A New Zealand honey giant fears a man who raided their hives may have also infected them.
Basil Martin Lawry, 64, stole brood (eggs, larvae and pupae), honey and bees worth more than $2000 from Arataki Honey and Kintail Honey from 2016 through to 2018.
He appeared before Judge Bridget Mackintosh in the Hastings District Court on Tuesday and admitted eight charges of theft.
Arataki Honey's John Walsh said he was relieved Lawry had been brought to account for the theft but was concerned that Arataki hives may have been infected with American foulbrood (AFB) - a highly infectious bee disease - through Lawry.
Arataki Honey describes itself as the number one beekeeping business in the southern hemisphere, with 20,000 hives across New Zealand.
"So while this theft was bad enough, the possibility that he's infected our hives has compounded the problem. We're just really concerned with this kind of behaviour."
Walsh said Arataki Honey hives were constantly checked for contamination, but the disease could take up to two years to develop.
Lawry began keeping his own hives in 2015.
He would separate the frames inside the hives and place them inside a nucleus (a wooden box with a queen bee and five wooden frames inside).
If successful, a nucleus can be sold for $250-$350.
Lawry had also been grazing stock on orchards where Arataki Honey hives were located and knew their locations.
Each year in 2016, 2017 and 2018 during pollination time, he drove on to different properties carrying his own nucleus full of new wooden frames.
He used a crowbar to lever open the lid of the Arataki hives and stole four frames from each one.
He put them in his nucleus and replaced the missing frames with empty ones.
He continued this every spring and did the same thing to four Kintail Honey beehives, near Hastings.
Lawry also continued targeting Arataki Honey and drove to their hive locations on Longlands Rd, Orchard Rd and Elwood Rd.
His luck finally ran out in November last year when he went to farmland on Taihape Rd.
Little did he know he was being watched by a hive manager and fled when he was approached - dumping his equipment on the way.
The manager took a photo of Lawry's vehicle and shared it on social media. Lawry was later located by police.
He admitted that between 2016/17 his hives were suffering and he became obsessed with helping his own bees regenerate.
Lawry will be sentenced in June.