Release Calculator and Guidelines for using Neoseiulus fallacis to Control Two-Spotted Spider Mites in Strawberry

No claims are made as to the appropriateness of any recommendations herein for regions elsewhere. These guidelines may apply to some extent, however, in regions of similar climate, such as Western Washington and British Columbia. Also, it has not yet proven whether N. fallacis can provide biological control of Cyclamen mites. While preliminary tests have shown that N. fallacis will eat Cyclamen mites, it is unknown at this time whether it can travel inside the budding leaves where Cyclamen mites are found.

Description and life cycle

Neoseiulus (Amblyseius) fallacis (Garman) is a predator mite of the family Phytoseiidae. These mites are very small in size (0.25 to 0.5 mm), and are somewhat similar in appearance to the twospotted spider mite, but are more flattened and lack spots (Figs. 1, 2 and 5). Phytoseiids pass through the same developmental stages as do spider mites (eggs, larvae, protonymphs, deutonymphs, and adults). Eggs of N. fallacis are nearly transparent white, shiny, and football-shaped (Fig. 3). Compare these to two-spotted spider mite eggs, which are rounder, and usually associated with the webbing produced by spider mites (Fig. 4). N. fallacis eggs hatch into a larval stage that have six legs. They then pass through the nymphal stages before becoming adults, which have eight legs. N. fallacis adults are somewhat shiny, tan to slightly orange in color, and are pear-shaped (Fig. 5).

Figs 1-5 (left to right).
1. N. fallacis (bottom) feeding on two-spotted spider mite
2. N. fallacis (right) feeding on spider mite
3. Eggs of N. fallacis
4. Eggs of two-spotted spider mites
5. Female adult Neoseiulus fallacis

There is a third type of leaf-inhabiting mite in strawberry that is sometimes confused with N. fallacis, the Tydaeid mite. These are not considered pests normally but they are not beneficial, either. They have a different color, shape, and behavior from either the pest or predator mites (photo not available). They are not marked and are dull-colored (not shiny), are smaller in size, have long front legs that are continually active rather like antennae, and they are very active but usually do not travel far, and almost act as if they are nervous.

The life cycle of predator mites is somewhat shorter than plant-feeding mites. N. fallacis, is very common in strawberry fields. They overwinter in dead leaves, on stems, and on green foliage. The developmental threshold of N. fallacis is near 50°F (10°C), versus the cooler threshold of the twospotted mite, which is around 47°F (8.3°C). At 70°F (21°C), N. fallacis eggs hatch in 2 days, and adults develop in 7 days. At 55°F, adults develop in 16 days.

Female predator mites each produce 30 to 60 eggs, which is generally equivalent to the number produced by their prey. This high rate of reproduction, plus a very high preference for spider mites help account for the effectiveness of N. fallacis. N. fallacis is known in particular for its affinity for humid environments such as in peppermint, strawberry, and sweet corn where the dense plant canopy sustains a high humidity.

Sampling guidelines

In determining need for controls, it is essential that predator mites be recognized and counted along with injurious mites when sampling strawberry leaves. The ratio of predator mites to plant-feeding mites and the population trends are very useful for determining whether or not expected injury justifies application of controls, whether chemical or biological. Unnecessary treatment with non-selective chemicals will reduce predator mite populations, which could result in a rapid buildup of injurious plant-feeding mites.

Populations should be sampled at least once before predator releases are made, and at least every 3-4 weeks thereafter (more often during critical treatment decision periods, less often during cool seasons). The only requirement for sampling is to have 3x or higher magnification. Hand-lenses, A sample may consist of at least 1 site per acre, at least 10 sites per field. A site may be several steps in circumference. Sample 6 trifoliate (whole) leaves per site, taking leaves from parts of the plant where spider mites are expected to occur (i.e., avoid sampling very young or very old leaves). Search leaf undersides for the presence/absence of predators, and count the total number of spider mites by 5s or 10s (0, 5, 10, 15, 20, 30, 40, etc.).

For spider mites, count a cluster of 5-20 eggs as 1 adult, and count a cluster of 5-10 immatures as 1 adult. For predators, count all adults, male and female, and larger nymphs. Training may be required in order to find and recognize predator mites. Since HIGH populations of predator mites occur at only about 1 predator for every 10 2-spot spider mites (1:10 predator:prey ratio), they can be very difficult to find because relatively few are needed to provide biological control. If you count predators based on their presence/absence, you may convert these counts to absolute density estimates using Table 1 below. Examination of each leaf should take about 20-30 seconds for trained observers.

At each site, record for each of the six leaves, the number of spider mites, and the presence or absence of predator mites. See our example short sampling form and longer sampling form (printable; may not work with all browsers) for the recording of sample data. Draw a map of the field and record the location of each sample site for determining and tracking "hot spots" in the field. You may want to release higher densities of predators or use selective acaricides in these "hot spot" areas of the field.

You may want to confirm the identity of your predator mites with the help of a specialist. Several consultants and Extension Entomologists can at least confirm whether you have predators in the mite family Phytoseiidae to which N. fallacis belongs. Identification to species will require more effort by facilities such as the Oregon State University Extension Entomology Insect ID Service (phone no. 541-737-5520). You can collect predators by saving up to 10 leaves that harbor them and storing under refrigeration (for a few days only) until delivery to a qualified identification specialist can be made.

Table 1. Conversion of predator presence/absence counts to an absolute estimate (N = 190 samples of 10 leaves). For example, if 45 out of 90 leaves were found to have predators present (50%), the overall average density would be about 0.95 or 1.0 per leaf. Note that these samples were taken in the field using 3X magnification; predators were missed that might have been detected in a more thorough search in the laboratory using a dissecting scope.

% leaves with predators	avg. # predators/leaf
10	0.12
20	0.29
30	0.50
40	0.70
50	0.95
60	1.25
70	1.60
80	2.00
90	2.50
100	3.00

Biological control guidelines

N. fallacis will provide excellent natural biological control of twospotted spider mites, if populations are conserved. When populations are disrupted due to incompatible pesticides, recolonization usually occurs from surrounding habitats. In addition, this species may be re-introduced manually. Several insectaries and biological control supply houses carry N. fallacis. Released predators will spread throughout the field over several weeks to months, depending on the number released, weather conditions, the condition of the foliage, .

Preliminary release recommendations are based on recent work with N. fallacis on strawberry and peppermint. Croft and Coop (1997) developed a model of how fast they disperse in strawberry in relation to degree-day (DD) accumulations. This model has been implemented as the predator mite release calculator (MITECALC), and the static information presented below is based on the calculator version of the model. We recommend you read all guidelines and then use the calculator to design a release program specific to your needs and field conditions.

The number to release depends in part upon whether more or less rapid control is desired. More-rapid control (3-5 weeks, longer for high spider mite infestations) can be expected with relatively high (innundative) release rates, about 7,000-12,000 predators per acre. The cost of these releases should be roughly equal to about 2 applications of conventional miticides. Less-rapid control (within 2-4 months, longer if releases are made in cooler months) can be expected with low to very low (inoculative) release rates, about 500-1,500 predators per acre. This approach takes advantage of the predators' high natural reproductive and dispersal abilities. The actual release rate depends on the density of 2-spot mites, the density of foliage in the field, the time of year, and the period of time allowable before control is desired. If available, predators may be taken from fields where spider mite populations have crashed and predators remain abundant, or when predator:prey ratios are 1:10 or better. Miticides including omite and vendex (both at lower rates) are compatible with predator mite releases and may be necessary to provide control while predator populations build up (see below for other pesticide compatibility).

Example release recommendations - use the Predator Mite Release Calculator (MITECALC) for recommendations when conditions differ from these examples :

Assumptions for using MITECALC (and for these examples):

1. This program was developed for estimating the number of N. fallacis to release in strawberry for the long-term control of Twospotted spider mites in Western Oregon.
2. Releases include healthy populations of adult and immature N. fallacis; only adults and older immatures (known as deutonymphs) are counted for the release rate however.
3. Weather data used in the MITECALC model are 20 year average from Corvallis, Oregon. Predator movement will be slower or faster depending on the season and the climate of the particular location.
4. Releases are shown to be evenly distributed: practical release strategies should place more predators wherever higher spider mites densities occur.

Map of releases: marks release sites:

row 1 ---><-------17 ft --------><-------17 ft --------> 2 ----------------------------------------------------------------- 3 ----------------------------------------------------------------- 4 ----------------------------------------------------------------- 5 ----------------------------------------------------------------- 6 --17 ft ---------><-------17 ft --------><---------

2. Example inoculative release for newly established plantings - late spring

Type of field:	Establishment
Date of release:	Third week of May
Date biological control is needed by:	Second week of August
Average spider mite density per (whole) leaf:	1.5 spider mites/leaf
Total release rate per acre:	1,100 adult females
No. release sites per acre:	44
Distance between release sites:	every 18 ft and every 6 rows
No. predators to release per site:	25 adults & large nymphs
Cumulative degree-days:	641 DD (50°F threshold)
Cost of mites @ $9.00/1000 (including shipping):	$9.90/acre (plus $3.00/acre estimated cost of application)

Map of releases: marks release sites:

row 1 ---><-------18 ft --------><-------18 ft --------> 2 ----------------------------------------------------------------- 3 ----------------------------------------------------------------- 4 ----------------------------------------------------------------- 5 ----------------------------------------------------------------- 6 --18 ft ---------><-------18 ft --------><---------

3. Example innundative release - spring:

Type of field:	Production
Date of release:	First week of April
Date biological control is needed by:	Third week of July
Average spider mite density per (whole) leaf:	15 spider mites/leaf
Total release rate per acre:	12,000 adults & large nymphs
No. release sites per acre:	480
Distance between release sites:	every 5-6 ft and every 2 rows; OR every 11 ft and every 1 row
No. predators to release per site:	25 adults & large nymphs
Cumulative degree-days:	529 DD (50°F threshold)
Cost of mites @ $9.00/1000 (including shipping):	$108.00/acre (plus $9.00/acre estimated cost of application)

Map of releases: marks release sites:

row 1 -><---11ft----><---11ft----><---11ft----> 2 --11ft----><---11ft----><---11ft----><--- 3 -><---11ft----><---11ft----><---11ft---->

Notes on handling of releases:

Be sure to follow directions supplied by the producer or distributor of the predator mites. These guidelines are very general and may not apply to the specific product you are dealing with.

1. Verification

   Because the actual number of predators can vary among shipments and individual containers, a verification sample is helpful to assure viability and quality. To take a verification sample: 1) Gently and thoroughly mix each shipment container and scoop out about 15 ml (if on vermiculite) and place in a vial with an air-tight seal; or place in a larger bottle with 70-90% alcohol, 2) Send the samples to a qualified expert to verify the species (or at least that they are in family Phytoseiidae) and the density per sample. Also, check the samples yourself. Reread notes on identification given above. The predators should be moving up the sides and on the lid of the container. Check for good, healthy female adults, which are easy to recognize from their robust, shiny appearance and very active search behavior. Small, dull-colored and sluggish females are not as likely to survive. Watch for 2-spotted (prey) mites, which are OK if present in the small numbers.
   
   

2. Storage

   Note that predator mites will not survive storage, except perhaps for a few days under refrigeration (best between 45° and 52° F). Avoid making releases when temperatures are very high (80+°F) and humidities are low, since A. fallacis is especially sensitive to dry conditions.
   
   

3. Dilution

   If predators arrive on vermiculite, it may be easiest to dilute 1,000 predators with extra vermiculite so that a standard 1/4 cup equals the 12 mites per site. Be sure to mix the predators gently and uniformly throughout the vermiculite. Do not try to dilute if predators arrive on leaves.
   
   

4. Placement

   For best results, apply the predators to the crop as soon as possible after receiving shipments. Make sure the predators make contact with the plant foliage (whether the carrier is vermiculite or bean plants). In general, space release sites evenly throughout the field, especially towards the upwind side and wherever spider mite densities are highest. If possible, scout the field and map spider mite hotspots just before or during predator releases. Apply predators more heavily wherever spider mite densities are high; apply fewer predators where spider mites are less dense. Predators will build up slightly faster wherever spider mite eggs are abundant. Predators will move more quickly wherever spider mites are sparse.

5. Validation of Establishment

   If you are making a release for the first time and you wish to be certain that they have survived after a short or long period, you may want to set out one or more marked release sites. To do this, find sites that have a sufficient population of spider mites to serve as food (0.5 or more per leaf are recommended). Then, release at least 50 predator adults and large nymphs at those sites. Next, mark the site(s) with a flag or similar marker so that you can sample immediately around them after 1 week, 2 weeks, 1 month, or sometime in the early spring (to determine if they have overwintered successfully). If you can find predators at these sites, then you can be fairly certain that they are surviving throughout the field. This method can also be used to save time in sampling the field while predator populations have not had sufficient heat units to easily be found throughout the remainder of the field.