The end of pear season is finally upon us. One topic of discussion is the potential need for and strategy for postharvest sprays to manage overwintering pear psylla. Here we will discuss some potential benefits and challenges to postharvest sprays.
A key benefit is that the psylla are still highly congregated on pear trees in the late summer and early fall. This means that the entire population can, in theory, be effectively targeted. This is not the case in late winter and early spring, when overwintering adults are gradually moving back into the orchard from various overwintering sites (native/landscape vegetation, fallow fields, etc.). So (again, in theory) if everyone sprays at a relatively similar time, the population can be significantly reduced.
In order for postharvest sprays to effectively reduce the overwintering psylla population, nearly everyone must a) participate, b) achieve adequate coverage, and c) use an effective material(s). Psylla adults disperse and intermix throughout the fall, winter and spring; so, the efficacy of postharvest sprays seen on an individual orchard will only be as good as the area average. Therefore, everyone needs to participate, and everyone should spray using whatever method (volume, speed, adjuvants, etc.) will achieve the best possible coverage. Better yet, everyone would coordinate sprays from helicopters to more efficiently cover a large area.
Choosing the right material(s) is always tricky. If using an adulticide only (such as malathion or lime sulfur), it may be necessary to spray twice (once prior to leaves changing colors and once as they are changing colors). Nymphs will continue to develop into adults until leaves start to drop. Therefore, spraying at these two timings should kill the main population of adults and any late developers (and probably some that were missed by the first spray). If spraying shortly after harvest when leaves are still green, it may be necessary to use a nymphicide as well. Options are more limited if helicopter sprays are used, as not all products have a ULV (ultra-low volume) formulation and aerial use on the label.
In summary, postharvest sprays can be a smart and effective way to reduce the number of adults we encounter next spring. This year has been much lighter for psylla pressure than other recent years, but that does not mean we as an industry can be complacent. It is much easier and more effective to maintain low pest populations, than to reduce high pest populations.
– Louis Nottingham, Washington State Entomology – TFREC