Tuesday, 23 July 2019

Insect of the Week (July 22): Doppelgangers: Lygus bugs nymphs vs. aphids

The case of lygus bug nymphs versus aphids: Small, green, soft-bodied, sucking insects - at first glance they could be either lygus bug nymphs or aphids. But spend a moment and look for the following characteristics, and you'll be able to tell which pest you are dealing with.
  • Size: depending on the species, aphids can reach up to 4 mm long, but most will be 1-2 mm. Lygus bug nymphs will be larger, 4-6 mm long
  • Cornicles (small upright backward-pointing tubes found on the back side at the rear of abdomen): aphids have them, lygus bug nymphs do not. In some aphid species, the cornicles or where they attach to the abdomen are black (e.g. corn aphid, English grain aphid). 
  • Markings: older lygus bug nymphs have five distinct black dots on their thorax and abdomen; aphids do not.

For more information about these species and more tips on telling them apart, see our Insect of the Week page! Also, see our Monitoring protocols for lygus bugs in canola

Tarnished plant bug nymph - note five black dots on
throrax and abdomen - Scott Bauer, USDA

English grain aphid - adult and nymphs - note black cornicles
(tubes) sticking out the back - Tyler Wist, AAFC

The case of the innocuous versus the evil twin: When making pest management decisions, be sure that the suspect is actually a pest. This can be challenge since insects often mimic each other or look very similar. An insect that looks, moves and acts like a pest may in fact be a look-alike or doppelganger.

Doppelgangers may be related (e.g. same genus) or may not be related, as in the case of monarch butterflies (Danaus plexippus) and viceroys (Limenitis achrippus).  Doppelgangers are  usually relatively harmless but sometimes the doppelganger is a pest yet their behaviour, lifecycle or hosts may be different.

Correctly identifying a pest enables selection of the most accurate scouting or monitoring protocol. Identification and monitoring enables the application of economic thresholds. It also enables a producer to select and apply the most effective control option(s) including method and timing of application.  For the rest of the growing season, the Insect of the Week will feature insect crop pests and their doppelgangers.

Review previously featured insects by visiting the Insect of the Week page.