Managing Silverleaf Whitefly (SLW) – Wet conditions, late crops and immigrant populations

Man­age­ment of white­fly this sea­son will be chal­leng­ing as a result of the high rain­fall and flood­ing in Novem­ber and  again in Feb­ru­ary which has result­ed in a wide spread of crop matu­ri­ty both between and with­in each region. While some cot­ton crops will mature on time, oth­ers could be set back by as much as six weeks by the cool over­cast and wet con­di­tions.

White­fly num­bers increased in most cot­ton grow­ing regions dur­ing Feb­ru­ary although infes­ta­tions report­ed vary great­ly from zero to 80% of leaves infest­ed. The recent warm tem­per­a­tures will like­ly lead to an increase of white­fly num­bers in crops as well as through migra­tion.  White­fly that migrate into cot­ton crops come from hosts sus­tained by reg­u­lar rain­fall but which are now becom­ing less palat­able to white­fly.

So giv­en the weath­er con­di­tions, crop matu­ri­ty and increas­ing white­fly num­bers – how do we man­age SLW? The fol­low­ing sce­nar­ios illus­trate how SLW may be man­aged in a range of sit­u­a­tions.

When crops are on time

The thresh­old matrix (below) should be used to guide white­fly man­age­ment deci­sions for crops that are on time, or close to it. This matrix pro­vides an excel­lent mea­sure of pro­ject­ed pop­u­la­tion build up and action thresh­olds for crops that are on time and not sub­ject to mass SLW migra­tions from sur­round­ing areas. The day degree cal­cu­la­tor found at http://CottASSIST.cottoncrc.org.au helps to assess whether crops are matur­ing on time. The 2011/12 Cot­ton Pest Man­age­ment Guide (p 26–28) pro­vides con­trol rec­om­men­da­tions using the thresh­old matrix.

When crops are late

White­fly pop­u­la­tions in late crops are like­ly to be a com­bi­na­tion of res­i­dent and immi­grat­ing white­fly. In this sit­u­a­tion, the thresh­old matrix may not be accu­rate.  as the matrix is based pri­mar­i­ly on SLW pop­u­la­tions build­ing up in crop with­out migra­to­ry influx­es. In addi­tion, the day-degrees that a crop accu­mu­lates may no longer align well with the matrix due to flood dam­age or water­log­ging. 

A crop man­ag­er faced with this sce­nario should aim to avoid hon­ey dew con­t­a­m­i­na­tion of open bolls. This means that pri­or to open bolls, influx­es of white­fly can be tol­er­at­ed.

Delay treat­ment?

The delay of treat­ment on late crops may mean that cool­er tem­per­a­ture in March/April slows white­fly activ­i­ty and hon­ey­dew pro­duc­tion. It also allows more set­tling time for migra­to­ry white­flies before treat­ment is applied. Treat­ing too ear­ly maybe par­tial­ly inef­fec­tive because of fur­ther crop re-inva­sion by adult white­fly as the treat­ments effi­ca­cy declines. Treat­ing too ear­ly increas­es the risk of hav­ing to retreat.

Con­trol deci­sions for late crops and/or where there are influx­es of SLW, should be based on:

  • Ori­gin (e.g. local build-up or mass immi­gra­tion) and num­bers of SLW
  • Pres­ence of open bolls
  • Expect­ed time until defo­li­a­tion leaf drop
  • The rate and lev­el of hon­ey dew accu­mu­la­tion on the crop canopy and lint
  • The like­ly effi­ca­cy and resid­ual impact of the cho­sen insec­ti­cide.
  • Rel­e­vant prod­uct with­hold­ing peri­ods (WHP’s).

>Prod­uct choic­es for SLW are pri­mar­i­ly lim­it­ed to knock­downs such as Pega­sus (Diafen­thi­uron) and Pyrethroids (Bifen­thrin) or the slow­er act­ing IGR’s such as Admi­ral (Pyr­prox­ifen) and Moven­to (Spirote­tra­mat). Resis­tance is a threat to these prod­ucts, so fol­low the IRMS. Admi­ral can only be applied once. 

Late crops with low SLW numbers

Crops with very low white­fly pop­u­la­tions, <10% infest­ed, and lit­tle hon­ey­dew at boll open­ing should not require con­trol. How­ev­er num­bers and hon­ey­dew should con­tin­ue to be mon­i­tored and if there is a sud­den increase in white­fly, due to an influx from neigh­bour­ing fields, con­trol with a knock­down may be war­rant­ed

Late crops with moderate to high SLW

If num­bers are mod­er­ate to high and defo­li­at­ed leaf drop is 3 or more weeks away an IGR may be the best con­trol option. Pyr­prox­ifen has excel­lent resid­ual and will most­ly pre­vent the con­tin­ued build up of res­i­dent and immi­grant SLW. The Cen­tral Queens­land expe­ri­ence strong­ly sug­gests that Pyriprox­yfen (Admi­ral®) works even bet­ter at low­er autumn tem­per­a­tures than it does in sum­mer and is capa­ble of clean­ing up dense white­fly pop­u­la­tions on late cot­ton effec­tive­ly with a sin­gle appli­ca­tion. Con­tin­ue to mon­i­tor white­fly num­bers and hon­ey­dew and if adult num­bers begin to rebuild — a knock­down may be required.

Crops less than 2 weeks from defoliation

If the crop is less than 2 weeks from defo­li­a­tion and an influx of adult SLW occurs, a knock­down type prod­uct may pro­vide bet­ter val­ue, keep­ing in mind rel­e­vant WHP’s. Con­tin­ue mon­i­tor­ing of hon­ey­dew and if white­fly num­bers start to recov­er and cause hon­ey­dew close to defo­li­a­tion — con­sid­er defo­li­at­ing a few days ear­li­er. Once defo­liant is applied adult white­fly will gen­er­al­ly leave the crop and falling leaves will take the nymphs with them.

Monitor for honeydew

It is dif­fi­cult to deter­mine at exact­ly what point, lev­els may become prob­lem­at­ic once bolls begin to open. In CQ and dur­ing lat­er out­breaks on the Dar­ling Downs, hon­ey­dew was con­sid­ered prob­lem­at­ic when leaves on the low­er canopy became heav­i­ly speck­led with hon­ey­dew. If the leaves are at or get beyond this lev­el and devel­op a hon­ey­dew “sheen” then cor­rec­tive action is required imme­di­ate­ly.

Other considerations

Some crops will have an ear­li­er and lat­er matur­ing phase of bolls. It is impor­tant to man­age white­fly to reduce the risk of con­t­a­m­i­na­tion of the ear­li­er bolls. The basic strate­gies out­lined above should be used in rela­tion to the ear­li­er bolls e.g., delay­ing the first treat­ment as late as pos­si­ble, but not lat­er than 5% open bolls. How­ev­er, with ongo­ing mon­i­tor­ing of white­fly and hon­ey­dew as a guide, a sec­ond appli­ca­tion of an insec­ti­cide from a dif­fer­ent group may be required. Ear­li­er defo­li­a­tion may also be ben­e­fi­cial.

If, despite efforts to man­age white­fly, crops end up with sig­nif­i­cant hon­ey­dew con­t­a­m­i­na­tion – e.g. low­er bolls dark with sooty moulds — then pick­ing should be delayed as long as pos­si­ble to expose the lint to max­i­mum weath­er­ing, espe­cial­ly rain­fall, which will help reduce hon­ey­dew lev­els. The long range fore­cast indi­cates a high prob­a­bil­i­ty of rain­fall late Feb­ru­ary and at var­i­ous inter­vals through­out March. In gen­er­al how­ev­er, the costs of pre­vent­ing the prob­lem will be far less than the poten­tial grade penal­ties for weath­ered lint – so proac­tive man­age­ment is the best option.

Arti­cle by Ian Tay­lor, Richard Sequeira, Paul Grundy and Lewis Wil­son.

This arti­cle first appeared in the Namoi Val­ley Cot­ton tales