Diamondback moth in canola - Grains Research & Development ...

PAGE 2Female moths can lay more than 100eggs in their lifetime, singly or in smallclusters along the leaf vein on bothsides of plant leaves. Eggs hatch afterfour to six days with the first stageof the larvae burrowing into the leaftissue.DBM larvae grown to about 12mm in length.The next three stages of the larvaefeed on the plant surface and areusually found on the underside of theleaves.The rate of development from eggsto moths depends on temperature;quicker in warm weather and slower incool weather.At 28 o C the lifecycle takes 14 days,whereas at 12 o C the lifecycle takesmore than 100 days.In warm weather there is oftenconsiderable overlap in generations,and all stages of the DBM lifecyclemay be present in a crop at anyone time.DBM populations can suddenly crashand the reason for this is only partiallyunderstood. One factor is likely to bethe outbreak of fungal diseases duringwet, warm weather.DamageDBM larvae can cause extensivedamage to canola, but this does nothappen in all years.The damage caused by newly hatchedlarvae appears as characteristic palewhite traces. Older larvae feed on theunderside of the leaves and causeholes with the upper leaf surface intact,often creating a ‘window’.Larvae can be found at any stage of acanola crop’s development, with theirnumbers often increasing in the leadup to flowering. Canola can tolerateconsiderable leaf damage before cropyield is affected. However, severeinfestations of DBM larvae can causecomplete defoliation and substantialyield losses.As flowering progresses, increasingnumbers of larvae move to the floralbuds, flowers and pods. Large larvaemay feed on small young pods whilemature pods are usually only surfacegrazed.Damage to mature canola plantsduring the late spring by risingpopulations of DBM often causesvisible scarring of the outer podwalls, but this rarely results in anyeconomic loss.Monitoring for infestationsCrops should be monitored using aninsect sweepnet at the first sign ofdamage and at intervals throughout thegrowing season from mid July through tolate spring/early summer. Numbers arelikely to increase quickly if DBM infestcanola early in the season and thereis a strong chance of prolonged warmweather for the pest to complete threeor four generations.Sampling should be carried out at aminimum of four separate locationswithin each canola crop to gain anestimate of DBM numbers and howthey vary throughout the paddock.Two sets of 10-sweep samples shouldbe taken at each of the four locations,giving a total of eight 10-sweepnetsamples. Empty the contents of eachset of 10-sweeps onto a white or lightcoloured surface, for example thebonnet of a white ute, and count thenumber of larvae. Note the sizes of theDBM larvae and the presence of otherinsects, especially native budwormcaterpillars.At least three estimates of larval densityover 12 days will determine whetherthe number of larvae is increasing ordecreasing. On each occasion, eightor more 10-sweep samples shouldbe taken throughout the crop. If noDBM are detected the crop should bemonitored again in a fortnight.Cool, wet, windy weather can reduceDBM numbers. Numbers of larvaecan also decline from the impact ofbeneficial insects and insect diseases.When to actThere is no simple ‘one figure fitsall’ for spraying. Thresholds varyingfrom 50 to 100 larvae per 10-sweeps,depending on the crop growth stage,grain price and the cost of spraying.The higher the grain prices the lowerthe threshold and conversely thehigher the spray cost the higher thethreshold. Variation in regional andseasonal conditions also influencesspray threshold levels.As canola develops, it can tolerateincreasing numbers of DBM withoutany significant yield loss. The followinglevels are a guide to spray decisions.■ Pre to early-flowering – if averagenumbers exceed 10 larvae in10-sweeps then a major outbreakcould develop. Continue weeklymonitoring if average numbersare less than 10 and twice a weekmonitoring if numbers exceed 10.■ Early to mid-flowering and podformation – if average numbersreach 50 or more larvae in10-sweeps, spraying should beconsidered. This threshold increaseslater in the season.■ Late flowering and most pods areformed – if the average numberof larvae is greater than 100 per10-sweeps, spraying should beconsidered.A high number of small larvae (lessthan 3mm long) indicates numbers arelikely to increase further.Photo: SARDI

PAGE 3Crops should be monitored with an insect sweepnet throughout the season.against DBM. The period betweenreapplication may vary with choiceof chemical. Users must abide byregistrations details on the product label.DBM larvae feed on canola foliage, stems, flower heads and pods.Monitoring should begin as soon as plant damage is detected.Photo: emma leonardSweepnet sampling of the crop three tofive days after the first spray should occurto assess the effectiveness of the sprayand to determine surviving numbersbefore the second spray is applied.Achieving good chemical penetrationinto a canopy crop is important asabout 20 per cent of DBM larvae arefound on the lower plant canopy.Trials show aerial (CP90 nozzles with30L/ha of water) and ground-basedapplications (flat fan 11015 nozzles with50L/ha of water) are equally effective atproviding water rates and droplet size toachieve good leaf coverage and spraypenetration to lower leaves.Where ‘soft’ chemicals are used thatare less disruptive to beneficial insects,such as Bt (Bacillus thuringiensis), themajority of larvae should be less than5mm in length. Bt is broken down byultraviolet light so the best results areachieved when applied at dusk.Integrated managementA number of beneficial insects attackDBM. These include parasitic waspsDiadegma semiclausum, Apantelesippeus and Diadromus collaris. Thesewasps lay their eggs inside DBM eggsor larvae (depending on species),where the developing wasp larvae feedinside. The parasitised egg or larvaeturns black and fails to develop.Moisture-stressed crops are moresusceptible to insect damage anda lower threshold may be used ifextended dry periods are anticipated.Control measuresNo insecticide registered for controlof DBM will completely eliminate thepopulation.DBM has developed widespreadresistance to older insecticidessuch as synthetic pyrethroids andorganophosphates, especiallyin horticultural areas, as a resultof generational turnover and thefrequent use of these insecticides.Poor application of insecticides isalso likely to have contributed to thedevelopment of resistance.Once sweepnetting indicates DBMlarval densities are at the spraythreshold, a quick response with twospray applications (three to sevendays apart) can give adequate controlof larvae and reduce yield losses.This two-spray strategy ensures DBMeggs and larvae that survive the firstapplication are controlled.Growers should note that withholdingperiods before harvest are the sameas for windrowing, as it is considered aharvest event.Research in the northern WA grain beltfound a single spray in mid-August hadlittle impact on the damage caused byDBM.Many chemicals are registered for usePredators including brown and greenlacewings, several predacious bugsand a range of spiders feed on DBMeggs, larvae and pupae. In many yearsthese insects can be enough to preventeconomic damage to canola crops.Beneficial insects should beencouraged by better targeting andreducing insecticide applications,as well as providing alternate foodsources and refuge habitat aroundcrops. A fungus, Zoophthera radicans,can cause a 90 per cent reduction inDBM larvae numbers. The fungus isfavoured by rainfall, humidity and warmtemperatures. Diseased larvae becomeyellowish, sluggish and swollen beforedying. Dead larvae are white, brittle, flatand covered with fungus and attachedto the plant leaves.Summer weeds provide a ‘greenbridge’ on which DBM can survive oversummer. Controlling these weeds nearpaddocks to be sown into canola canhelp break the green bridge.