PDA Links

To: Tree Fruit Growers
From: Doug Pfeiffer Fruit Entomologist
Date sent: June 17, 2000

Woolly Apple Aphid: Many have seen woolly apple aphid (WAA) in greater numbers than usual in recent weeks. It is common to see WAA colonies in the canopy in June, but if numbers are very high, and especially if arboreal colonies are seen at other times of the year, it is a sign that colonies could be moving to roots, where they are more damaging. If WAA needs control, there are several options. Lorsban has been effective, but is not attractive to many growers because of recent publicity. Thiodan (1 lb 50WP or 2/3 qt 3EC). Malathion is also moderately effective against WAA.

A critical issue is spray timing. Mid-late June is the time of greatest activity of first instar nymphs (the youngest stage, and the primary dispersal stage). If populations are very high in the canopy, more crawlers may be available for root colonization. At low densities, most arboreal colonies are restricted to the trunk and larger branches. At high densities, colonies may move out onto limbs. So presence of arboreal colonies at unusual times, or distributed over exterior parts of the canopy, are signs of high populations. If growers have noted such conditions, they may now consider spraying WAA, since this would be the most effective timing, as crawler move to other parts of the tree.

We have been using a provisional threshold of 50% of pruning cuts infested with WAA. Unfortunately there is not a good correlation between the number of aerial colonies and root infestation. One West Virginia study found a $118/acre loss from root-feeding WAA; tat the time there was only an 11-12% infestation rate of aerial WAA in the orchard. It is also difficult to control root infestations through aerial sprays.

It is worth considering other aspects of WAA management. Host plant resistance has been established for years for WAA, using Malling Merton rootstocks, which have inherited resistance from Northern Spy parentage. There are biotypes of WAA in some parts of the world that can partially overcome this resistance (South Africa and North Carolina); nevertheless, these rootstocks are still considered resistant. There are also lines being investigated with stronger resistance than that provided by North Spy lines. Biological control can also be very effective against WAA. The introduction of the parasite Aphelinus mali was a very successful case. There are also predators of WAA that can be important, such as syrphid flies. The role of such natural enemies needs to looked at further as our chemical control strategies change. Outbreaks of aerial WAA can be induced by the use of pyrethroid insecticides.

Lorsban update: It now appears that the agreement posted earlier between Dow AgroSciences and EPA does eliminate post bloom uses of Lorsban on apple (http://www.epa.gov/pesticides/op/chlorpyrifos/agreement.pdf). Any changes in this policy will be forwarded in a late message. The change takes effect after December 2000.

Relative control of ERM and TSM: The question has come up several times this spring on the relative degree of control given by acaricides for twospotted spider mite (TSM) versus European red mite (ERM). In the pest efficacy table given in the tree fruit spray guide ( http://www.ext.vt.edu/pubs/treefruit/456-419/efficacy.pdf), it can be seen that the degree of control is sometimes less for TSM than for ERM; TSM are harder to kill. In a year like this, when TSM may predominate over ERM, it will be important to (1) use the upper end of the recommended rate, and (2) be more careful in using an action threshold. It is often more difficult to reduce a population that has already reached high levels. This is especially critical given that Tetranychus species (like TSM) are more damaging than Panonychus species (like ERM).

References Take care, Doug

Douglas G. Pfeiffer
Department of Entomology
205C Price Hall
Virginia Tech
Blacksburg VA 24061
ph: (540) 231-4183
fax (540) 231-9131