Scouting for and treating aphid infestations in wheat

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

This week there are reports of sprays going on for aphid infestations in wheat. Commercial companies are promoting intensive wheat management in Michigan, including managing fertility, weeds, diseases and insects. It is rare to have an aphid population in wheat in Michigan that merits spraying.

It is typical to first see aphids in late April or early May in wheat. The oat bird-cherry aphid is the first aphid we find in April or early May. It is olive green, and has an orange (rusty) patch on its behind. You can actually see this orange patch with your naked eye. In late May and early June, English grain aphid appears. This is a large green aphid with long black cornicles (“tail-pipes”) and long legs with “black knees.” You may also see corn leaf aphids (blue-green, short black cornicles, short black legs) and the greenbug (light green with a dark stripe on its back). Oat bird-cherry aphid may overwinter in Michigan in the egg stage. That may be why we find this species early in the spring. The other species probably move north as the weather warms, similar to the movement of potato leafhopper into Michigan. In fact, we often begin to see potato leafhopper and the other grain aphids at about the same time in the spring. The Russian wheat aphid (a devastating aphid with a salivary toxin) is found in the western United States, but is not present in Michigan.

Aphids impact wheat in several ways

Direct feeding damage. Aphids feed by sucking plant juices. A few aphids per plant do not cause a problem, but if populations are very high, enough water and nutrients can be removed to effect yield.

Salivary toxin
. A few aphids inject a toxin into plants as they feed. I already mentioned Russian wheat aphid, which is not found in Michigan. However, the greenbug also injects a toxin into wheat as it sucks plant juices. This causes a yellowing or tan discoloration of the leaf tissue. Greenbugs are not our most common aphid species in wheat, and we do not experience the feeding damage seen in western and southern wheat-producing areas.

Virus transmission
. All of the aphids mentioned above can transmit barley yellow dwarf virus (BYDV) to wheat, barley and oats. For fall-planted winter wheat, the source of the virus is most likely grasses outside the field. Transmission in the fall is more important than transmission in late spring, since the virus infects the plant for a longer period of time.

In my ten years in Michigan, I have seen only one field close to the treatment threshold. This was a research plot south of the MSU campus, not a production field, so there were unusual conditions associated with the high aphid infestation. As far as virus levels, BYDV is not a common problem in Michigan wheat. Fields in the state are planted later in the fall after the fly-free date when the weather is cooler and aphid populations tend to be low. Unlike states to the south, aphids do not overwinter in Michigan on wheat. They die as the weather turns cold. Fall spraying of wheat is not justified in Michigan, as aphid populations are low and then die in the fall as temperatures drop. Spring treatments should be made based on scouting and a threshold.

Scouting for aphids in wheat

Unlike scouting and thresholds for some insects, scouting methods and thresholds for aphids on wheat are well-researched and reliable. Thresholds are based on examining tillers for aphids. Examine the upper and lower leaf surface, as well as rolled-up portions of the leaves and stem. Don’t just walk the edge of the field (aphid infestations often start at the edge.) Walk 30 to 50 paces into the field, sample a tiller, then walk another 30 to 50 paces, and so forth. Finding the aphids on each tiller is more important than distinguishing species. Depending on your sample scheme (see Table 1), you can count aphids or simply note the presence or absence, on 100 stems per field.

The threshold is 12 to 15 aphids per tiller up to the boot stage. The aphid species does not matter, only the number of total aphids per stem. If you don’t want to count aphids, you can use a presence or absence sampling method, determining the number of infested tillers. Species does not matter. Total number of aphids per tiller does not matter. You only want to know if the tiller is infested with at least one aphid (presence) or not (absence). Once you know the proportion of tillers with aphids, you can use a decision table to determine the need for treatment.

Pick 25 tillers in the field and examine them for aphids. According to the table, if 25 (100 percent) of the tillers are infested, you stop sampling because you are over the threshold and should treat.  If zero to 18 of the tillers have aphids, you also should stop sampling because you are below threshold. You should not treat. However, if 19 to 24 of the tillers have aphids, you can’t make a decision yet. You must keep sampling. Pick another 5 tillers, examine them for aphids, and add the result to the total. Now your decision is based on the second line of the table (30 tillers examined). Continue until you make a decision or you sample 100 tillers. Usually, you will make a decision very quickly within the first 25 or 30 tillers examined.

Table 1. Decision table for presence/ absence sampling of cereal aphids

 

                   Number of infested tillers

Total number of tillers examined

Stop sampling…
Do Not Treat

Keep sampling
(pick 5 more tillers)

Stop sampling…
Treat

25

0-18

19-24

25

30

0-22

23-29

30

35

0-27

28-34

35

40

0-31

32-39

40

45

0-35

36-43

44- 45

50

0-40

41-48

49-50

55

0-44

45-53

54-55

60

0-48

49-58

59-60

65

0-53

54-62

63-65

70

0-57

58-67

68-70

75

0-61

62-72

73-75

80

0-66

67-77

78-80

85

0-70

71-81

82-85

90

0-75

76-86

87-90

95

0-79

80-91

92-95

100

 0-84

If 84-100 tillers infested,
then treat

Here are two examples

  1. You examine 20 tillers and find that four tillers have aphids. Your decision? Stop sampling! Don’t treat.
  2. You examine 25 tillers and find that 20 tillers have aphids. Your decision? Keep sampling! Pick five more tillers.

The five new tillers all have aphids. Your decision? Keep sampling, pick five more tillers. Twenty infested tillers plus five infested tillers equals 25 infested tillers out of 30 total. According to the table, you keep going (between 23 to 29 out of 30).

The five new tillers all have aphids. Your decision? Keep sampling. 25 + 5 = 30 infested tiller out of 30 total tillers. You are still within the “keep sampling” column.

The five new tillers are not infested. Your decision? STOP SAMPLING. 30 + 0 = 30 infested out of 40 total tillers. You do not have to treat.

Dr. DiFonzo’s work is funded in part by MSU‘s AgBioResearch.

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