April 20, 2012

Natural Enemies Likely Also Survived the Winter Well

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With this year’s unusually warm March, growing degree days are ahead of normal for much of North America. Because insect activity is largely determined by temperature, this means some insects are becoming active earlier.

Last week Jeff Whitworth (K-State Extension Entomologist) reported cereal aphid activity in winter wheat in Kansas. He noted that while the number of cereal aphids is increasing, so is the number of natural enemies. He reported that cereal aphid numbers in Kansas generally are well below economic thresholds and are being held in check by an abundance of natural enemies. It’s important to consider the potential benefit of natural enemies since improper timing or use of some chemicals can reduce natural enemy numbers disproportionately to the pest and set back biological control activity.

Potential Damage

Aphids damage crops such as wheat through several mechanisms, including virus transmission and direct damage.

Virus Transmission. In recent years in Nebraska aphid damage has occurred more commonly through virus transmission. Winter wheat can be at high risk for viruses (such as barley yellow dwarf) when aphids move to wheat after planting in the fall. This risk can be especially high for early-planted fields. That is, early-planted winter wheat can receive aphids from over-summering hosts that may also harbor wheat viruses. Viruses can be transmitted in the spring; however, it is thought that these older plants have greater tolerance to virus infection.

Direct Damage. Some aphids cause direct damage by their sap sucking activities. For example, Russian wheat aphids are not known to transmit viruses to wheat; however, their numbers can build up rapidly to cause significant economic damage to wheat. Damage from Russian aphids can be particularly apparent as a red to purple coloration on flag leaves around boot stage. Large populations of Russian wheat aphids during boot-stage wheat can seriously hinder head emergence. Again, wheat stages are advancing ahead of normal, so keep a close eye on wheat fields in May!

The Situation in Nebraska

Our mild winter is a cause for concern as some cereal aphids apparently overwintered well and growers have reported finding early populations of Russian wheat aphids. We have already received some unsubstantiated reports of bird cherry-oat aphids, Rhopalosiphum padi, (Figure 1) in south central Nebraska. This aphid is a vector for barley yellow dwarf virus in wheat, but rarely reaches an economic threshold requiring treatment. However, at least one field in southern Nebraska was reported to have high enough numbers that a chemical application was needed to prevent direct damage from sap feeding (not from virus transmission).

In western Nebraska, we started finding Russian wheat aphids, Diuraphis noxia, in our surveys. This aphid is not known to transmit any viruses, but can cause direct damage. To date we have found few aphids and the sampling method we use is very efficient for finding even a small number.

Aphid numbers do not appear to be high enough in western Nebraska to warrant treatment; however, considering the mild winter, conditions may have been adequate for survival or even overwintering of some cereal aphids in Nebraska. Keep a close eye on your wheat fields in May for the development of aphid populations.

More Information

See insect/wheataphids for details on aphid thresholds and a partial list of chemicals labeled for use against cereal aphids in wheat. Additionally, see NebGuide G1284, Cereal Aphids, for aphid identification.

Jeff Bradshaw
Extension Entomology Specialist, Panhandle REC