Written by Christopher Strohm, WSU Extension. April 19, 2018
As part of a Pear IPM project we are tracking natural enemies in pear orchards and we wanted to share who is active in the orchard now with you. As you manage your blocks this year, consider your natural enemies and what they can do for you. Here we describe the parasitic wasp Trechnites that is abundant during bloom in organic pear orchards. Look out for descriptions of other “good guys” Deraeocoris, green lacewings, and Campylomma as they come out this year.
Why Trechnites are important?
The Trechnites wasp (Trechnites insidiosus or T. psyllae) is the most common parasitoid of pear psylla in the Pacific Northwest. These wasps are important because they are a true specialist of pear psylla and are active early in the season. Unlike predators, Trechnites wasps do not feed on other pests or fellow predators in orchards systems. The only goal of these wasps is to parasitize new psylla hosts and reproduce. In unsprayed orchards, the rate of psylla parasitism by Trechnites can exceed 70%, in an organic orchard this rate can be as high as 50% (Unruh & Brunner, 1993).
Trechnites in Wenatchee Valley Pear Orchards
During the 2017 season we scouted for Trechnites adults using beat trays and yellow sticky card traps. There was a large emergence of wasps around bloom, a drop in numbers in early summer, followed by a steady build in population during July, August, and September.
During bloom, up to 48 wasps per trap per week were found in orchards using all types of pest management. However, during summer, more wasps were collected in orchards using organic or selective spray products compared to orchards using the typical conventional products. To see the 2017 Trechnites data check out the scouting results page here: http://treefruit.wsu.edu/crop-protection/pear-ipm/scouting/
The adult wasp is very small (~1mm) with a dark body but can be seen on a white beat tray sheet. With magnification it is possible to see dark legs with yellow knees and a char
acteristic blue-green iridescent patch visible on the wasp’s back (Photo 1). Parasitized psylla nymphs (mummies) contain the developing eggs, larva, or pupa of the wasp. Psylla mummies are golden brown with a hump-backed shape (Photos 2 and 3).
Trechnites primarily reproduces by making clones of itself (Unruh & Brunner, 1993). Female wasps lay eggs into the haemocoel (blood) of psylla by piercing the abdomen of nymphs with their ovipositor. What we know from lab studies is that it can take 90 seconds or more for a female to successfully oviposit in a psylla nymph. There may be a preference for nymphs in their third or fourth instar that are encapsulated in honeydew (Gutierrez, 1966).
Parasitized psylla nymphs become immobile and will not progress through normal development. Once wasp eggs hatch, larvae will develop inside the host psylla. By the time the wasp larva reaches its second instar, the psylla’s internal organs are consumed and it becomes mummified (Unruh & Brunner, 1993). Trechnites wasps overwinter as fully developed larvae inside of the host psylla mummies. In spring, these larvae will pupate and adults will emerge around the finger to bloom stage of pears and begin parasitizing new psylla hosts. There are three to four generations per season (Unruh & Brunner, 1993).
Unruh, T. & Brunner, J.F.., Trechnites insidiosus (1993). from Orchard Pest Management (Beers, E. H. & Brunner J. F., editors). WSU/Washington State Fruit Commission.
Gutierrez, A.P. (1966) The bionomics of two Encyrtid parasites of Psylla pyricola Förster in northern California (Unpublished M.S. Thesis). Retrieved from University of California Berkley Interlibrary Services, Berkley, CA.
This project was funded in part by a grant from WSDA Specialty Crop Block Grant K1986.
Extension Assistant, Pear IPM
WSU Tree Fruit Research and Extension Center
509-663-8181 ext. 233
Tree Fruit Extension Specialist