The commercial orchards require a lot of chemical input. There used to be a factory at Mapua that made agricultural chemicals, many of which have been banned.
As reported by New Zealand Geographic:
…this sad and barren land is a highly toxic waste area laced with agricultural poisons strong enough to kill the hardiest plague of insects. Lying empty and abandoned since 1988 when the Fruitgrowers Chemical Company (FCC) finally closed after 56 years of manufacturing a cocktail of toxic agricultural chemicals and pesticides, the site has sat unused and unusable.
The legacy of the factory is still being felt with contaminated land that cannot be used for home-grown vegetables (as instructed by a recent letter from the council to residents). And it can’t be a coincidence that there are very few mosquitoes or sandflies around Mapua…one of the nicer side effects of being near a toxic chemical plant that has contaminated land and water far beyond the confines of the area that has been remediated .