Aphid Alert 2000, No. 9, August 6

Update: Summary of virus vector situation through 6 August

Captures of winged aphids in the Aphid Alert network traps were roughly double that of last week (74.3 per trap compared to 38.2). The most abundant species was corn leaf aphid (71.4% of all captures). Corn leaf aphid is capable of vectoring PVY, but it does so very inefficiently (see Table 1 below for comparison of vector efficiency of various aphid species). Thistle aphid aphid was another common species (4.0% of all captures) but like corn leaf aphid is a very inefficient vector of PVY. Turnip aphid (12.5% of all captures) was common at many locations. Turnip aphid is relatively efficient as a vector of PVY and thus its presence is cause for concern. Turnip aphid like green peach aphid reproduces on canola and wild mustard and in many locations is present in high numbers on these hosts. As canola is swathed we anticipate increased numbers of turnip aphid and green peach aphid will be captured.

Captures of green peach aphid were comparable to that of last week (about 0.6 per trap). However, there are still large populations of green peach aphid present on canola and wild mustard. Earlier this week, Robert Suranyi sampled green peach aphids developing on the remaining foliage of canola stubble. He found that 21% of large nymphs (4^th instar) were developing wing pads (i.e., will mature into winged adults), whereas 60% of 3^rd instars and 76% of 2^nd instars had wing pads.

Several growers have reported dissatisfaction with the aphid control they obtained using Fulfill (pymetrozine). In our research trials over the past several years, Fulfill has always provided excellent control (> 98%) of green peach aphid. Fulfill kills aphids very slowly (5-7 days may be required before the insect dies), but feeding activity ceases almost immediately.

In our insecticide efficacy trials, we use a ground applicator at 40-50 gallons per acre at 65-80 psi. This eliminates spray coverage and canopy penetration as variables permitting us to compare different active ingredients head to head. Of course, most growers are not applying Fulfill with ground applicators but are using aerial application with spray volumes of 5-7 gallons per acre. Observations we made in several commercial fields this past week suggested that where ground applicators were used Fulfill provided good control while in those fields where Fulfill was applied by air (helicopter or fixed wing) control was less than that seen with Monitor, although generally aphid densities were below threshold. It is worthy mentioning that in Fulfill treated fields there was an abundance of predators and parasites while in those treated with Monitor very few living insects of any kind were found.

An issue that needs further research is whether the potato aphid is as susceptible to Fulfill as is the green peach aphid. In previous years, we have not had sufficient potato aphids in our plots to permit critical evaluation of efficacy against this aphid. In pretreatment counts made August 9, 2000; we had an average of 0.5 potato aphids per leaf along with 100+ green peach aphids per leaf. These plots will be evaluated next Monday, so may be able to report on the efficacy of Fulfill against both aphid species in next week’s Aphid Alert.

Published data available on efficacy of Fulfill on potato aphids suggests inconsistent control of potato aphid with Fulfill. However, it is hard to draw conclusions from many of these studies because the frequency of application rates, surfactants, and evaluation dates vary greatly. We will continue to investigate this issue.

Our current recommendation is to use Fulfill early while aphid densities are still low (> 1 per leaf). Always use a non-ionic surfactant with Fulfill and use the highest spray volume as possible (up to 80 gal. per acre). Consider using ground application if possible. Most importantly, do not expect a rapid kill from this product. Make a judgement on efficacy at 7-10 days after treatment, never earlier. For heavy aphid infestations (>1 aphid per leaf) Monitor will perform better.

We have had reports of melon aphid in Wisconsin and Michigan. In Michigan, melon aphid has been found on soybean (a crop not previously considered to be a host). Melon aphid is a very efficient vector (comparable to green peach aphid) of PVY, Moreover, melon aphid tends to be resistant to Monitor. Provado (imidacloprid) is effective against melon aphid. To recognize melon aphid look for a small dark aphid with black cornicles.

Weekly plot of captures of green peach aphid per trap, 1992-1994 & 1998-2000.

Weekly plot of captures of PVY vectors other than green peach aphid per trap, 1992-1994 & 1998-2000.

Table 1: Efficiency of various aphid species as vectors of PVY (mosaic) and PLRV (potato leafroll), summary prepared by D. Ragsdale from review of published literature)