Whitefly management options

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In the last week, reports from the Downs are sug­gest­ing that sil­ver­leaf white­fly (SLW) num­bers have increased rapid­ly, and now there are a num­ber of fields that have a pop­u­la­tion at or exceed­ing the treat­ment thresh­old.

This post­ing has been com­piled from infor­ma­tion and dis­cus­sion with Richard Sequeira (Prin­ci­pal Ento­mol­o­gist, Emer­ald) and Paul Grundy (Senior Ento­mol­o­gist, Ayr) who have con­sid­er­able expe­ri­ence in man­ag­ing SLW. The aim is to pro­vide infor­ma­tion that may help in mak­ing deci­sions about the need for and tim­ing of con­trol, and the appro­pri­ate con­trol option.

It appears that there is a larg­er than pre­dict­ed pop­u­la­tion of SLW in cot­ton this sea­son, espe­cial­ly as the sea­son is aver­age in terms of tem­per­a­ture. Why is this so par­tic­u­lar­ly on the Downs where it is pre­dict­ed that out­breaks would only occur in hot­ter than aver­age sea­sons?
The answer to this is that out­breaks of SLW are dri­ven not only by tem­per­a­ture, but by two fac­tors, the size of the ini­tial pop­u­la­tion in spring and the sum­mer tem­per­a­tures. In the 2007-08 sea­son we have expe­ri­enced tem­per­a­tures that are close to the long term aver­age, but we would have start­ed the sea­son with a large car­ry­over from the out­break in 2006-07. The rain­fall and weed growth in win­ter and spring would have pro­vid­ed hosts for the SLW to car­ry­over from last sea­son to this one.

Over the last week or so, SLW pop­u­la­tions seem to be increas­ing with the per­cent­age infes­ta­tion ris­ing rapid­ly. In Cen­tral Queens­land (CQ) it is usu­al to see a rapid increase in the per­cent­age infes­ta­tion at the 5th node once the crops cut out. It looks as though the pop­u­la­tion is increas­ing in size, but what is actu­al­ly hap­pen­ing is move­ment of the pop­u­la­tion up towards the top of the plant as it stops putting on nodes. In CQ the qual­i­ty of the leaves low­er on the plants also starts to decline around this time and becomes unsuit­able for SLW, forc­ing them high­er in the canopy. Paul sug­gest­ed that there are visu­al clues to whether the pop­u­la­tion is increas­ing in num­ber. Observ­ing the amount of hon­ey­dew on the low­er leaves can be instruc­tive. Over a week, an increase from a light speck­ling to larg­er droplets is indica­tive of a pop­u­la­tion increase.

As a SLW pop­u­la­tion approach­es thresh­old nat­ur­al ene­mies will not con­tain it. It is inter­est­ing to note that there has been very lit­tle par­a­sitism record­ed from sam­ples tak­en in St George, and on the Downs this sea­son. Par­a­sitism lev­els in CQ are also low. Richard’s inter­pre­ta­tion of this is that wet weath­er and high humid­i­ty has a neg­a­tive impact on the par­a­sitoids.

As SLW pop­u­la­tions approach, or exceed, the treat­ment thresh­old, ques­tions arise around the issues of whether to con­trol, when and what prod­uct to use. The SLW thresh­olds are avail­able in the Cot­ton Pest Man­age­ment Guide.

To use the thresh­old rec­om­men­da­tions it is nec­es­sary to cal­cu­late the day degree (DD) accu­mu­la­tions for the sea­son, this will enable you to match the crop stage to the appro­pri­ate thresh­old and man­age­ment options. The cur­rent DD data for a range of cot­ton-grow­ing regions is pre­sent­ed in the table below.

Weigh­ing up the options
In mak­ing a deci­sion about SLW con­trol, it is impor­tant to con­sid­er not only the lev­el of infes­ta­tion but also the age of the crop (DD accu­mu­la­tion) , how long it has to go until open cot­ton or leaf drop, and what is going on in neigh­bour­ing crops.

The sit­u­a­tion on the Downs cur­rent­ly is that there are crops at SLW thresh­old that do not yet have open cot­ton. In this sit­u­a­tion, Paul sug­gests that a sec­ond check be made on the pop­u­la­tion a week after it is first record­ed to be at thresh­old, just to make sure the pop­u­la­tion real­ly is at that lev­el. If it is, it is nec­es­sary to weigh up the options for con­trol.

Richard’s sense was that with­out open cot­ton, there is no urgency to con­trol the pop­u­la­tion. Poten­tial­ly you can wait a week or two until clos­er to the first open boll before apply­ing Admi­ral®. Cer­tain­ly, the appli­ca­tion of Admi­ral® should be clos­er to 1600 DD than 1500 DD (first open boll is at 1650 DD). The aim of this strat­e­gy to decrease the chance that there could be a resur­gence of the pop­u­la­tion post treat­ment, still with time to con­t­a­m­i­nate open cot­ton. How­ev­er, on the Downs, the expec­ta­tion is that the tem­per­a­tures will be cool­ing through March and into April, and the risk of a sec­ond large pop­u­la­tion is unlike­ly.

Anoth­er approach is to treat pop­u­la­tions at thresh­old now with Admi­ral®, par­tic­u­lar­ly know­ing that it has 3 week resid­ual, and that SLW will not start to breed in the crop until April. With this time­frame, par­tic­u­lar­ly in cool­ing con­di­tions, it is unlike­ly that SLW will build up to thresh­old before the end of the sea­son.

Pega­sus® will knock a pop­u­la­tion of SLW, and con­trol aphids in the crop. How­ev­er, giv­en the length of sea­son many crops still have to go, the use of Pega­sus ® now may require a re-treat­ment with Admi­ral ® lat­er. Whilst Pega­sus ® is the less expen­sive option, it is more like­ly to require re-treat­ment if used now than the appli­ca­tion of an Admi­ral ®. Pega­sus ® is an option for late crops where SLW pop­u­la­tions do not reach thresh­old until there is open cot­ton and a quick knock­down is need­ed late in the sea­son. If using Pega­sus® remem­ber that it is a con­tact prod­uct and good cov­er­age is essen­tial for good con­trol.