Mite outbreaks usually are associated with hot, dry weather, which accelerates their reproduction and development. During periods of high humidity and field moisture, a fungal disease can reduce populations, but high temperatures can nullify these effects. Outbreaks also are associated with the application of certain insecticides that kill natural enemies and seem to make the soybean plant more nutritionally suitable for mites.
Check weekly for mites starting in early July through August, especially during a hot, dry season. Concentrate on the field borders and look for the early signs of white stippling at the base of the leaves. Do not confuse mite damage with dry weather injury, mineral deficiencies, or herbicide injury. If feeding injury is evident, press the underside of a few damaged leaves on white paper to reveal any crushed mites. Determine the extent of the infestation and assess the level of injury by examining 20 to 30 plants in the infested area. Field infestations often show defoliated or injured plants at some localized point, with injury becoming less evident and extending in a widening arc into the field.
If isolated spots of mite activity are confined to the perimeter of the field, spot-treatment using ground equipment is recommended to prevent further spread of mites into the field. If the infestation is distributed throughout the interior of the field, treatment of the entire field is suggested if live mites are numerous (20 to 30 per leaflet) and more than 10 percent of the plants show stippling, yellowing, or defoliation over more than one-third of the leaves. If rains come, mite development and survival will decrease but may not drop to economic levels if heavy populations are developing under high temperatures. Early detection is critical for effective control. Once yellowing occurs, plants will not recover and significant yields may be lost.
Control failures with dimethoate have occurred in recent years for several possible reasons:
1. diminished systemic activity in drought-stressed fields due to poor absorption and translocation of the chemical into leaf tissues;
2. inadequate spray coverage by aerial application, especially if “kill by contact” is the primary means of control;
3. accelerated degradation due to alkaline hydrolysis and high iron in the water used as the carrier. (Note that original strength of dimethoate is reduced by 50 percent in 48 minutes at a pH of 9, 12 hours at a pH of 6, and 21 hours at a pH of 2, and degrades more rapidly in the presence of iron); and
4. reduced strength of the product due to storage at high temperatures.
Many control failures can be avoided if management actions are taken before mite populations become well established, thus early spot treatment of localized infestations is highly recommended. Optimum spray coverage is achieved with at least 20 gallons of diluted spray at 40 psi delivered by ground with the boom adjusted as close to the canopy as possible to achieve forceful but uniform penetration. During drought conditions, the addition of a crop oil or organosilicone can improve coverage and penetration, and reduce evaporation of spray droplets. The organosilicones are more expensive but are much more effective.
To reduce alkaline hydrolysis, use a high quality pH meter to obtain accurate readings of alkalinity. Note that a pH reading taken in the spring may be different than a summer reading when the water is used for spray operations. If a high pH situation exists, the alkalinity of the water in the spray tank can be lowered by adding an acid-based buffer. The buffer should be added to the spray tank first, before the addition of dimethoate. Also do not tank mix dimethoate and then store for an extended period of time before spraying. To safeguard against storage problems, do not store dimethoate in areas where temperatures may exceed 95°F for extended periods of time.
You are viewing records 1 - 4 of 4.| Insecticide and formulation | Rate of active ingredient per acre | Rate of formulation per acre | Time lmits: Days before harvest | Remarks |
|---|---|---|---|---|
| chlorpyrifos (Lorsban 4E) |
0.5-1.0 lb | 1.0-2.0 pt | 28 | Restricted Use: |
| dimethoate (Dimethoate 4EC) |
0.5 lb | 1.0 pt | 28 | Restricted Use: |
| bifenthrin (Brigade 2EC) |
0.08-0.10 lb | 5.12-6.4 oz | 18 | Restricted Use: |
| zeta-cypermethrin plus bifentrhin (Hero) |
0.10 lb | 10.3 oz | 21 | Restricted Use: |
