• Spider mites in Indiana field corn are uncommon.
• Use caution while interpreting spider mite information from western states.
• Mite species/environments vary considerably across the Corn Belt.
• While scouting, must go beyond the end rows to determine infestation.

Spider mite damage in field corn is a rarity in the Eastern Corn Belt, and very little is understood about their potential effect on yield in the Midwest even after some experience in the drought of 2012. As we visit with, or read our colleagues’ information out west, we feel that we’re comparing apples to oranges. Mite species and humidity levels are two of the glaring differences.

Two-spotted spider mites (Tetranychus urticae) are our enemy in field crops, including soybeans, as well as many homeowner and greenhouse plants, while out in the Western Corn Belt, their primary species is the Bank’s grass mite (Oligonychus pratensis) with some two-spotted mites mixed in. There are several subtle differences in identification, biology, and damage between the two species, but the most important one to readers is that two-spotted spider mites are more difficult to kill. In addition, two-spotted mites are prone to building pesticide-resistant populations. In short, we’ve got the tougher pest. And because of that, pesticide carrier volumes are recommended to be increased to at least 5 gpa by air and 20 gpa by ground.

Drought is the root of spider mite problems in Indiana. Though not all areas of the state are impacted by the dry conditions, all counties have had higher humidity in recent days. Though high humidity (>50%) doesn’t stop spider mites, it certainly slows down water loss from plants, and therefore lowers plant stress and the mites’ reproductive rates. It also makes mite populations prone to epizootics (“plagues” of fungal disease), which we saw locally during the last week.

Spreading through a cornfield is not as easy for spider mites as in soybeans. Mites can either walk from plant to plant via touching leaves or they “balloon” with spun webs, allowing the wind to transport them. Certainly this must be a slow process, because wind movement in the lower canopy of cornfields is quite limited. Make certain that spider mites have moved beyond the end rows, walking well into the field to determine their presence and colonization of leaves is crucial.

We still remain doubtful that spider mite treatments in yellow-dent corn are warranted. However, if fields with decent yield potential (150+ bu/a) have spider mite colonies established on lower leaves (discolored), and spreading to the ear leaf or above, then treating before the dent stage may be justified. It will not be easy to get treatments down to where the mites are most numerous though. As previously mentioned, high amount of carrier (5 by air, 20+ by ground) is strongly recommended. Consider that spider mites are usually most actively colonizing the underside of lower leaves so canopy penetration is necessary. We have no experience to draw from in treating corn for spider mites, but those products registered for use in Indiana are dimethoate, etoxazole (Zeal), hexythiazox (Onager), propargite (Comite), and spiromesifen (Oberon).

Fall armyworms are only able to survive the winter in extreme southern US, along the Gulf Coast and in Florida. Fall armyworms tend to migrate northward gradually, with each successive generation moving several hundred miles further north. They reach Indiana every year, but their populations are unpredictable in timing, numbers, and location. This week we have received reports of fall armyworm infestations in corn from southwest Indiana and northern Elkhart County, so they are throughout the state. However, infestations tend to be spotty, with individual fields or even portions of a field showing damage, with neighboring fields uninfested.


Fall armyworm caterpillar labeled to show diagnostic features.

Fall armyworms will feed on corn at all stages of development and will feed on all above ground plant parts. However, their typical late arrival in Indiana (July-August) means that we are mostly concerned about feeding on kernels in the ear. Because of the higher value of the crop, sweet corn, seed corn, and popcorn growers should be particularly observant for possible fall armyworm infestations. Late planted sweet corn can be attacked during the whorl stage and may require an insecticide application in the late whorl/early tassel emergence stage in addition to the normal earworm spray program. Seed corn and popcorn are beyond the whorl stage now but may benefit from an insecticide application if damaging populations are observed. Very late-planted (e.g., replated drowned out areas) field corn is rarely economic to treat. The pyrethroid insecticides still provide good control of fall armyworm in the US, although resistance has been observed in Puerto Rico.


Fall armyworm feeding on kernels.


Fall armyworm damage to whorl.

The Purdue Weed Science group is again offering herbicide resistance screening for Palmer amaranth, waterhemp, and giant ragweed for the 2016 growing season.  The resistance screens include glyphosate (group 9) and ALS-inhibitor (group 2) assays for giant ragweed, as well as glyphosate (group 9) and PPO-inhibitor (group 14) resistance screening for waterhemp and Palmer amaranth

Leaf tissue samples can be submitted for molecular DNA analysis that will allow results to be generated within a few weeks of submission.  It is important to follow the directions on the submission form for collecting, storing, and shipping leaf tissue samples as this can have a large impact on the accuracy of the results.

Seed samples can also be submitted for analysis of herbicide resistance.  This allows us to also screen for glyphosate resistance in giant ragweed.  It is also important to follow the directions on the submission form for seed collection from the appropriate number of plants to assure quality results.  The seed samples will take several months to return results as plants will need to be grown from seed in the greenhouse.

The submission form with instructions for collection, storage, and shipping can be found at the following link: https://ag.purdue.edu/btny/weedscience/Documents/2016HerbicideResistancescreeningform.pdf.  The submission form can also be found on the front page of the Purdue Weed science website: https://ag.purdue.edu/btny/weedscience/Pages/default.aspx.

Please contact Julie Young (young294@purdue.edu, 765-494-0891) with any questions or concerns you have when sampling or shipping a sample.