Baan, H.E., van de

Field, R.P.

Groot, J.J.R.

Hoy, M.A.

Wageningen Yield 

Hungry adult female spider mite predator prepares for dispersal by 
standing on its rear legs and raising its first two pairs of legs (see 
drawing), enabling it to be carried from tree to tree by low to moderate 
winds. Well-fed females lower their bodies to the leaf surface (right), so 
they cannot be picked up by breezes. The dispersal trait permits growers 
to inoculate almond orchards by placing predator mites in one tree in nine. 
Aerial movements of mites in almonds: 
implications for pest management 
Marjorie A. Hoy 0 Hugo E. van de Baan 0 J. J. Rob Groot 0 Ross P. Fi~ld 
spider mites and predatory mites can learn how aerial -dispersal might influ-
live all year on deciduous trees and ence a pest management program in 
vines. During the growing season they almonds that focuses on inoculative re-
colonize the foliage, and during winter leases of pesticide-resistant predators. 
they overwinter under bark and in crev- During 1982, clear plexiglass panels 
·ices in .a state of dormancy (diapause). (about 3 by 6.5 inches) were greased on 
Spider mites and predatory mites can one side and placed vertically on towers 
move from plant to plant by walking or outside two orchards. The tovvers were 
they can be accidentally transferred by on all four sides of the two orchards, and 
other organisms. Spider mites are also panels were installed about 6, 12, and 18 
known to disperse aerially, and clusters feet above ground level on each tower, 
of spider mit.e females can sometimes be with two panels at each height directed 
leaving Livingston-! during the season 
were trapped on panels facing the or-
chard on its south and east sides. Like-
wise, 96 percent of the spider mites and 
82 percent of the predators leaving Liv-
ingston-III were trapped on the south 
and east towers on panels facing the 
orchard. We found no consistent differ-
ences in numbers of mites trapped at 
the three heights. 
Movements within the orchard 
seen on the tips of branches before their toward the orchard and two directed A total of 96 panels were hung during 
1 dispersal. Some spider mite species drop away from it. The 12 panels of each 1982 in each orchard, four each at two• 1 
from leaves on thin silk strands and are tower were replaced every week or t\.vo heights- about 15 and 8 feet above the 
picked up by the wind. weeks and, with the aid of a dissecting ground- between tree rows at six sites j 
The predatory mite Metaseiulus occi- microscope, spider mites and predators in each orchard. The greased surfaces at 
dentalis (Nesbitt) can move from leaf to were counted. Panels were continuous- each height were oriented north, south, j 
leaf and from branch to branch by walk~ ly present from May 5 until September 1 east, or west to determine whether mite 
ing. Because this mite is so tiny, the outside the Livingston-! orchard and movements within the orchard were 1 
distances it trav~ls are relatively sma)l. from May 26 until August 11 outside the random. Panels were replaced each I 
Yet releasing pesticide-resistant strains second orchard, Livingston-III. Foliage week or every two weeks. 1 
of M. occidentalis into individual al- counts were also taken to estimate spi- Consistent differences in numbers ofl 
mond trees (one tree in nine) successful- der mite and predator densities in each. spider mites or predators trapped on 
ly inocplated entire orchards. Walking, Peak dispersal of spider mites oc- panels at either height facing the fou~ 
alone, did not seem sufficient to account curred in late July to early August in directions did not occur, although sig-
for the relatively rapid spread of this Livingston-! and in mid- to late July in nificant differences were sometime~ 
wingless predator. Livingston-III. Peak dispersal of pred- found. The results suggest that prevail 
Hungry predator females have an ex- ators occurred in early August in Liv- ing winds from the northwest some! 
plicit dispersal behavior in the laborato- ingston-I and in mid-July in Livingston- times influence mite movements withi} 
ry. They stand on their rear legs with III. Few predators dispersed immediate- the orchard but that wind turbulencl 
their first two pairs of legs off the sub- ly before those dates. Adult females within the orchard usually results ij 
strate in ]ow to moderate winds. In constituted more than 90 percent of the relatively random movement of spid~ 
contrast, well-fed females lower their spider mites and predators on the mites and predators throughout the oJ 
· o the substrate and cease mov- greased panels. This result suggests that chard. t3~~ 'Gk-:etb.Jlp.;;;~~;;,;;;-c:JlLf~rnales are the I 
Seasonal dispersal patterns 
In a series of experiments in two Liv-
ingston, California, almond orchards 
during 1982 and 1983, we sought to 
- ~ ~~;.-·c:~:::c,,~0"~" Movement in and out of orchard 
Mostrn1 e ~~~~~~~~~~~;;~~~~~~~~~~~~~ via prevailing win~s from the north- 1 • occJ 
west. About 92 percent of the spider facing away from the south side o 
mites and 94 percent of the predators ingston-I during .the entire 1982 sea~ 
CALIFORNIA AGRICULTURE, SEPTEMBER 1984 
was 22; two were trapped on panels 
facing west, non13 on panels facing 
north, and three on panels facing east. 
These 27 predators were presumed to be 
entering · the orchard, . although some 
could have been leaving but were 
trapped by eddying winds. In contrast, 
103, 5, 6 and 84 predators were trapped 
over the season on panels facing toward 
the orchard south, west, north and east, 
respectively; and these 198 predators 
presumably represent the proportion 
leaving. No significant differences in 
catches at the three heights were found. 
Our findings suggest that relatively 
few pesticide-susceptible M. occidenta-
lis entered Livingston-! during 1982. 
Similar results were obtained for Liv-
ingston-III. 
If we conservatively assume that each 
almond tree has half a million leaves 
and that on each leaf there is an average 
0.1 predator, we c·an conclude that there 
are at least 50,000 predators per tree. 
Each acre contains 70 trees, or a mini-
mum of 3.5 million predators. Very 
large numbers of susceptible predators 
would have to enter the orchard over 
the season to have an effect on the 
Sevin-resistance level in the orchard 
population, and a Sevin ~pplication dur-
ing the season would select against the 
immigrants into the orchard. 
To estimate the number of spider 
mites leaving each side of the orchard, 
we assumed that the numbers trapped 
on the panels were directly proportional 
to the riumber leaving the air space on 
the entire side of the orchard. If these 
assumptions are correct, then (lt least 
170 million spider mite females left the 
south side of and 30 million left the east 
side of the 14-acre Livingston-I orchard. 
Likewise, 230 and 1,100 million spider 
mite females left the south and east 
sides, respectively, of the 45-acre Liv-
ingston-III orchard during 1982. 
Making the same assumptions, we es-
timated that 8.1 and 8.3 million M. occi-
dentalis females left the south and east 
sides of Livingston-! during 1982, while 
40 and 97 million predators left the 
larger Livingston-III orchard. 
These large numbers may or may not 
have an impact on recipient orchards or 
vineyards. Mortality during dispersal is 
likely to be high, and only a few may 
successfully colonize a new site. Fur-
thermore, it is likely that 200 million, or 
even 1,100 million spider mites repre-
sent only a small proportion of the spi-
der mites in most orchards. If we con-
Resistance level of 
carbaryi-OP-sulfur-
resista.nt laboratory 
strain relative to above 
Guthion resistance levels in nine native populations of M. occidentalis from almond orchards. The fully 
shaded circle indicates the highest LCso value (10.31b. a.i./100 gal. water). The partially shaded circles 
indicate proportionally lower LCso values. Resistance levels of a Guthion-resistant laboratory colony 
and a vineyardacollected colony are shown for comparison. Each dot represents 100 acres of almonds. 
22 CALIFORNIA AGRICULTURE, SEPTEMBER 1984 
servatively assume that there is one 
spider mite on each leaf over a season, 
and each almond tree has about half a 
million leaves, then an almond orchard 
vdth 70 trees per acre may conservative-
ly have 35 million spider mites per acre 
over a season. Arrival of additional spi-
der mites may go unnoticed unless the 
orchard lacks predators or has low den-
sities of spider mites because of acari-
cide applications. 
Daily dispersal patterns 
Greased Perspex plastic panels mea-
suring 2 by 4-feet were placed on two 
towers on the south side of Livingston-! 
during the weeks of July 18 through July 
25 and July 25 through August 1, 1983, 
to learn the predators' daily dispersal 
patterns. Panels were replaced every 2 
hours for 24 hours d~ring the first week 
and every 4 hours for 48 hours during 
the second week. The plastic was cut in 
strips and the mites were counted with 
a dissecting miscroscope. \!Veather data 
(temperature, relative humidity, wind 
speed, and wind direction) were record-
ed each time the panels were replaced. 
Neither spider mites (Panonychus 
ulmi and Tetranychus pacificus) nor M. 
occidental is dispersed uniformly 
throughout the day. More than 80 per-
cent of the dispersal occurred between 
4:00 and 10:00 p.m. During this time, 
temperatures declined from the daily 
peak, wind speeds increased from a low 
of 200 feet per minute to 1,000 feet per 
minute, and relative humidity in-
creased from 30 to 46 percent. 
Survey for Sevin resistance 
in surrounding orchards 
Fifteen almond orchards surrounding 
Livingston-! and III, where Sevin-OP-
resistant predators were released in 
1981 and 1982, were surveyed to deter-
mine whether Sevin-resistant predators 
had successfully colonized them. Colo-
nies v,rere collected between July 19 and 
July 26, 1983, and 40 to 50 adult females 
from each colony were tested with 2.4 
grams active ingredient carbaryl (Sevin) 
per liter distilled water. Seven-OF-
resistant and Sevin-susceptible labora-
tory colonies \vere tested at the same 
time as the controls. Native M. occiden-
talis typically exhibit 0 to 4 percent 
survival rates at this dosage, as do sus-
ceptible laboratory colonies. The Sevin-
resistant colony typically has a 70 to 85 
percent survival rate. 
exhibited 
via prevailing winds from the north-
west. These four orchards are about 0.5 
mile from the nearest release site. In 
other tests. both spider mites and pred-
ators were· trapped in large numbers at 
least 640 feet southeast of Livingston-! 
in 1983, and 70 percent of the hungry 
adult females survived for at least 12 
hours at 86°F under 30 percent relative 
humidity. 
Guthion resistance in 
native predators 
Native M. occidentalis were collected 
from nine almond orchards from 1980 
through 1982. Colonies were tested in 
fall 1982 with Guthion (azinphos-
methyl), using a slide dip technique. 
Doses tested \•Vere 0.5, 1, 2, 4, 8, and 16 
pounds active ingredient per 100 gallons 
distilled water. At least 100 females 
'"'ere tested at each dose, and females 
\.vere scored after 4B hours at 78°F under 
an 18-hour daylength. LC50 values were 
calculated using a logit analysis. Colo-
nies \•vere obtained from Modesto in 
Stanislaus County; Wasco in Kern 
County; North Avenue, Caruthers, Los 
Banos, Raisin Citv, and Three Rocks in 
Fresno County; Livingston in Merced 
County; and Road 20 in Madera County. 
A colony collected from a Fresno Coun-
ty vineyard in June 1980 and the Sevin-
OP-sulfur-resistant laboratory colony 
were compared at the same time. 
Variability in Guthion LC50 values is 
high in native predators from these al-
mond orchards (see map). The lowest 
{1.2 pounds .active ingredient per 100 
gallons water) and the highest (10.3 
pounds) LC50 values are from orchards 
located near each other in the central 
part of the valley. LC.li0 values from the 
other orchards range tram 2 to 4 pounds 
active ingredient per 100 gallons of wa-
ter. These values probably reflect differ-
ing pesticide treatment histories for the 
orchards, although treatment histori~s 
were not obtained. Because growers 
typically apply Guthion at rates of 1.5 to 
2 pounds 50 wettable powder (VVP) per 
acre, these resistance levels are surpris-
ingly high. 
Conclusions 
· Peak aerial dispersal of spider mites 
and predators occurred in July and early 
August of 1982 and 1983. Most predators 
and spider mites left the Livingston al-
mond orchards on prevailing winds 
from the northwest. VVithin the orchard, 
;:::::=~~~;;;;;~ehad less influence, and 
pears to occur 
early evening. 
The Sevin-resistant predators collect-
ed from four almond orchards 0.5 mile 
from sites where Sevin-OP-resistant 
predators were released in 1981 and 
1982 document the aerial movement 
and establishment of the Sevin-resistant 
strain. However, growers wishing to use 
the Sevin-OP-resistant strain of M. occi-
dentalis in their orchards should release 
it, as dispersal is too slow, and the 
numbers dispersing are too lmv relative 
to the densities of native predators that 
are often already present, to provide a 
useful inoculation level. 
Few predators were trapped on the 
panels facing away from the orchards. 
suggesting that relatively fe\v predators 
entered compared \•vith the numbers 
resident in the orchard. This suggests 
that dilution of the Sevin-resistant pop-
ulations is minimal, particularly if Se-
vin is used once everY few vears. Simi-
lar conclusions '"'e~e rea'ched after 
comparison of Sevin-resistance levels in 
six to eight other release sites. 
OP-resistance levels in native M. occi-
dentalis are sufficiently high that grow-
ers should be able to use low-to-moder-
ate rates of Guthion, Diazinon, or 
Imidan (phosmet) without disrupting 
their predator populations. The variable 
OP-resistance levels, however, indicate 
that the San Joaquin Valley does not 
contain a single interbreeding popula-
tion of M. occidentalis. 
In orchards where abunda'nt native 
predators are well distributed, growers 
may be able to adopt the mite manage-
ment program. This involves monitor-
ing spider mites and predators and use 
of lower-than-label rates of selective 
acaricides, if predator populations have 
successfully survived previous Guth-
ion, Diazinon, or lmidan applications. 
Greased panels inside Livingston almond 
are checked by former research assistant Darryl 
Castro to determine if spider mites and predators 
disperse aerially between rows of trees. 
Sevin cannot be used with native popu-
lations in the central and southern San 
Joaquin Valley, because they are very 
susceptible to it. 
Aerial movements of spider mites and 
predator mites provide a mechanism for 
between-tree, between-row, and longer 
range movements within and out of al-
mond orchards. Recognizing that such 
movements occur is important for pest 
management programs. Orchard "hot 
spots" can serve as the infestation foci 
for the entire orchard, justifying spot 
treatments of spider mites, particularly 
when predator densities are low. Large-
scale aerial movements of spider mites 
could also explain the rapid buildup of 
infestations in almond orchard treetops 
occasionally seen in July and August 
when densities during the preceding 
two weeks were exceptionally low. 
While rapid population increases can be 
due to the very high reproductive rates 
of spider mites in the hot, dry San Joa-
quin Valley climate, they may be due in 
some cases to immigration of large num-
bers of spider mites into the orchard 
from surrounding crops with outbreak 
populations. 
Aerial movements are hazardous to 
both spider mites and predators, and 
probably relatively few are able to colo-
nize successfully a new site a long dis-
tance away. A colonizing predator must 
find prey upon its arrival; it must sur-
vive hot, dry conditions during the jour-
ney; it must not be subjected to pesti-
cides it cannot tolerate; and it must 
compete successfully with any native 
predators in the recipient orchard. If 
Sevin is not applied in the newly colo-
nized orchard, the Sevin resistance gene 
cannot increase in frequency. There-
fore, rapid natural spread and increase 
of the Sevin-resistant predator strain is 
unlikely unless Sevin is applied to re-
cipient orchards. It is unknown whether 
these predators (or spider mites) can 
disperse very long distances (hundreds 
1 
of miles) in a single dispersal. It will be I 
interesting to learn if isolated popula-
1 tions of the Sevin-resistant predators 1 
are detected such distances from release 
sites. j 
Majorie A. Hoy is Professor and Entomologist, 
Department of Entomological Sciences, Universityl 
of California, Berkeley; Hugo E. van de Boon and}. 
]. Rob Groot are graduate students from Agricul· 
tural University, Wageningen, The Netherland~ 
who visited Berkeley for six months during 198 
and 1983, respectivelv; and Ross P. Field is 
former graduate student, Department of EntomojL 
ogy, UC Berkeley, now Senior Research Office 
Keith Turnbull Research Institute, Frankston, Vi 
Ioria, Australia. The authors thank Frances Cav 
Darryl Castro, and Dan Cohn for their assistan 
C. V. Horton, who made his orchards available 
· and WilJiam W. Barnett. Area Speci 
ty, and 
CALIFORNIA AGRICULTURE, SEPTEMBER 1984 
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Aerial movements of mites in almmonds: implications for pest management

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Wageningen Yield

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