Di­a­mond­back Moth in 2018 Grow­ing Sea­son

Moose Jaw Express.com - - FACES OF THE REGION - Maryna Van Staveren, Sum­mer Stu­dent, Moose Jaw Saskatchewan Min­istry of Agri­cul­ture Di­a­mond­back Moth

Di­a­mond­back Moth is one of the ma­jor early sea­son pests of the bras­sica fam­ily. Its damage in­ten­sity varies de­pend­ing on the den­sity of its pop­u­la­tion and pres­sure from its nat­u­ral en­e­mies. It may over­win­ter in the prairies; how­ever, it mainly mi­grates into Canada through May to July from the south­ern or west­ern U.S. Di­a­mond­back moth, be­ing a mul­ti­vol­tine species, pro­duces as many as four gen­er­a­tions per year. Early ar­rival of these pests re­sults in a higher num­ber of gen­er­a­tions and there­fore a higher risk of eco­nomic damage. Adult pests do not cause eco­nomic damage to the crop but they lay eggs on the leaves of canola. The lar­vae then hatch within five or six days and be­gin to feed on the leaves and later on flower buds and pods, caus­ing sig­nif­i­cant damage to the fu­ture yield. The adult moth is small, ap­prox­i­mately 10-12 mm in length and grey­ish-brown with a 12 mm wing­span cov­ered in long hairs. The moth’s defin­ing phys­i­cal char­ac­ter­is­tic can be ob­served when it is at rest, for as it holds its wings to­gether, a pat­tern of three yel­low di­a­mond-shaped spots can be seen along the top of the moth’s body. Its lar­vae are yel­low-green in colour, cov­ered with short hairs and are five to 12 mm long. Upon phys­i­cal contact, the cater­pil­lars tend to de­scend off of the plants and dan­gle from a silken thread. The cater­pil­lar’s defin­ing char­ac­ter­is­tics are its ta­pered ends and forked pos­te­rior. The larva pu­pae is ini­tially light green how­ever upon ma­tu­rity a brown adult moth be­comes visible through a del­i­cate white co­coon. This stage usu­ally lasts five to 15 days, with warmer con­di­tions aid­ing in faster ma­tu­rity.

The worst damage oc­curs in the sec­ond and third gen­er­a­tions, dur­ing mid-July to early-Au­gust, where the older lar­vae feeds on canola flow­ers, pods and stems. Lar­vae feeds on the in­ter­nal leaf tis­sue and upon ma­tu­rity move onto the out­side of the leaf, leav­ing tan-coloured blotches on the plant. Feed­ing tends to last for 10 to 30 days, de­pend­ing on out­door tem­per­a­tures. Feed­ing dur­ing the early flow­er­ing stage will de­lay plant ma­tu­rity and cause un­even de­vel­op­ment of the crop canopy. Lar­vae will typ­i­cally pre­fer to feed on the flower bud prior to feed­ing on the pods. Dam­aged pods may be sub­jected to pre­ma­ture shat­ter­ing.

Scout­ing for di­a­mond­back moth should be done weekly from mid-July to early Au­gust. When scout­ing the field, mon­i­tor at least five one-square-me­tre sec­tions of the crop. Early damage can be ob­served in the ridges and knolls of the field in a form of ab­nor­mal whiten­ing. Vig­or­ously shake the crop canopy or pull plants in each of the cho­sen sec­tions and count the lar­vae on the plants and the ground, as well as un­der the leaves and in the plant de­bris. It is im­por­tant to keep in mind that the en­vi­ron­men­tal con­di­tions will de­ter­mine the amount of eggs laid and the chance of lar­vae’s sur­vival. Heavy rain­fall washes young lar­vae off the leaves to the ground with a chance of drown­ing the pest all to­gether; cold and windy tem­per­a­tures slow the eggs’ ma­tu­rity and re­duces the adults’ ac­tiv­ity. On­go­ing hu­mid con­di­tions may cause the out­break of En­to­moph­tho­rales, a fun­gal dis­ease that oc­curs in the later grow­ing sea­son dur­ing high di­a­mond­back moth pop­u­la­tions, lim­it­ing the de­vel­op­ment of lar­vae into adults. The eco­nomic thresh­old for di­a­mond­back moth varies upon the stage of the crop; with it be­ing 25 to 30 per cent leaf damage at the seedling stage, 100 to 150 lar­vae per square me­ter dur­ing the flow­er­ing stage and 200 to 300 lar­vae per square me­tre at the pod stage.

There are cur­rently three par­a­sitoid species of par­a­sitic wasps that ag­gres­sively prey upon di­a­mond­back moth. Cres­son ( Di­adegma in­su­lare), Mue­se­beck (Mi­cropli­tis plutel­lae) which prey upon the lar­vae, and Graven­horst ( Di­adro­mus sub­til­icori­nis); which feeds upon the pre­pu­pal and pu­pal stages. Other nat­u­ral en­e­mies in­clude flies, lacewings, pi­rate bugs, bee­tles, spi­ders and birds. Tim­ing of fo­liar ap­pli­ca­tion is key in suc­cess­fully re­duc­ing the pest’s pop­u­la­tion. In­sec­ti­cide ap­pli­ca­tion tar­get­ing the lar­vae should be ap­plied once the eco­nomic thresh­old is ex­ceeded. Once an in­fes­ta­tion is suc­cess­fully con­trolled at the pod­ding stage, a new in­fes­ta­tion is un­likely to oc­cur due to the later stage of the crop. Con­trol­ling vol­un­teer canola and other weeds of the bras­sica fam­ily will rid of ad­di­tional hosts for the di­a­mond­back moth adults to con­tinue their life cy­cle. Keep­ing up­dated with pro­vin­cial agri­cul­tural web­sites for on­go­ing fore­cast­ing of the pest ac­tiv­ity will aid in de­ter­min­ing the early num­bers in the pop­u­la­tion.

For fur­ther in­for­ma­tion, please re­fer to:

• 2018 Guide to Crop Pro­tec­tion avail­able from the Saskatchewan Min­istry of Agri­cul­ture. www.saskatchewan.ca/agri­cul­ture

Prairie Pest Mon­i­tor­ing Net­work Blog, www.prairiepest­mon­i­tor­ing.blogspot.com

• Agri­cul­ture Knowl­edge Cen­tre at 1-866-457-2377 or by email at ag­info@gov.sk.ca

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