Soybean stem fly on the move in soybeans in the Northern Rivers NSW

Out­break report: Soy­bean stem fly (Melana­gromyza sojae) was detect­ed in late Jan­u­ary, 2018 in a 6-week old soy­bean crop near Casi­no, NSW. At the time of detec­tion, most stem fly in the crop were in the pupal stage. There were no plant ill health symp­toms (e.g. wilting/death), despite numer­ous exit holes made by lar­vae pri­or to pupa­tion. These holes are made by ‘about to pupate’ lar­vae to allow adult flies that emerge post pupa­tion, to exit the stem.

StemFlyCrownRotDSCF2799

Plant death symp­toms in soy­beans severe­ly infest­ed with stem fly and char­coal rot.

In the last major out­break (Casi­no 2013), many crops were also infest­ed with char­coal rot, which pro­duces sim­i­lar plant death/wilting symp­toms (see image below), and which made the attri­bu­tion of dam­age to stem fly quite dif­fi­cult. Growers/consultants are there­fore asked to close­ly watch any stem fly infect­ed crops and report any dam­age symp­toms in crops with and with­out char­coal rot.

 

Note that two oth­er pests (lucerne crown bor­er or Zygri­ta, and etiel­la) can also tun­nel in soy­bean stems and cause sim­i­lar dam­age, i.e. eat­en out and brown, dis­coloured pith (see below). Any agro­nom­ic mea­sures to reduce plant stress, par­tic­u­lar­ly irri­ga­tion, will also help mit­i­gate the impacts of stem fly, crown bor­er and char­coal rot.

Insec­ti­cide options: Cur­rent­ly there are no insec­ti­cides cur­rent­ly reg­is­tered or under per­mit for soy­bean stem fly con­trol in soy­beans. In 2013, an emer­gency per­mit (PER 14121 – now lapsed) was issued for dimethoate, in response to the major out­break at Casi­no. The per­mit rate of 800 mL/ha was based on lev­els reg­is­tered for ordi­nary bean­fly (Ophiomyia phase­oli) in oth­er sus­cep­ti­ble puls­es such as navy beans.

Expe­ri­ence from the 2013 out­break showed that dimethoate at this high rate did not reverse the dam­age symp­toms in severe­ly dam­aged crops, but did flare sil­ver­leaf white­fly. Unknown at this stage are whether dam­age would have been reduced with more time­ly spray appli­ca­tions (i.e. ear­li­er in the pest’s infes­ta­tion cycle), the poten­tial for more selec­tive insec­ti­cide options for this pest, and if the dam­age symp­toms were pri­mar­i­ly caused by the stem fly or char­coal rot.

Spray tim­ing: No mat­ter which prod­ucts are even­tu­al­ly reg­is­tered against or secure a per­mit for stem fly con­trol, spray tim­ing is like­ly to be impor­tant. In a crop where most stem fly are in the pupal stage (as report­ed at Casi­no), spray­ing would be inef­fec­tive, as the pupal stage does not feed, and there­fore would not take up any insec­ti­cide. How­ev­er, as the crop could be rein­fest­ed with the off­spring of flies emerg­ing from these pupae, sys­temic insec­ti­cides tar­get­ing these lar­vae (while they are still young and before they reach the main stem) may have poten­tial, but tri­als are required to ver­i­fy this hypoth­e­sis.

The cur­rent detec­tion will be close­ly mon­i­tored to see if any crop wilting/death symp­toms devel­op. If the crop is rein­fest­ed and devel­ops dam­age symp­toms, and if sim­i­lar dam­age is wide­ly report­ed in oth­er crops with lit­tle obvi­ous char­coal rot, then an emer­gency use per­mit appli­ca­tion will sub­mit­ted for stem fly con­trol. If this occurs, details will be includ­ed in a future Beat­sheet arti­cle.

Check your crops: Grow­ers and con­sul­tants in all coastal soy­bean regions are advised to check their crops for stem fly. In the severe 2013 out­breaks, swarms of the small black adult flies were read­i­ly observed, both visu­al­ly and in sticky traps. Check stems for tun­nelling in the pith, and for the tell-tale exit holes. Inspect dam­aged plants close­ly to deter­mine which pith-tun­nelling pest is respon­si­ble. Note that plants can be infest­ed by more than one pest, and that lucerne crown bor­er is very com­mon in many regions, Check that pupae in the stem are those of soy­bean stem fly, as small native par­a­sitic wasps were record­ed in sig­nif­i­cant num­bers in the lat­ter stages of pre­vi­ous stem fly out­breaks. The adult parasitoid’s shape is evi­dent in the wasp pupa.

Please report any sus­pect­ed soy­bean stem fly infes­ta­tions to both your local agron­o­mist and one of the researchers below:

Arti­cle by Hugh Brier and Liz Williams. Insect pho­tos by Joe Wes­sels.