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   Pest Management
 Home>Lawn & Garden>Commercial Horticulture>Pest Management>

Scouting and Biological Control of Two-spotted Spider Mite

IPM (integrated pest management) is a comprehensive approach to pest control that uses all control measures, from cultural management to the release of natural enemies or the application of pesticides to reduce pest populations. IPM programs are becoming more attractive to ornamental growers because of the negative aspects associated with sole use of chemical controls.

Scouting is a key component of any IPM program. It helps you to (1) detect when pests are present, (2) make proper control decisions and (3) monitor whether your control practices are effective. Here are some tips on how to scout and practice IPM for the control of one of the top greenhouse pests - twospotted spider mite.

Twospotted spider mite (TSM) is a troublesome pest that infests more than 200 ornamental crops. It is also a pest notorious for developing resistance to traditional miticides. Spider mites are tiny but have the ability to increase quickly. Their presence in a crop is often easy to miss until large populations build or feeding damage appears.
 
Scouting is critical for successful control of this pest. A weekly or bi-weekly scouting program is needed for early detection and implementation of IPM options. Spiders are not insects because they have eight legs. Most insecticdes will only maintain infestations and not control them.

Identification: TSM can be recognized by the presence of two large dark-green spots on the abdomen of mature females. Their body color varies from light green, dark green, brown to orange, depending on the host plant and environmental factors. Their eggs are round and colorless when first laid, but become white before hatching.

Damage symptoms: Spider mites primarily feed on the bottom surface of the leaves by removing water and cell contents using their piercing-sucking mouthparts. On ivy geranium and impatiens, they are often found feeding on young, fully expanded leaves that are rich in tissue nutrients. On many crops, TSM damage symptoms include white or brown stippling in pockets around the infested areas, followed by puckering of the leaf. These symptoms are similar to those caused by western flower thrips. Therefore, look for specks of black feces produced by thrips, for correct diagnosis.

Scouting: Basic scouting tools include a 10-15 X hand lens or head-magnifier(optivisor), production area maps, flagging tape to denote hot spots, data sheets and pencil or a Palm Pilot to record data, and a white sheet of paper that can be placed underneath when striking a leaf or plant to dislodge mites. Without the ability to fly, mites tend to have a spotty distribution within the production area. Mapping the growing area into blocks and recording plant species in each block helps communication between scout and manager.
 
When scouting, walk at random through the blocks in a zigzag pattern, stop after every predetermined number of feet and examine the undersides of 2-3 randomly selected leaves. Be on the lookout for leaves with TSM damage symptoms, and make sure they are examined thoroughly. Identify and flag infested ‘hot spots,’ where control measures should be implemented quickly or further intensive monitoring is needed.

If spider mites are the primary concern, grouping crops based on their susceptibility can make scouting and control more efficient. Bedding plants such as impatiens, sweet potato vine, and marigolds, herbaceous perennials such as aquilegia, hedera, hibiscus, Russian sage and salvia are more susceptible and thus should be checked first and, if possible, more frequently.
 
On some other plants, damage symptoms are delayed because of thick or hairy leaves (ivy geranium, succulent plants, buddleia and perennial salvia S. transylvanica) or purple/red foliage color (some varieties of eupatorium, guara, penstemon and heuchera). These plants can tolerate a low level of spider mite infestation without showing damage that leads to economic loss. Therefore, they have higher economic thresholds that give the manager a little bit more time to take control action, for example using natural enemies.

For tolerant plants, trying to kill every spider mite is unnecessary if mite levels are not high enough to result in economic loss. Even on susceptible plants, a few spider mites (below action threshold) do not necessarily spell disaster because, unlike thrips, spider mites do not spread viruses.

Biological control and action threshold: Many growers have successfully controlled twospotted spider mite by using one of its natural enemies, the predatory mite Pytoseiulus persimilis. Each P. persimilis can consume five spider mites or 20 mite eggs per day; they reproduce fast when prey is available. It is unnecessary and uneconomical to release predators preventively because these control agents rely on the presence of prey to be effective. A sampling plan and action thresholds developed by researchers at Kansas State University on ‘Amethyst 96’ ivy geraniums makes using this predator easier for growers.

Twospotted spider mite sampling is done by looking at the undersides of 60 randomly chosen young, fully expanded leaves from different plants in an area of about 100 sqyare feet around the “hot spot.” This translates to a circle with a radius of about 6 feet around the infested plant(s). If 0 to five TSM are observed on a leaf, the leaf is counted as no mites. If five or more TSM are observed on a leaf, the leaf is counted as infested.

Calculate the proportion of leaves that are infested. For example, if nine of the 60 leaves that you sample have more than five TSM, then 15 percent of the leaves are infested. Use Figure 1 to estimate how many TSM are present per leaf so you can compare the result to the action threshold.

The action threshold for control of TSM using P. persimilis is two TSM per leaf for ivy geraniums that are within the first five weeks of a 10-week production cycle. This means that when two spider mites are found per leaf, predatory mites need to be released at a predator:prey ratio of 1:4 to suppress mite population. Action threshold increases to three TSM per leaf during the second five weeks of the production cycle because TSM levels will not increase quickly enough within these remaining five weeks of the production cycle to lead to economic damage.

For plants susceptible to TSM damage such as impatiens, a lower action threshold is needed for releasing biological control agents. Some of our studies have showed that predatory mites work well when they are released at a ratio of 1:4 (predatory mite:TSM) on plants artificially infested with TSM at both low and high levels of infestation.

It is logical to use chemical control when the mean number of TSM per leaf is found to be much higher than the action threshold, for example four per leaf on ivy geraniums during the first five weeks of production. A targeted miticide such as Floramite (befenazate) or Akari (fenpyroximate) or the use or addition of Ultra Fine Oils is recommended. After taking control measures, either the release of natural enemies or application of miticides, weekly monitoring of TSM should be continued to provide information on the effectiveness of the control measures taken.

Last Updated: 6/21/2013 2:16:38 PM
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