Monarch butterflies don’t like aphids any more than humans do

The common aphid, a diminutive pest that frequently wreaks havoc in gardens, has been identified as a significant threat to the life cycle of the cherished monarch butterfly. This disturbing connection was recently discovered in a study carried out by scientists from the University of Florida.

These small pests, specifically oleander aphids, were observed in this new research to interfere with the development of monarch butterflies on tropical milkweed. This non-native milkweed species is extensively used across the southern U.S., stretching from California to Florida.

Monarch butterflies rely heavily on milkweed and similar plants to complete their life cycle, which is now being compromised due to aphid infestation.

“When aphids attack tropical milkweed, they compromise this monarch resource,” says the study. These findings paint a concerning picture for efforts to sustain monarch butterfly populations across the country. Many initiatives are focusing on planting milkweed in urban areas to support monarchs. But aphids and similar pests often reach high densities on plants, including milkweed, particularly in these urban environments.

“Efforts are underway to plant milkweed in urban areas to support monarch populations. Our study helps us better understand how such pest outbreaks may affect monarch survival on the most common ornamental milkweed species produced and planted in the South,” Adam Dale, senior author of the study and an associate professor in the UF/IFAS entomology and nematology department, elaborated on the implications of their findings.

Aphids, tropical milkweed, and monarch butterflies

Tropical milkweed is widely used in the southern U.S. to both attract and sustain monarchs, and as an ornamental plant. Yet, it often becomes a hub for oleander aphids that target both oleander and milkweed plants. These pests drain the sap from the plants, resulting in stunted growth and leaving behind a moldy residue.

Bernie Mach, first author of the study and a postdoctoral researcher in the UF/IFAS entomology and nematology department, expressed, “It’s long been known that oleander aphids flock to milkweed, especially in nurseries and urban areas, and that led us to wonder if and how that affected the monarchs who used these plants.”

The scientists did not identify precisely why aphid-infested plants prove less accommodating for monarchs. Nevertheless, they suggested that the combination of past research on aphids’ effects on tropical milkweed and their current findings offers some valuable insights.

“Tropical milkweed has particularly high levels of cardenolides that ramp up even more when it is attacked by large infestations of oleander aphids. We think that these ramped-up levels may deter monarchs from laying eggs on these plants and also affect their caterpillars,” Mach further explained.

What the researchers learned

However, a critical discovery from the study was clear – monarch butterflies have a higher chance of survival on aphid-free tropical milkweed. When the researchers cultivated tropical milkweed in a nursery setting, they found that the butterflies laid three times as many eggs on aphid-free plants.

The resulting caterpillars on the aphid-free plants also ate twice as much leaf material and reached their full size, unlike those on the aphid-infested plants, many of whom fell behind in growth or died.

For gardeners in the southern U.S. looking to support monarch butterflies through their landscaping efforts, the researchers recommend considering native milkweed species like swamp milkweed. These species have lower cardenolide levels, and other studies have shown that monarchs thrive on these plants even in high aphid densities.

For those keen on using tropical milkweed as a monarch aid, an effective way to control oleander aphids is suggested by the researchers: insecticidal soap. Bernie Mach clarified, “Spraying the aphids directly with insecticidal soap — while avoiding monarch caterpillars and butterflies — is an effective way to keep oleander aphids down and help tropical milkweed stay in better shape.”

Nonetheless, implementing this solution on a larger scale, such as in nurseries where there are hundreds or thousands of plants, isn’t always feasible. As a response to this challenge, in the next phase of their research, Dale and Mach plan to explore pest management options that maintain low aphid levels and pose no harm to monarch butterflies.

More about aphids

Aphids are a type of insect belonging to the superfamily Aphidoidea, known for their small size and extensive damage they can cause to plant life. These tiny, soft-bodied insects, also called plant lice, greenflies, or blackflies, are among the most destructive pests to both cultivated and wild plants in temperate regions.

Identification

Aphids vary greatly in color, from green and black to yellow, red, or brown, depending on their species and diet. Adult aphids generally measure between one and ten millimeters long, and their body shape is pear-like. They possess two compound eyes, a pair of antennae, and two tubes, known as cornicles, protruding from their hind end, which is a unique feature among aphids.

Aphids life cycle

Aphids exhibit a fascinating and complex life cycle which includes both sexual and asexual reproduction. In spring, female aphids hatch from overwintering eggs and begin to reproduce asexually, giving birth to live young through a process called parthenogenesis. These offspring, called nymphs, mature in about a week and then start producing their own offspring. This rapid, asexual reproduction leads to quick population growth, often leading to infestations.

As summer turns to fall, aphids switch to sexual reproduction. The females produce eggs which overwinter and hatch the following spring, starting the cycle anew. This pattern allows aphids to adapt to changing environmental conditions.

Diet and damage to plants

Aphids are sap-sucking insects that feed on the juices of plants, specifically the phloem sap, which is rich in sugars. The feeding of aphids can lead to the yellowing and curling of leaves, stunted growth, and in severe infestations, the death of the plant.

In addition, aphids secrete a sticky substance known as honeydew, which can lead to the growth of sooty mold, further damaging the plant and reducing photosynthesis.

Role in disease transmission

Apart from the direct damage, aphids are notorious for transmitting plant viruses, acting as vectors. There are hundreds of plant diseases known to be spread by aphids, causing significant agricultural losses every year.

Natural enemies and control

Aphids have many natural enemies, including ladybugs, lacewings, and parasitic wasps, which help keep aphid populations in check. Aphids also fall prey to birds and other insects.

In agriculture and gardening, aphids can be controlled using various methods. Biological control measures involve introducing natural predators. Chemical control includes using insecticidal soap or synthetic insecticides, although care must be taken due to the potential impact on non-target species. A combination of these methods often provides the most effective control of aphid populations.

Relationship with ants

Aphids have a unique symbiotic relationship with ants, in which ants protect aphids from predators and parasites in return for honeydew. This association, although beneficial for aphids and ants, further contributes to the harmful impact of aphids on plants.

In summary, aphids, while small in size, can have a major impact on plant health due to their feeding habits and ability to transmit diseases. Understanding their life cycle and behavior can aid in the development of effective strategies to control their populations, benefiting both agricultural and natural environments.

More about monarch butterflies

The monarch butterfly, scientifically known as Danaus plexippus, is one of the most recognized and studied butterflies worldwide, renowned for its vibrant orange and black wings, as well as its extraordinary migration pattern.

Identifying monarch butterflies

Monarch butterflies are easily distinguishable by their wing color and pattern. Adults exhibit bright orange wings, with black veins and a black border dotted with white spots. Monarchs typically have a wingspan of 3.5-4 inches, and males can be differentiated from females by the two black spots on their hindwings, which are scent glands used during mating.

Life cycle

The life cycle of a monarch butterfly consists of four distinct stages: egg, larva (caterpillar), pupa (chrysalis), and adult. Female monarchs lay their eggs on the underside of milkweed plants, which upon hatching, serve as the sole food source for the caterpillars. After two weeks, the caterpillar forms a chrysalis, within which it metamorphoses into a butterfly over a period of about two weeks. Adult monarchs live for 2-6 weeks, except for the generation that migrates, which can live up to 8 months.

Migration

One of the most remarkable characteristics of the monarch butterfly is its long-distance migration. Each fall, millions of monarchs travel up to 3,000 miles from North America to specific sites in central Mexico (or coastal California for a smaller population) to overwinter. In spring, they return north. This migratory generation goes through a unique state of reproductive diapause, delaying reproduction until the journey back.

Diet

As adults, monarch butterflies primarily feed on the nectar from flowers, providing them with sugars for energy and other nutrients necessary for reproduction. As larvae, they feed exclusively on the leaves of milkweed plants, which contain toxic compounds known as cardenolides. This diet renders monarchs unpalatable to predators, providing them with a potent defense mechanism.

Conservation

Monarch butterflies are currently facing several conservation challenges. The primary threats include habitat loss, both in their summer breeding grounds due to the decline of milkweed and in their overwintering sites due to deforestation. Other threats include climate change, pesticides, and disease. Despite these challenges, monarchs are not currently listed as endangered, though conservation efforts are ongoing.

In summary, monarch butterflies, with their striking appearance and phenomenal migration, play a significant role in pollination and serve as indicators of environmental health. Their survival hinges on the conservation of their habitats and the milkweed plants they rely on throughout their life cycle. Understanding and appreciating these fascinating creatures can aid in their conservation and ensure they continue to be a cherished part of our natural world.

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