Monday, 18 July 2016

Rearing parasitoids that attack Lygus

As mid-June approaches, Lygus in the Peace Region will be starting to lay their eggs into the stems and leaves of canola, alfalfa, and other plants. Later in the summer, when then the canola and other crops are more developed, adults and nymphs (Fig. 1) can damage crop yields when they feed on the juicy plant material of these crops. They pierce through the plant tissues with their sucking mouthparts- which often leave visible lesions on stems, buds, flowers and pod surfaces. Buds and flowers drop as a result and the seed pods often turn brown and shriveled from being fed on. The economic damage on canola in the Peace region is all caused by Lygus species native to North America.
Figure 1. Lygus nymph on the left and adult Lygus on the right (image retrieved from$department/deptdocs.nsf/all/agdex741).

Lygus can be pale green to reddish-brown to black and have a distinct V-shape centered on their dorsal side (Fig. 1). Lygus overwinter as adults.  Theyemerge early in spring.  Canola is most susceptible to damage from Lygus at late flowering through the early pod stages.
The phenology of Lygus varies by region with 2-3 generations per year in the south of the Canadian prairies to only one generation per year in the Peace River region.  Lygus feeding damage will depend upon growing conditions. Abundant rainfall helps canola compensate, whereas hot, dry growing conditions favour Lygus development and often result in high populations capable of causing damage and yield losses.  If canola fields are adjacent to hay fields, early season monitoring of Lygus densities within the alfalfa can often help growers anticipate Lygus in bolting and early flowering canola.
The economic threshold for Lygus in canola varies by canola crop stage and is applied at late flower or early pod (Table).  It depends on sweep-net monitoring at multiple sites within a field and counting both adults and nymphs (3rd-5th instar stages). When densities exceed the economic threshold is exceeded registered foliar-applied insecticides are used to reduce feeding damage and protect the developing pods and seeds which then results in quality and yield being maintained.  There are a number of natural enemies that help reduce and regulate Lygus populations; notable wasp parasitoids include species from the Family Mymaridae and Braconidae.  In North America, the braconid, Peristenus pallipes has been reared from Lygus lineolaris and L. keltonii as well as L. shulli. A potential biological control agent also worth mentioning is Peristenus digoneutis which is a parasitic wasp or European origin that can attacks nymphal stages of Lygus rugulipennis  Poppius but has shown a degree of host preference for North American species of Lygus and has been released to in eastern Canada in strawberry production systems.   Historically, Peristenus digoneutis was released on the Canadian Prairies in the 1970’s but failed to establish in detectable numbers.  Efforts still continue to investigate P. digoneutis as a biocontrol agent for Lygus found in canola grown in Canada.

Figure 2.  Peristenus digoneutis here is shown laying an egg in a lygus nymph (image retrieved from: ).
In our lab, we have prepared Peristenus rearing cages and mass collections were performed during the last week of June.  Mass collection involves sweeping the canopy then retrieving live specimens back for processing.  The density of Lygus is recorded and nymphs are isolated for rearing.

Peristenus wasps attack and lay an egg within 1st-2nd instar stages of Lygus nymphs (Fig. 2).  Once the egg hatches, the Peristenus larva develops within the body of a Lygus nymph so we rear the nymphs until they either mature to adults or a Peristenus larva emerges from the host to then drop down to the soil and spin its cocoon (Fig. 3).  We monitor the number of Peristenus cocoons that arise from the Lygus nymphs then the cocoons will be transferred to collaborators at AAFC-Lethbridge who will continue to rear and obtain the live wasps the following spring.

Figure 3.  Peristenus rearing cages at Beaverlodge Research Farm.

Haye, T., et al. "Controlling Lygus Plant Bugs (Heteroptera: Miridae) with European Peristenus relictus (Hymenoptera: Braconidae) in Canada – Risky Or Not?" Bulletin of Entomological Research 96.2 (2006): 187-96.