Monitoring Aphids in Mississippi Wheat

Several aphid species can be found in wheat throughout Mississippi each year.  The most common are bird cherry-oat aphids, green bugs, and corn leaf aphids.  Of the three, bird cherry-oat aphids have been the most common in recent years and there have been sporadic reports of very low numbers being found in the Delta region of the state but most consultants have reported that none have been observed to date.  Bird cherry-oat aphids cause very little direct damage to wheat in most instances but are vectors of Barley yellow dwarf virus (BYD) which can be yield limiting if transmitted in the fall of the year.  Mississippi does not have a threshold established for treating bird cherry-oat aphids since yield limiting infections of BYD are uncommon in MS.  BYD can also be transmitted in the spring but rarely does this impact yield.  Use of insecticide seed treatments or foliar applications of insecticides in the fall have resulted in a yield advantage in several studies, however, foliar applications past early to mid February have generally not shown any economic benefit in MS. In northern and eastern states where BYD occurs more commonly than in MS, economic returns from treating to prevent BYD have been more common.

Bird Cherry-Oat Aphid

Of the three aphids we encounter in MS, greenbugs are the most serious threat.  Greenbugs transmit a toxin in their saliva when they feed that results in localized chlorotic or reddish brown lesions on the leaves.  If enough greenbugs are present, damage can result in plant death. Greenbugs can be distinguished from other aphid species by their “lime green” appearance.  Also, on the larger greenbugs a thin black line can be seen along the center of their back.

Greenbug

Greenbug Symptomology

Greenbug Threshold:  When plants are 4 to 6 inches tall, treat when there are 50 aphids per linear foot. When plants are 6 to 10 inches, treat when there are 200 aphids per linear foot. When plants are 18 to 20 inches, treat when there are 300 aphids per linear foot. When plants are 30 inches or taller, treat when there are 800 aphids per linear foot.

 As temperatures warm up in coming weeks, I would expect to see increased aphid activity in the wheat.  From this point forward our concerns and scouting should be focused around greenbugs.  Keep in mind that there are a number of beneficial insects that do a great job of keeping aphid populations in check, particularly Lysiphlebus testaceipes, a small parasitic wasp.  The female wasp lays an egg inside the aphid where the developing young feed inside the live aphid causing what is called a “mummy” eventually killing the aphid.  I have seen wheat field completely covered up in bird cherry-oat aphids crash in a matter of days when this parasitic wasp is present in high numbers.

Parasitized Aphids by Lysiphlebus testaceipes