By Robert Wright and Julie Peterson et.al
Spider mite damage to corn and soybeans is being reported in Nebraska. The following story addresses management. The companion story, Identifying Spider Mite Damage and the Species Responsible, includes a guide for distinguishing Banks grass mites, found in grass crops, and twospotted spider mites, found in soybean and grass crops.
Impact of Beneficial Insects, Disease and Insecticide Applications
Several species of insects and mites prey on spider mites. These predatory insects and mites play a major role in suppressing spider mites in most years. Many spider mite problems in corn and soybeans may be traced back to an earlier application of a broad spectrum insecticide that reduced populations of these natural controls. The most important of these include a predatory spider mite, the mite destroyer beetle, six-spotted thrips and the minute pirate bug. In addition to these predators, a fungal disease also may be important in reducing spider mite populations.
Insecticide Choice is Critical
Most pyrethroid and organophosphate insecticides used in corn and soybeans have severe, detrimental effects on spider mite predators. Additionally, pesticides differ a great deal in their effects on BGM and TSM. Some cause little mortality of either species, while others are somewhat toxic to BGM. Fewer are toxic to TSM. Thus, great care should be taken to evaluate the benefits of an insecticide application before any material is applied for insect control in a field that also has spider mites. Even small numbers of mites can rapidly increase to damaging levels when conditions are favorable. In many cases, it is an earlier treatment for western bean cutworms or corn rootworm beetles that leads to a later spider mite problem in corn.
If a decision is made to treat an insect pest in a field that also has spider mites, the choice of products becomes very important. Because TSM and BGM differ in their susceptibility to various pesticides, it is important to determine which species is present. Spider mites (particularly TSM) are noted for their ability to develop resistance to chemicals that were once toxic to them. For this reason, it is very likely that some products now toxic to spider mites will become less toxic in the future.
Figure 2. Moderate and severe damage to soybean leaves from twospotted spider mites.
Treatment Thresholds
According to Colorado State University, in corn “treat if damage is visible in the lower third of the plant, and mite colonies are present in the middle third. Once the crop has reached the hard-dough stage, no economic benefit will result from the treatment.”
No research has been conducted that would allow calculation of an economic injury level for twospotted spider mites on soybeans. On soybeans, the following scale is suggested to assess damage by spider mites:
Table 1. Rating Scale to Assess Damage by Spider Mites
| Rating | Damage Observed |
|---|
| 0 | No spider mites or injury observed. |
|---|
| 1 | Minor stippling on lower leaves, no premature yellowing observed. |
|---|
| 2 | Stippling common on lower leaves, small areas or scattered plants with yellowing. |
|---|
3
Spray Threshold | Heavy stippling on lower leaves with some stippling progressing into middle canopy. Mites present in middle canopy with scattered colonies in upper canopy. Lower leaf yellowing common and some lower leaf loss. |
|---|
4
Economic Loss | Lower leaf yellowing readily apparent. Leaf drop common. Stippling, webbing and mites common in middle canopy. Mites and minor stippling present in upper canopy. |
|---|
| 5 | Lower leaf loss common, yellowing or browning moving up plant into middle canopy, stippling and distortion of upper leaves common. Mites present in high levels in middle and lower canopy. |
|---|
Table Source: Managing spider mites on soybeans, Ostlie and Potter, 2018, University of Minnesota.
Damage from mites may be confused with that caused by drought and several foliar diseases, so be sure to base treatment decisions on the presence of mites, rather than just apparent injury symptoms. Fields may be spot treated if the infestation is localized, but check other areas for mites (especially downwind of infestation) and extend treatments into these areas if large numbers of mites are found. Although late-season infestations may accelerate soybean senescence and increase pod shattering, caution should be used in deciding to treat with pesticides because many of the pesticides used for mite control have 21-28 day preharvest intervals.
Control
Spider mite control can be tricky.
For effective control, spider mites must come into contact with the miticide. Since mites are found primarily on the underside of the leaves, they are difficult to reach with low volume applications. Using three or more gallons of water per acre by air to carry miticides may increase effectiveness. Aerial applications are generally more effective if applied very early in the morning or in the late evening. Applications made at these times avoid the upward movement of sprays, away from the plants, on hot rising air.
Eggs are difficult to kill with pyrethroid or organophosphate miticides, so reinfestation is likely to occur seven to 10 days after treatment as a result of egg hatching. The reinfestation is frequently heavy because natural enemies have been reduced or eliminated. A second application may be necessary to kill newly hatched mites before they mature and deposit more eggs.
Miticides with activity against eggs and immature stages include Zeal, Oberon and Onager.
In many cases, especially with the twospotted spider mite, slowing the rate of population increase is all that can be accomplished with a miticide application.
Table 2. Product options for spider mite treatment.
| Mode of Action | Active Ingredients | Product Names
(examples) | Crops Labeled | Notes | Stages Targeted |
|---|
| | | | | | Eggs | Immatures | Adults |
|---|
| 1B: Organophosphates | Dimethoate | Dimethoate, Dimate | Corn + Soybeans | Difficult to kill mite eggs and removes natural enemies; infestations can recur in 7-10 days | X | ? | ? |
| 3A: Pyrethroids | Bifenthrin | Bifenture, Brigade, Sniper |
| Zeta-cypermethrin + bifenthrin | Hero |
| 6: Chloride channel activators | Abamectin | Agri-Mek | Soybeans | Targets active stages of mites | X | ? | ? |
| 12C: Inhibitors of ATP synthesis | Propargite | Comite | Corn |
| 10B: Mite growth inhibitors | Etoxazole | Zeal | Corn + Soybeans | Active against eggs and immatures | ? | ? | X |
| 23: Tetronic/tetramic acid derivatives | Spiromesifen | Oberon | Corn | ? |
| Hexythiazox | Onager | Corn | X |
Source : unl.edu