Larva

larva [ LAHR-vuh ] noun, plural larvae [ LAHR-vee ], also larvas: wingless immature insect, often worm or caterpillar shaped, that hatches from the egg and turns into a pupa or chrysalis before becoming an adult insect

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Larvae are part of the process of complete metamorphosis, in which insects develop through four basic life stages: egg, larva, pupa, and adult. The larvae emerge from eggs laid by mature insects such as butterflies, moths, beetles, and others. (Read about The Life Cycle of Butterflies & Moths.) The larvae then often feed exclusively on the host plant where the eggs were laid. Gardeners interested in helping pollinator insects like butterflies need to be aware of the host-plant relationship between the desired species.

One of the first of these relationships that many of us learn about is that between the monarch butterfly (Danaus plexippus), a species in decline, and milkweed (Asclepias), the only plants eaten by the monarch’s caterpillars (larvae). But, as shown in the lepidopteran host plant charts referenced below, other butterflies and moths can be similarly specific about their larval hosts.  When planning pollinator gardens, essentially collections of larval host and nectar plants, gardeners should choose plants that provide nectar and pollen to attract desired adults, who can then lay their eggs on nearby larval host plants.  Massing plants for visibility from the air is a consideration. Several plants are generally more visible and better attractors than lone specimens. Another consideration is locating host plants where they can remain undisturbed, as defoliated stems, dead foliage, or flowers, which may harbor eggs or developing larvae, should not be tidied during the season. (See Creating a Monarch Waystation and Gardening to Attract Butterflies and Moths.)

Monarch Larvae Exclusively Eat Milkweed Species
while Black Swallowtail Larvae Eat A Variety of Umbellifers

Monarchs left to right: Adult on Asclepias syriaca, early instar on A. syriaca, late instars on A. incarnata and A. tuberosa, and larva ready to pupate.
Black swallowtails left to right: adult on rue, early instar on fennel, late instars on dill and parsley, larva ready to pupate.

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Larvae can be herbivores, predators, or scavengers, but gardeners are largely concerned with the herbivorous ones. The fact that the larvae emerge from their eggs hungry and needing nutrition means that some parts of the host plants, often leaves but also other parts, will get eaten. In the case of many pollinators the damage is usually short-lived and largely cosmetic, seldom a threat to the health of the plant. But this is not always the case. Many plants do suffer considerable damage from caterpillars, worms, the larval forms of beetles, and also grubs and borers in lawns or some garden soils, where they attack the roots and nutrient systems of plants including grasses, vegetables, ornamentals, and even woody plants.  

Herbivorous larvae depend on their host plants for adequate nutrition, and studies have shown that host plant quality matters to the survival and reproductive rates of the adult insects that eventually develop from the larvae. If plants are weakened by poor soil or adverse weather conditions to the point where they provide fewer good nutrients to the larvae, the larvae may have less effective immune systems and lower survival rates, and even if they survive to mate, may reproduce at lower levels. Larvae on very healthy plants with high nutrient values may face a different problem: those plants may attract higher levels of parasites, which may also harm the larvae.

Insect Larval Types

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While many gardeners may grow milkweed, fennel, pipevines and other specific host plants to attract desirable butterflies, moths, and other pollinating insects, vegetable gardeners, for example, often take a different view, regarding the larval stage of certain insects as pests that are a major threat to their crops. Virginia Tech has information on numerous larvae of the non-beneficial sort for those seeking to identify and deal with pests of shrubs, trees, and many vegetables or ornamental plants. The publications listed below describe specific insect pests whose larvae can do serious damage destroying crops or even trees. These include vegetable weevils that attack cole crops (e.g., broccoli, Brussels sprouts, cabbage, cauliflower, collards, kale), tomatoes, root vegetables, and beans; lilac/ash borers that tunnel through sap wood; black vine weevils that are destructive on yews, hemlocks, rhododendrons, cherry laurel and other broad-leafed evergreens; eastern tent caterpillars; fall webworms; celery leaftiers; and more. The larvae can take the form of large or small grubs, caterpillars, hornworms, and other wormlike creatures. While both adults and larvae may attack leaves and other surface parts, some larvae also live below ground and attack roots and tunnel further through stems.

To limit plant damage, it is not only important to be able to identify larval pests but to distinguish larval predators, such as lacewings, lady beetles, and syrphids. Instead of eating plant parts, these beneficial larvae feed on soft-bodied garden pests like aphids, mealy bugs, mites, and thrips that can drain plants of nutrients, discolor foliage, cause plant decay, or spread disease. Some farmers grow flowering plants near their cash cops specifically to attract these adult beneficials, which then lay their eggs nearby where their larvae can prey on aphid and other crop pests.

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References

Lepidopteron Host Plants

• Carman D. 2017. Butterfly Larval Host Plant List. York County Master Gardener Program. PennState Extension.
• Free M. 2022. Nectar and Host Plants for Selected Mid-Atlantic Butterflies and Moths. Master Gardeners of Northern Virginia.  
• Pettis S. 2021. Plants That Host Butterfly Larvae. Henderson County Center. North Carolina Cooperative Extension.

Virginia Tech publications on harmful insects whose larvae can damage the leaves, roots, stems, or sapwood of plants seriously enough to even kill them in heavy infestations

• Day E. (Dellinger TA. Revised 2020). Bagworm. VCE Publication 2808-1008 (ENTO-351NP). Virginia Cooperative Extension. 
• Day E. (Dellinger TA. Revised 2020). Black Vine Weevil. VCE publications 444-210 (ENTO-354NP). Virginia Cooperative Extension. 
• Day E. (Dellinger TA. Revised 2020). Eastern Tent Caterpillar. VCE Publication 444-274 (ENTO-356NP). Virginia Cooperative Extension. 
• Day E. (Dellinger TA. Revised 2020). Fall Webworm. VCE Publication 2808-1013 (ENTO-357NP). Virginia Cooperative Extension.   
• Day E. and Dellinger TA. 2021. Fungus Gnats. VCE publication 3104-1579 (ENTO-442NP). Virginia Cooperative Extension. 
• Day E. and Dellinger TA. 2021. Rose Chafer, VCE publication 3104-1564 (ENTO-436NP). Virginia Cooperative Extension.   
• Day E. and Dellinger TA. 2021 Vegetable Weevil. VCE publication 3104-1569 (ENTO-425NP). Virginia Cooperative Extension. 
• Dellinger TA and Day E. Parsleyworm. Virginia Cooperative Extension.
• Wiseman E, Gugercin S. Close D. 2013. Emerald Ash Borer. VCE Publication HORT-69NP. 

Types of insect larvae 

• Gossling N. 2023. NHAES Research for Using Flowering Plants to Attract Hover Flies as Biological Pest Controls. National Institute of Food and Agriculture. USDA.
• Syrphids (Flower Flies, or Hover Flies). Natural Enemies Gallery. University of California Integrated Pest Management Program.
• Townsend L. Revised 2010. ENTFACT-017: Recognizing Insect Larval Types. University of Kentucky College of Agriculture.