It’s not the opposite that is true but that the net effect is not that negative.
Anecdotally, from my backyard food growing experience, biodiversity increase means more pests, but once more species come in, there comes a balance that exists in nature. Year 1 of my backyard farming, I had plentiful bounty. All my crops did extremely well, and I thought I was some kind of genius that disproved the decades of agricultural progress re:chemicals in farming. Year 2, was my biggest and most ambitious growing season and I could barely get anything out of it. The pests came, from insects to rodents and nothing survived. Year 3 and year 4 were my learning days. I introduced more plants to my garden, native plants, plants that provided food in late summers to next spring for the fauna, more varieties of food crops, etc. I welcomed everything in my garden, including pests. Eventually I hit a balance. Aphids suddenly affecting plants? The wasps are here. Rats and squirrels eating my strawberries? The owls and crows came to rescue. Slug problem? The neighborhood possum took care of it (though I had to put up a fence around some of my veggies for the possum). Overall I think with the constraints of the size of my backyard, I still need some proactive measures of pest control, but most of the time nature takes care of it and makes sure the problem is only temporary.
The exact opposite is true.
https://www.caryinstitute.org/science/research-projects/biod...
In ecosystems where predators are removed, the population of their prey, in this case, mice, often increases due to a lack of natural control. This process can lead to an increase in the tick population, as ticks often feed on mice. This is especially true for the white-footed mouse, which is a common host for ticks in the eastern and central United States. When the mouse population increases, ticks have a higher chance of obtaining their first blood meal from a mouse, which often carry the bacterium that causes Lyme disease. (less predators => more mice => more ticks)
https://theconversation.com/as-biodiversity-declines-on-corn...
As biodiversity declines on corn farms, pest problems grow
"We found that communities with stronger networks – that is, more linked species – had fewer pests. Not only this, but network centrality was important; communities with several groups of highly linked species, including insects that prey on others, don’t have the pest problems that loosely linked networks have. Thus, in addition to species diversity, the strength of interactions among species within a community seem to be related to when and where pest outbreaks occur. This research suggests that practices that reduce diversity in cornfields will aggravate pest problems over time."
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4646789/
Decisions that reduce insect diversity and decrease the network strength in insect communities result in higher pest abundance.
https://www.pnas.org/doi/10.1073/pnas.1821039116
Tree diversity regulates forest pest invasion
https://onlinelibrary.wiley.com/doi/book/10.1002/97811182318...
Biodiversity offers great potential for managing insect pests. It provides resistance genes and anti-insect compounds; a huge range of predatory and parasitic natural enemies of pests; and community ecology-level effects operating at the local and landscape scales to check pest build-up.