Whitefly in establishing canola crops — Central NSW

Over the past cou­ple of weeks there have been reports of white­fly adults infest­ing canola in the Cen­tral West and Lach­lan regions of NSW. This is not an occur­rence that has been record­ed before, so we don’t know con­clu­sive­ly what the out­come will be of these infes­ta­tions. What I have done is pulled togeth­er infor­ma­tion on white­fly that will help you make an assess­ment of what is hap­pen­ing in your crop, if you have a white­fly infes­ta­tion. Thanks to Richard Sequeira (Research Sci­en­tist, DAFF Queens­land) for his insights into white­fly behav­iour and poten­tial impact on crops.

Where are the white­fly com­ing from?

It is like­ly that the white­fly are Sil­ver­leaf White­fly, and are mov­ing out of cot­ton crops as they are defo­li­at­ed and picked. The warm sum­mer and autumn have allowed white­fly num­bers to build up in cot­ton crops this year. Large scale move­ment of white­fly from cot­ton was observed in Emer­ald dur­ing the ear­ly years of out­breaks. White­fly will fly some dis­tance them­selves, but can also be car­ried long dis­tances by wind. It is less like­ly that they are com­ing from weed hosts in these high den­si­ties.

What impact will the white­fly have on my canola (and pos­si­bly oth­er broadleaf win­ter crops)?

We con­sid­er it high­ly unlike­ly that infes­ta­tions of adult white­fly will cause sig­nif­i­cant crop dam­age. We do not know how good a host canola is for white­fly, although hor­ti­cul­tur­al bras­si­cas (e.g. broc­coli) are good hosts. Regard­less of how good a host canola may be, con­sid­er the fol­low­ing:

1) White­fly is a warm weath­er species, and does not per­sist dur­ing cold weath­er. Even if day­time tem­per­a­tures are warm, the cool nights will slow the rate of feed­ing and repro­duc­tion; slow­ing the poten­tial impact on the crop.

2) Adult white­fly will live for 2 to 3 weeks under cool­er autumn con­di­tions (no more than a week in sum­mer).

3) Influx­es of adults may con­tin­ue for some time, which will make it dif­fi­cult to deter­mine whether it is sur­vival or immi­gra­tion con­tribut­ing to the pop­u­la­tion.

4) The feed­ing done by juve­nile white­fly (nymphs) is far more dam­ag­ing to plants than adult feed­ing. If the white­fly are not repro­duc­ing, the poten­tial for crop dam­age is low­er.

5) Eggs may be laid on plants that are unsuit­able for the sur­vival of nymphs. The pres­ence of eggs on canola, lupins, faba beans etc does not nec­es­sar­i­ly mean that the white­fly pop­u­la­tion is going to estab­lish in that crop.

What to look for to make an assess­ment of the dam­age poten­tial of a white­fly infes­ta­tion in win­ter crops.

1) Con­firm iden­ti­ty of the white­fly. Use the images below to exam­ine adult and juve­nile white­fly and deter­mine if they are green­house or sil­ver­leaf. Green­house are sig­nif­i­cant­ly less dam­ag­ing than Sil­ver­leaf.

2) Is there evi­dence of feed­ing? Look for shiny, sticky speck­ling on low­er leaves. This is hon­ey­dew that is secret­ed by both adults and nymphs as they feed on the plant. The more hon­ey­dew, the more feed­ing is going on.

3) Look for signs of crop stress: wilt­ing, red­den­ing in canola, retard­ed growth. Be real­is­tic about what may be being caused by the white­fly, and what may be mois­ture stress.

4) Look for the pres­ence of eggs and nymphs on the under­sides of leaves (see pic­tures below). A han­dlens or micro­scope will be nec­es­sary to see these on the leaves. Select the low­est leaves on the plants; those that have been exposed to the adult white­fly the longest.

Distinguishing Greenhouse and Silverleaf Whitefly nymphs and adults.

Dis­tin­guish­ing Green­house and Sil­ver­leaf White­fly nymphs and adults.

Silverleaf whitefly adult with eggs.

Sil­ver­leaf white­fly adult with eggs.