What ‘bugs’ New Zealand’s hop growers?

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Many people would be aware that New Zealand has been isolated from the rest of the world until relatively recent times. What the visitor sees is an abundance of unspoilt natural beauty and picture-postcard scenery.

By Doug Donelan

Fortunately, as well, there are pristine fertile valleys situated at forty one degrees south, so for those who don’t know, that means you can grow hops there! But wait it gets better… disease-free hops. That’s right; none of the fungal diseases such as the mildews or wilt that can have significant impact on crop health in other parts of the world. These diseases can have a devastating effect on hops and are generally combated by spraying with approved fungicides.

This affords the New Zealand growers the opportunity to supply spray-residue-free hops, except there is one drawback, diseases are not the only enemy lurking in the garden, hops also fall prey to pests. One that is a particular nuisance is the two-spotted mite (Tetranychus urticae) a very tiny pest, but a really big problem. And not just in middle earth either; these mites reap havoc through hop gardens across the planet.

 

 

 

 

Predaory mite Phytoseiulus persimilis
 

Figure 1. Predaory mite Phytoseiulus persimilis (right) feeding on the twospotted mite (Tetranychus urticae).
Photo courtesy of Zonda Resources New Zealand.

So, how do you get rid of them, remain residue-free, and retain the balance in the garden? Strangely, the answer is to introduce more mites. That’s right more mites, but in this instance Phytoseiulus persimilis (Figure 1). A ferocious predator of the harmful mite!

The predators are grown up in hothouses and applied to the plants as they’re approaching “burring”. This is just prior to the hop cones forming while the warm, late summer conditions are really beginning to spark up the two-spotted mites (TSM). The predators go to work to dramatically reduce the numbers of the TSM. The numbers are monitored and it may be that outbreaks occur in ‘hot spots’, but these can be brought under control by introducing more predators to those areas.

A balance is required however, as it’s necessary to have sufficient TSM for the predators to feed on or they’ll die out and this would then open up an opportunity for the TSM to regather numbers and fight on.

North Westerly aspect of New Zealand’s pristine hop growing region at Forty One degrees South  

Figure 2; Northwesterly aspect of New Zealand’s pristine hop growing region at Forty One Degrees South.

 

Application consists of two methods. The original method employed has been to grow up whole colonies, which include adults, nymphs and eggs of predator mites on bean leaves and place these into a hop bine (Figure 2) in the garden, which need only be every fourth bine as the colony will spread and multiply as it hunts.

The pickings are good so this colonisation is fairly rapid. A newer method currently being trialled consists of adult mites being supplied in a media that can be sprinkled onto the bine (Figure 3) again every fourth plant. Application is easier and only when results are in from the trials will it be seen which of the two methods is most effective.

 

 

Bean leaf placed on a vine in the garden
 

Figure 3; Original method of application has whole colonies of mites that have been grown up on a bean leaf placed on a bine in the garden.

Anecdotally it has been said that the whole colonies on the bean leaves have a more readily available, longer-term impact due to having maturing juveniles at the ready. Other growers have said that getting a fighting force of adults immediately started into the carnage is preferable, however as previously stated, this is anecdotal and only further analysis of the trials will tell. One thing’s for certain though; it works.

On the question of economics within the agronomics it can be said that this approach is more expensive than the alternative, which of course is to spray. It’s also more labour intensive, and some may argue that it’s not as effective as pesticides. The truth is that pesticides are too effective; they indiscriminately kill everything, the beneficial insects, mites and beetles as well as the harmful ones. And they also present the very real potential of leaving chemical residues in the final product.

One grower was quoted recently as saying, “after seven years of being totally pesticide free" within his gardens there are "armies of natural predators". This of course includes everyone’s childhood favourite the Ladybird (Hippodamia convergens). These marvellous little creatures are frontline casualties when the sprayers get wheeled out of the shed and hitched up to the tractor. Who wants that?

Adult mites supplied in a media that can be sprinkled on to the bine  

Figure 4. Newer (trial) method with adult mites supplied in a media that can be sprinkled on to the bine.

 

The decision taken by the New Zealand growers to be residue free has been unanimous and this practice is employed because their environment allows them to, and they want to do it. Admittedly not everyone else in other regions around the world has that luxury. The New Zealand hop grower is uniquely placed to be able to make a philosophical commitment while facing commercial realities in a highly competitive world market. Forty One Degrees South retains its pristine valleys (see Diagram 4) and the hops are just a little greener for it.

 

The author

Doug Donelan is the recently-appointed Chief Executive Officer of New Zealand Hops Limited. Doug’s background is in brewing and prior to taking up his new role he was Head Brewer for Lion Nathan’s Malt Shovel Brewery.

Acknowledgements

Technical, Hort Research New Zealand and Zonda Resources, New Zealand.