When Plants Get Sick, Pests Thrive: The Unseen Battle in Cranberry Bogs

Imagine a world where a plant infection doesn't kill its host but instead turns it into a gourmet feast for insects. This intriguing, and concerning, scenario is playing out in cranberry bogs, according to a recent study led by Professor Rodríguez-Saona, Rutgers University — P.E. Marucci Center, Chatsworth, NJ. The researchers explored how a specific phytoplasma infection, known to cause false blossom disease in cranberries, dramatically alters the plants, making them more vulnerable to both aboveground and belowground insect herbivores.

            Phytoplasmas are Mollicutes, unique bacteria (including mycoplasmas) that lack a cell wall and exclusively infect a plant's phloem, the tissue responsible for transporting sugars and nutrients. They are transmitted by phloem-feeding insects like leafhoppers. In cranberries (Vaccinium macrocarpon), infection by the phytoplasma Candidatus Phytoplasma sp. subgroup 16SrIII-Y leads to false blossom disease. This disease causes a range of debilitating symptoms, including deformed flowers, sterile ovaries, and "witches' brooming" (abnormal clusters of shoots), ultimately resulting in significant yield losses for the cranberry industry.

            Prior research has shown that this phytoplasma can manipulate its cranberry host, benefiting both itself and the insects that feed on the plant. This phenomenon aligns with the "host manipulation" hypothesis, which suggests that pathogens can modify their host's characteristics for their own benefit.

            The recent study specifically investigated the impact of this phytoplasma infection on two key insect herbivores:

• The spongy moth (Lymantria dispar), a well-known foliar (leaf) feeder.

• The oriental beetle (Anomala orientalis), a root feeder.

Researchers used both phytoplasma-infected and uninfected cranberries of two common genotypes, "Ben Lear" and "Crimson Queen," to see if the effects varied. Key Findings on Herbivore Performance:

• Spongy Moth (L. dispar): Larvae feeding on infected cranberry plants generally exhibited larger sizes and more efficient tissue consumption.

  • They were twice as big on infected plants.

  • They damaged 70% more leaves and consumed 56% more leaf material on infected plants.

  • Their efficiency of ingested food conversion was 51% higher on infected plants.

  • These effects varied by cranberry genotype, with some impacts, like increased larval growth, being more pronounced in the "Ben Lear" genotype.

• Oriental Beetle (A. orientalis): This study provides the first case demonstrating that phytoplasma infection can benefit a subterranean herbivore.

  • Larval survival was 15%–30% higher on infected plants.

  • Despite consuming 50% less root tissue on infected plants, their size increased by 37%–55%.

  • This led to 2.2 times higher ingested food conversion, indicating that infected roots offered improved nutritional quality to the oriental beetle larvae.

  • Unlike the spongy moth, genotype had minimal influence on the oriental beetle's survival and growth.

    
    

The reason for this increased herbivore performance lies in the profound changes the phytoplasma induces in the cranberry plants.

• Infected plants showed stunted shoot and root growth. Uninfected plants were 40% longer and 25% heavier in shoots, and their roots were 43% heavier than infected plants.

• Critically, phytoplasma infection was associated with elevated nitrogen content in both the shoots and roots of infected plants. Nitrogen is a key nutrient for insect herbivores, and its availability often limits their growth.

• Infected plants also had significantly lower levels of defensive compounds called proanthocyanidins (PACs). PACs, also known as condensed tannins, are important antiherbivore compounds. Specifically, PACs were 50% lower in leaves and 67% lower in roots of infected plants.

These findings suggest that the phytoplasma "manipulates plant chemical composition by increasing nutrient levels and decreasing defensive compounds," thereby enhancing herbivore performance both above and below ground. Interestingly, the study found that the "Ben Lear" genotype, a native wild selection, had a higher phytoplasma titer than the cultivated "Crimson Queen".

            From an applied perspective, the results are clear: high levels of phytoplasma infection could significantly worsen pest problems in cranberry beds. This increased susceptibility to a community of insect herbivores, both above and below ground, may necessitate more intensive pest monitoring and stricter control measures. This research highlights the intricate and often detrimental interplay among plants, pathogens, and insect herbivores in agricultural systems. Future studies are crucial to develop methods that can prevent phytoplasma from manipulating the host plant or enhance the plant's resistance to both the disease and its associated herbivores.

Rodriguez-Saona C, Salazar-Mendoza P, Holdcraft R, Polashock J. Phytoplasma infection renders cranberries more susceptible to above- and belowground insect herbivores. Insect Sci. 2025 Jun;32(3):957-972. doi: 10.1111/1744-7917.13444. Epub 2024 Sep 15. PMID: 39279278; PMCID: PMC12175984.

https://onlinelibrary.wiley.com/doi/10.1111/1744-7917.13444