTROL OF SPIDER MITES IN WALNUTS . ... interior portions of the tree canopy. ... populations