Integrated Control of Twospotted Mite in Orchards
Note Number: AG0157
Published: June 2000
Integrated control of twospotted mites is probably the best known integrated pest management system in horticultural crops. Twospotted mite is the main mite pest in pome and stone fruit orchards in Victoria. Integrated control, or integrated pest management, is a system in which chemical, biological and cultural methods of control are integrated in such a way that pest populations are prevented from causing economic damage. Better timing, selection and application of pesticides reduces the amount of chemical residues on crop plants at harvest. The use of biological control agents and less pesticide reduces the chances of pests developing resistance to pesticides.
There are, however, problems associated with the use of integrated control. Growers need to have a good understanding of pest biology, monitoring techniques and threshold sizes of pest populations. They must become more aware of the implications of each course of action they take.
Trials to assess predatory mites for biological control of twospotted mite in Victoria began in 1972 and since then introduced predatory mites have been mass-released into all apple and peach growing areas in the state. The predators can keep twospotted mite populations below damaging size if chemicals of low hazard to the predators are used for the control of other pests and diseases. There are two main species of predatory mite in use against twospotted mite in Victoria. Both are available commercially.
T. occidentalis is a creamy-white, pear-shaped predacious mite about the size of the adult twospotted mite. It is often found on the undersides of leaves, close to the mid-rib. T. occidentalis eggs are oval, cloudy white and slightly larger than those of twospotted mite. The young predators that hatch from the eggs pass through three growth stages before maturity: a larva with six legs, and the protonymph and deutonymph, each having eight legs. The adult has eight legs. The immature stages tend to be clear in colour.egg production depends on the food supply. T. occidentalis is capable of rapidly increasing in numbers when twospotted mite numbers increase. T. occidentalis prefers to eat strongly webbing, plant-feeding mites and will eat all stages of twospotted mite including the eggs, which it prefers. It feeds by clasping its prey and sucking out the liquid contents.
Adult T. occidentalis are each capable of eating the equivalent of about 50 adult twospotted mites during their lifetime and can survive for about 10 days without food. Only mated females overwinter, and these shelter in the same protective situations as the twospotted mites. Winter mortality may be high, but conditions that are unfavourable for the predators will generally be unfavourable for the twospotted mites as well.
Another predatory mite, Phytoseiulus persimilis was found in Victorian orchards in 1981. Strains of this predator have been used overseas for mite control in glasshouses. It is available commercially in Australia and has been used in orchards in the Goulburn Valley.
The adult female is an orange-red, pear-shaped mite about one-and-a-half times bigger than twospotted mite. It has long legs and moves rapidly. The eggs are about twice the size of twospotted mite eggs and are laid singly on the undersides of leaves among twospotted mite colonies. The immature predators are similar in appearance to T. occidenialis except that they are more active and have a pinkish tinge. P. persimilis is a very voracious predator and is capable of rapid population increase.
Dispersal and behaviour
The predators range over leaves seeking out twospotted mites. They are capable of rapidly controlling infestations of twospotted mites and will continue feeding and reproducing until the mite population is reduced to a low level. Predators will not completely eliminate mites from an orchard but they will reduce them to a level where mites will be hard to find. Predator numbers will then drop and remain at a low level until the mite population begins to build up again. The predators can spread rapidly within an orchard and into neighbouring properties. By using only chemicals of low hazard to the predators, growers will encourage the predators to spread.
Effects of chemicals
Agricultural chemicals may have drastic effects on predatory mites by killing them, or reducing their fertility, or reducing the amount of their prey. The predators may die or be unable to complete their life cycle depending on the amount of food available. Information on chemicals toxic to the predators can be obtained from the suppliers of the predators.
Balancing predators and prey
The success of predators in saving miticide applications will depend on how well the orchard is managed. In many orchards the predators will have established well and may have spread to all trees on the property; on others they will occur only in small numbers on the original release trees.
Occasionally during the growing season the predators may need assistance to control twospotted mite. The following simple mite/predator assessment guide for apple and peach orchards will help growers to decide when to begin a sampling programme but mite populations will be easier to manage if sampling is performed on a regular basis. Sampling once a fortnight should be sufficient but, if populations are close to the thresholds, weekly sampling may be required until the person doing the monitoring develops confidence in the system.
Mite sampling guide for apple and peach orchards
- Select 10 trees at random in each block.
- Sample five leaves from low down in the centre of these trees; that is, sample a total of 50 leaves.
- Use a hand lens to find out whether any twospotted mite and predators are on each leaf (most T. occidentalis tend to be found close to the midrib on the underside of the leaf). You do not have to count the numbers present, just the number of leaves they are on.
- Record mite presence or absence for each leaf and
- if less than 45 leaves (90%) are infested by active stages of twospotted mite, a miticide application is not required.
- if more than 45 leaves (90%) are infested by twospotted mites and less than 35 leaves (70%) have active stages of predators (this includes small and, adult predators), apply a miticide.
- if more than 45 leaves (90%) are infested by twospotted mite and more than 35 (70%) have active stages of predators, a miticide is not required because control of twospotted mite by the predator should occur within two weeks.
Remember, your orchard mites must be "managed"; you should not spray at the first sign of mite activity. Be prepared to suffer minor mite damage in the centre of your trees; this will not affect your present or future crops. During the time spent in sampling for mites you can also check the orchard for other pests and problems. Pear growers should use the same sampling technique but convert the % leaves infested value to CLIDS (Cumulative Leaf Infested Days).
How to calculate CLIDS
It is not difficult to calculate the CLID for each leaf sample. The arithmetic is simple and can be performed quickly using pen and paper only.
Calculation of CLIDS for the period between each leaf sample:
(add this number to the numbers taken from previous monitoring data)
This calculation may be done in an exercise book containing all monitoring data or a computer can be used to do the work for you.
|Date||% Leaves infested LI||Average % LI||No of days between samples||Average % LI No Days||Running (cumulative) Total (CLID)|
|Weekly||10/01/93||2 (1 leaf)||1||7||7||7|
|17/01/93||4 (2 leaves)||3||7||21||28 (21+7)|
|24/01/93||6 (3 leaves)||5||7||35||63 (35+28)|
|31/01/93||14 (7 leaves)||10||7||70||133 (70+63)|
Table 1. Cumulative Leaf Infested Days (CLID)
These units are calculated for each leaf sample and a running total kept for the season (for the above figures it would be 20 + 40 + 80 = 140 CLID). The CLID figure is an excellent guide to the mite pressure experienced by trees in a block. Leaf scorch develops when certain CLID levels are exceeded. The graph below shows the relationship between CLID and the level of leaf scorch that develops.
This relationship was then used to develop damage thresholds. These will indicate when leaf scorch damage will develop before it actually occurs and allow appropriate control to be utilised only if necessary.
- 1% leaf scorch* will develop at approximately 1000 CLID
- 5% leaf scorch will develop at approximately 1500 CLID
- 10% leaf scorch will develop at approximately 2400 CLID
- 20% leaf scorch will develop at approximately 3000 CLID
These CLID levels are damage thresholds, ie., they give an infestation pressure at which a known amount of leaf scorch will develop. We can also put an approximate cost on the damage levels shown above.
*Remember that the % leaf scorch damage figure is averaged over the whole block and, for example at 5% damage, some individual trees may have significantly higher levels of damage and others lower or nil damage.
The original author of this Agriculture Note was David Williams, Tatura, in June 2000.