Jenny Dymock has provided this update:
“TGW is (thus far) only a problem north of Awanui where a combination of kikuyu dominance (only feeds on kikuyu), sandy soils, warmth (TGW only develops if temperatures are over 11 C) and sufficient moisture and humidity. All will depend on the weather is good (Indian Summer with more rain/moisture) through into April. If so – large numbers of moths emerging from the current “crop” of pupae will lay a large number of eggs giving rise to more damage.
Yesterday I visited sites and there has been an increase in area affected by TGW since last week (6 March). TGW populations are at various stage of the life cycle.
Paparore: increased areas damaged – late instar larvae and pupae with lots of moths sheltering in the rank kikuyu.
Waiharara monitoring sites (2) at Turk Valley Rd: One site the kikuyu is recovering from last week’s damage and there were 8 larvae +4 pupae /square metre (25 x 0.25m2 quadrats). At the end of Turk Valley Rd, damage has appeared at the site. Numbers up from 19 larvae/m2 to 80 larvae + 4P/m2.
Motutangi: population has pupated compared to last week (37 larvae + 38 pupae/m2) with kikuyu damage and 2larvae + 23 pupae/m2.
All of the larvae will move into pupae and then large numbers of moths will emerge over the next weeks and if the weather is still warm enough they will lay a huge number of eggs.
When the population gets to over 30 larvae/m2 then pasture damage is obvious – brown, chewed out patches, as if treated by herbicide.
Harrowing is the only control option if contour allows. Not possible with long rank kikuyu so can graze off pasture ahead of the TGW “wave”. At least the stock can get some of the feed before the TGW does. Rips open the kikuyu thatch, changing the microclimate for TGW larvae – do not like sunlight and exposure to dry conditions. Also some collateral damage occurs – some larvae and pupae get squashed. More than one pass with the harrows may be necessary.
Following my recommendation last week Lynn Macrae has had considerable success with harrowing. There was kikuyu recovery and low numbers of TGW in the harrowed pasture. More than one pass with the harrows may be necessary.”
For advice or to report an infestation, contact:
Dr Jenny Dymock, independent entomologist contracted to Northland Regional Council. Home/Work (09) 4060033 Mobile 021 02210323. firstname.lastname@example.org
AgPest is a free website, with information on life-cycle, monitoring and management of many species of pest and weeds: http://agpest.co.nz/?pesttypes=tropical-grass-webworm
Webworm population explosion may be imminent in Far North: http://www.voxy.co.nz/national/5/306117