The Green and Brown Mirid are both pests of cotton in Australia. Green Mirid is the most abundant of the two and may can be present right through the season. Green Mirids are pests in cotton and a range of legume crops.
Identification: Adults are about 7mm long, and both adults and nymphs are pale green with long antennae. Newly hatched nymphs are about 1.4- 1.5mm long and larger nymphs (4th and 5th stage) can be up to 7mm long. They are characterized by their distinctive red tipped antennae.
May be confused with: A related species, the Brown Mirid (Creontiades pacificus) is sometimes found in cotton and is similar in appearance to the Green Mirid although slightly larger, darker and with antennae that are striped with red and white. Green Mirid nymphs may also be confused with Broken-backed Bug and Predatory Black Mirid nymphs. Nymphs of these insects are much smaller (e.g. 5th instar Broken-backed Bug and Black Mirids are about 3mm long). Broken-backed Bugs nymphs have shorter light green antennae while Black Mirid nymphs have dark red eyes and black and white striped antennae.
Lifecycle: The Green Mirid has an egg and five nymphal stages. Eggs are 1.5mm long, banana shaped and laid singly within the plant tissue with only an oval egg cap showing above the leaf or petiole surface. At optimum temperatures (30 - 32°C) eggs hatch after 4 to 5 days and each nymphal stage takes 1.5-3 days to develop. Adults can live for three to five weeks and females can lay up to 80 eggs in this time.
Host range: Green Mirids can feed and develop on a wide range of hosts, including sunflower, safflower, lucerne and other legume and weed species. Safflower in particular builds up large Mirid populations which can move into adjacent cotton as the safflower dries out in early summer – leading to damage and insecticide applications. Application of pesticides to safflower pre-emptively to control the Mirids and prevent them moving to cotton must be done carefully as broad-spectrum insecticides such as pyrethroids will decimate beneficial populations and lead to build up of large populations of Two-Spotted Spider Mite which can be blown into adjacent cotton crops and require control.
Damage: During feeding Green Mirids pierce the plant tissues with their sharp mouthparts or stylet and release a chemical (pectinase) which destroys cells in the feeding zone. The affected tissue rapidly dulls in colour, then blackens, desiccates and dies. Favoured feeding sites include plant terminals (particularly on seedlings) which often results in death of the growing tip (tipping out), small squares and young leaves. If seedling tipping out and early square loss caused by Green Mirids is excessive, crop maturity can be delayed and yield reduced. Damage at early crop stage (up to first flowering) can be compensated for under ideal growing conditions (no stress from water or nutrient).
Green Mirids can also feed on large squares and small bolls, causing shedding if the bolls are young (< 10 days old) or if the bolls are larger, individual locks may be damaged. This damage is characterized by shiny black spots on the boll wall and brown staining of the developing lint. Bolls over 20 days old are protected by their thick boll coat.
Adult Green Mirids can cause more damage than nymphal instars 1 - 3 and similar to or slightly more damage than the final instars 4 and 5.
Monitoring: Mirids are extremely mobile and populations can fluctuate rapidly, so regular sampling is important. Sampling is more effective if carried out using a beat sheet or a sweep net. The beat sheet method has proven to be the best at finding nymphs, while the sweep nets are best for finding adults.
It is also important to monitor plant damage, as Mirids can be present yet there may be little damage.
Natural enemies: A number of generalist predators are known to feed on Mirids. These include Spiders (especially the Lynx spiders), Ants, Damsel bugs, Big-eyed bugs, Assassin bugs and Predatory Shield bugs. Green Mirids are also predators and will eat spider mites and eggs of Helicoverpa species.
Newly laid Green Mirid eggs are a light blue colour, changing to a pale yellow prior to hatching 1.5mm Photo: M. Khan
Newly hatched Green Mirid nymphs are approximately 1.5–2mm in length, with red tipped antennae that are much longer than their body. 2mm Photo: M. Khan
Adult Green Mirids are approximately 7–9mm in length with an elongated body, long antennae and long legs (especially the hind legs). 7mm. Photo: C. Mares
Green mirid damage is characterized by shiny black spots on the boll wall. Photo: L. Wilson