Aphid Alert 1998, No. 2, August 10
Features of this issue
- Aphid flight activity 20-29 July
- Control of green peach aphid
- Scouting for aphids
- Identification of potato colonizing aphids
In the previous issue of Aphid Alert we focused on how aphids transmit potato virus Y (PVY) and potato leaf roll virus (PLRV). Since PLRV can be acquired and transmitted only by those aphid species that colonize potato, the goal of this issue is to provide information on how to identify potato colonizing aphid species, when to apply insecticides, and which insecticides to use.
In the Red River Valley the most important potato colonizing aphids (those aphid species that reproduce on potato) are green peach aphid (Myzus persicae), buckthorn aphid (Aphis nasturtii), and potato aphid (Macrosiphum euphorbiae). Rarely, foxglove aphid (Aulacorthum solani) can also be found to colonize potato in the Red River Valley.
Among these aphid species, green peach aphid is generally the most abundant and is by far the most efficient PLRV vector.
Control of PLRV spread
Control of within-field spread of PLRV lies in reducing inoculum within the field (roguing) followed by effective control of green peach aphid. As discussed in the previous issue of Aphid Alert, insecticides can reduce within-field spread of PLRV – spread that is attributed to wingless aphids walking from plant to plant. However, only a few insecticides reliably control green peach aphid.
Worldwide, green peach aphid has developed resistance to at least 69 different insecticides representing all major classes of insecticides (Georghiou 1990). Insecticide induced aphid outbreaks are most likely to occur after multiple applications of the same class of insecticide, e.g., pyrethroids, organophosphates, or carbamates, used to control other insect pests such as the Colorado potato beetle and potato leafhopper. Examples of insecticides used on potato that can induce aphid outbreaks are Asana®, Sevin®, Guthion®, and Penncap M®. An insecticide-induced aphid outbreak is caused by selectively killing the predators and parasites that naturally keep green peach aphid populations under control.
Currently, the only insecticides registered on potato that provide effective control of green peach aphid are Admire® applied at planting and Monitor® and Provado® as foliar sprays.
When should control applications be made?
In addition to choosing an appropriate insecticide for aphid control, timing the application is just as critical. Research at the University of Minnesota established that once green peach aphid populations exceeded 3 to 10 wingless green peach aphids per 100 lower leaves, in-field spread of PLRV by wingless aphids increased. In a few experiments, spray applications on a threshold of 3 aphids per 100 leaves tended to give better control than when applied at higher thresholds. For seed growers, caution suggests treatment whenever aphids exceed 3 apterae per 100 lower leaves.
How to scout for green peach aphid?
To determine aphid density in a potato field, select 25 lower leaves (be sure to sample full compound leaves - not single leaflets) in each of four areas in the field and count the number of aphids found on the underside of the leaf. Sampling lower leaves is important because green peach aphids prefer to feed on older senescing leaves because they provide the highest food quality in terms of the balance of nitrogen and sugars. Green peach aphids will not be found in the upper canopy during the early stages of population growth. However, avoid leaves that are completely yellowed or in contact with the soil.
Historically, green peach aphids can be found in potato fields by mid-July in the Red River Valley. In the last issue of the Aphid Alert we reported finding winged ("alatae") green peach aphids in some traps and wingless ("apterae") in some fields. Seed growers should now begin scouting fields weekly for green peach aphids. Remember, all aphids are female, they give birth to live young (rather than laying eggs) and under favorable environmental conditions, a newly born aphid becomes reproducing adult within 7 days, and can produce up to 5 offspring per day for up to 30 days. This remarkable reproductive potential can lead to aphid populations that double every two days. Once the threshold of 3 apterae per 100 lower leaves is reached, insecticides should be applied to prevent further spread of PLRV by wingless aphids.
Unfortunately, no insecticide can stop the spread of viruses by winged aphids. The most critical period for virus spread by winged green peach aphid is in late August. Seed fields that are still green and growing into September are at increased risk of late season spread of viruses. Late season infection shows no foliar symptoms and research at the University of Minnesota demonstrated that tubers become infected in as few as 7 days following inoculation. Those seed lots to be recertified should be harvested first and as early as possible. One purpose of the Aphid Alert is to warn growers when winged green peach aphids are present.
How to identify wingless forms of potato colonizing aphid species?
Wingless forms of the four most important species, green peach aphid, potato aphid, buckthorn aphid and foxglove aphid, can be easily differentiated in the field using a 10X hand lens. Overall body shape and color, length of the legs and cornicles (tube-like structures on the abdomen) and shape of the head and cauda (tail) (Figure 1, Figure 2, from Radcliffe et al., 1993, Management of aphids and leafhoppers, pp 117-126. In Potato Health Managment, Rowe [ed.], American Phytopathology Society) are used in the following key, based on MacGillivray (1979).
To identify wingless aphids we have provided a simple key. The key uses pairs of statements called couplets. Compare your specimen to the description provided in couplet 1a followed by 1b. Continue this process through couplets 2 and 3 or until the description leads to a species name.
Key for distinguishing the wingless forms of the primary aphid pests on potato:
1a. body outline egg or teardrop shaped, cauda short - 2
1b. body outline elongate, antennal tubercles large, pointing outward, cauda long and pointed, cornicles longer than the distance between their bases (Fig. 2a), legs prominent, color green, yellow, or pink, may have a darker dorsal stripe, highly mobile aphids - potato aphid
2a. body thick, head with prominent antennal tubercles, antennae as long or longer than body . .3
2b. body flattened, head without prominent antennal tubercles, antennae shorter than length of body, cornicles almost as short as cauda (Fig. 2b), color opaque lemon yellow to green in color, black in autumn - buckthorn aphid
3a. body pear shaped, widest at base of cornicles, antennal tubercles prominent and almost parallel sided, cornicles tapered with prominent flanges on the dark tip (Fig. 2c), color light yellow green to dark green, with dark areas around base of cornicles, legs and antennae with dark joints - foxglove aphid
3b. body egg shaped, almost the same width from base of middle legs to base of cornicles, antennal tubercles prominent and pointing inward, cornicles unevenly swollen (Fig. 2d), color light green to almost translucent, pink, or peach, legs and cornicles the same color as the body - green peach aphid
If using the key to identify the green peach aphid, you would make the following choices in the key:
1a. body outline egg or teardrop shaped, cauda short. -- 2
2a. body thick with prominent antennal tubercles ("head bumps"), antennae as long or longer than body. -- 3
3b. body egg shaped, almost the same width from base of middle legs to base of cornicles, antennal tubercles prominent and pointing inward, cornicles unevenly swollen, color light green to almost translucent, pink or peach, legs and cornicles the same color as the body. -- green peach aphid
Trapping sites this past week show an increase in winged aphid capture (Figure 3, Table 1). This increase is associated with ripening of small grains and other crops throughout the Red River Valley. A few winged green peach aphids were found in Hoople, Cando and Rolette. This may indicate the beginning of summer migration of green peach aphids. Apterous green peach aphids have been reported in fields near Grafton (Duane Preston and Willem Schrage, personal communication). Numbers of green peach aphids in the traps are very low, but this insect is seldom abundant compared to grain aphids. Because of the importance of green peach aphid as a vector of PLRV and PVY, detection at any density is cause for increased vigilance. Begin scouting fields for green peach aphids now. Last minute note: on 5 August, the numbers of wingless green peach aphids found at the Hoople site have exceeded the threshold!
Robert Suranyi, Dr. David Ragsdale, and Dr. Ted Radcliffe
Department of Entomology University of Minnesota, St. Paul
Table 1. Aphid species (% of total capture, 07/20-07/29)
|green peach aphid||-||-||-||-||-||0.5||1||0.7|
|corn leaf aphid||0.9||-||-||1.1||5||0.5||17.6||8.1|
|English grain aphid||1.8||-||1||-||10||2.7||2||-|
|Identified non-vector species||7.2||0.5||2||0.5||25||3.7||5.9||3|
|Total # captured||187||643||211||365||20||210||no data||102||13|
*: in traps with >50 aphids / trap a sub-sample of 50 aphids