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Bug Word of the Day: Phytophagous

The word “phytophagous” is an adjective that means “plant-eating,” and it’s typically used to describe arthropods that feed on the tissues of living plants, but not those that consume pollen or nectar.

You’ve probably also heard the term “herbivorous,” which also means “plant-eating.” But the term “herbivorous” is usually applied to hooved mammals, such as cows, sheep and goats, whereas “phytophagous” almost exclusively is used for invertebrate organisms.

Lots of insects are considered to be phytophagous – at least 500,000 species, representing most of the insect orders. Many of them have little or no impact on human activity, but some phytophagous arthropods are serious crop pests. For example, the Asian citrus psyllid, which transmits the pathogen that causes citrus greening disease, is a phytophagous insect that feeds on the sap of citrus trees. (Read more about the Asian citrus psyllid in our Big Money Bugs profiles series.)

All arthropods that feed primarily on plants earn the designation “phytophagous,” regardless of whether they attack roots, stems, leaves, flowers and fruits; whether they feed on plants during larval stage, adult stage, or both; and regardless of whether they feed on external or internal plant materials.

Phytophagous arthropods can also be grouped into three classifications, depending on the number of plant species they consume.

The pickiest eaters will only feed on one plant species, or, at most, a small group of closely related species – we call these species “monophagous.” Most of the arthropods used in biocontrol efforts against weeds have this characteristic, which is important because the people using biocontrol want to minimize the chances of unintended damage to non-target plants. A good example here is the tropical soda apple leaf beetle, one of our Bug of the Day selections this week.

At the other end of the spectrum are the generalists, which feed on a broad range of plants. One example here is the twospotted spider mite, which will feed on more than 200 plant species – it’s another Bug of the Day selection. We refer to these species as “polyphagous.”

Finally, there are the bugs in the middle, arthropods that can feed successfully on a modest range of plants – perhaps some or all of the plant species in one family, for example – if the plants have some similarities. These arthropods are called “oligophagous.”

Why some phytophagous insects are monophagous (specialists) and others are polyphagous (generalists) is a much-debated question in entomological circles.

Some scientists believe that dietary preferences evolved in tandem with arthropod reproductive habits, and the result was that generalists tend to produce more offspring, but the offspring of specialists tend to have higher survival rates. Other scientists believe that specialized dietary habits evolved as a response to plant chemistry – this hypothesis suggests that certain insect species developed a stronger and stronger association with certain plants because the insects were better adapted to feeding on those plants, compared with other insect species. Over time, spontaneous mutations and natural selection in the plants and in the insects could cause one insect species to feed more and more successfully on one plant species, and over many generations that insect would stop feeding on other plant species altogether.

Phytophagous insects and other arthropods, regardless of whether they’re generalists, specialists or somewhere in the middle, can have huge impacts on agricultural production. As mentioned previously, there are some phytophagous insects that help people manage weeds and save money. But, for every phytophagous insect that helps humans keep money in their pockets, perhaps 100 other species do just the opposite.

But take heart, Florida. By uncovering new facts about the eating habits of bugs, UF/IFAS experts can help determine why insects are attracted to certain plants and then develop ways to discourage the attacks. These results are used to inform efforts to produce resistant crops, to help make IMP plans with better management techniques (read about IPM in another of this week’s Bug Word of the Day selections), and even to help develop strategies that use insect behavior to coax pests away from valuable crops and into places where they won’t cause economic damage.

This write-up was informed by material from Encyclopedia of Insects, 2nd edition, 2009; Academic Press, edited by Vincent H. Resh and Ring T. Cardé.

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Photo by USDA-ARS