In this section:
Pest control decisions
A. Using crop monitoring to guide pest control decisions
WHAT’S GOING ON IN MY CROP?
If sticky traps, plant scouting or plant damage show that thrips have started to appear in your crop you should check two things:
- use a small lens to check a sufficient sample of plants to thoroughly assess what is happening by looking at flowers, shoots and leaves for thrips levels and estimating the relative proportions of adults and larvae (adult thrips have wings and ‘jump’ or fly, larvae are slower and can’t fly)
- get the thrips species identified by an expert to find out if they are WFT
If the thrips present are mostly larvae then they are breeding steadily in the crop. If they are all adults they have just flown in from outside or from another area in the crop where they are breeding. If they are WFT it is important to begin control early whereas other thrips species can be ‘knocked back’ fairly easily once they reach a level that causes commercial concern.
HOW DO I KNOW WHEN TO SPRAY?
In some plant species and climates thrips breeding rates may not be significant and spraying is unnecessary even though low numbers of adult WFT may be present. Thrips control is more of a priority with seedlings and plants that are susceptible to Tomato Spotted Wilt Virus that is spread by several thrips species including WFT, tomato and onion thrips.
If adult and larval thrips numbers are low and you are not sure what to do you may decide to monitor the situation more closely and wait. If the situation remains unchanged and you are confident that your monitoring is giving an accurate picture it is safe to hold back on spraying. If monitoring identifies either a sudden or a steady increase in thrips levels then you should again consider getting the thrips species identified, especially if a reliable report can be quickly obtained. This should be done if you are considering using an application of 3 sprays to try to ‘knock out’ a suspected breeding population of WFT or if damage levels are higher than normal. If Western Flower Thrips are not the culprit then one, or perhaps two sprays should be enough to gain control.
A sudden increase in adult thrips numbers may be due to a wave of thrips flying in (not necessarily WFT), whereas a steady increase despite a spray program indicates a probable infestation of resistant Western Flower Thrips. A shift from cool to warm temperature can also stimulate thrips breeding rates. This will cause a noticeable increase in thrips larvae first and then in adults a few days to one week later. A heat wave may bring the numbers down if temperatures rise above 45deg.
If adult thrips numbers are high and you are not sure what to do you should spray and monitor the results.
DECIDING IF THE SPRAY APPLICATION WORKED
If there were high levels of WFT breeding steadily in the crop large numbers of thrips may reappear just a few days after a spray application. This does not mean that the chemical has failed but that more thrips have emerged from eggs and pupae that were not killed by spray applications. One or two more spray applications will be required 3-6 days apart. If however the spray is not effective in killing most adult thrips and larvae in 1-2 days then there is clearly a problem with the spray program to be diagnosed and repaired!
WHAT ABOUT OTHER PESTS?
These monitoring principles will give you a much better basis for making spray decisions concerning most pests. Watch the pest levels and try to determine their breeding activity from the proportion and distribution of different life stages present in the crop (adult, larvae, egg, pupae etc.). Look for signs of any spray failure to alert you to the need to take corrective action.B. Examples for diagnosing management of flying pests
(thrips and whitefly)1) FIND OUT WHEN AND WHERE PEST NUMBERS ARE INCREASING IN THE CROP TO KNOW IF YOU NEED TO TAKE ACTION.
To check for any new pest populations moving into the crop you can use yellow sticky traps for thrips and whitefly. To see if they are already breeding in the crop it is better to inspect flowers for adults, and leaves for adults, eggs, pupae or larvae. This information can tell you when and where to keep a closer watch or if you need to treat now.ACTION:
- If pests (thrips or whitefly) remain below action threshold (*) on sticky traps, flowers and leaves, no problem!
- If there is a gradual increase they are probably breeding in crop. Spray when thrips exceed maximum limit decided for crop safety !!!!!
2) CHECK THE LEVEL OF PEST KILL AFTER APPLYING CHEMICALS TO SEE IF IT HAS BEEN EFFECTIVE
Pest levels generally behave in about 3 different ways after a spray application
- Plant checks show that pest numbers are low the day after spraying and they remain low
- Pests numbers are low after the spray but go up again within the week
- Pest numbers are still too high the day after the spray
This critical information still needs to be interpreted to know what is actually happening and what the best course of action is. Here is a guide for how to interpret changes in Western Flower Thrips levels (similar for whitefly)
ACTION:
Check flowers and leaves to see if any pests have survived.
- If thrips levels are very low stop spraying and keep checking flowers/sticky traps every 2-3 days
- If thrips are still high then you need to identify and fix problems by making adjustments to your spray program. There could be an error or fault in some part of the spray application methods. If the chemical has been working well before but suddenly has no effect this is most likely. This is not uncommon. Poor spray coverage can arise in many ways including worn spray jets, pressure too high or low, correct chemical rate/ha but wrong water volume, inefficient control of spray direction. The only way to be sure that your coverage is good is to do a dye test. Tank mixing problems can weaken chemical effectiveness. (see insecticide diagnosis and correction)
Spray program weaknesses are more likely than resistance as a cause of spray failure. Resistance would usually show as a gradual failure of the chemical and spray program error would usually show as a sudden failure of chemical that has been working well. However there may have been a long-term flaw in the spray program causing inconsistent results.
3) KEEP CHECKING TO SEE IF PESTS INCREASE AFTER A SUCCESSFUL SPRAY APPLICATION
ACTION:Keep monitoring, especially on leaves and flowers to see how quickly pest levels increase again.
- If they increase significantly in the next 2-5 days it is likely they are emerging from a build up of eggs and pupae in the crop, especially if they have been left to build up in the crop for a while. If this happens 2 more sprays of the same chemical are needed close together
- If they increase later than one week after spraying there has probably been a new flight of pests into the crop. If you spray soon enough one spray should kill the new adults before they start breeding. Keep monitoring to see if this cleans them up.
Sudden increase, probable new flight into crop
4) IF NONE OF THESE IMPROVE PEST CONTROL THEN PROBABLY YOU HAVE A RESISTANCE PROBLEM, ESPECIALLY IF YOU HAVE NOT BEEN ROTATING YOUR CHEMICAL GROUPS CAREFULLY ENOUGH OR TANK MIXING DIFFERENT GROUP
