Tropical milkweed herbivore and predator dynamics in suburban South Texas

Authors: RIVERA, DANIELA - University Of Texas Rio Grande Valley; NOVAL, JOSE - University Of Texas Rio Grande Valley; ELLIOTT, LILLY - University Of Texas Rio Grande Valley; Penn, Hannah

Submitted to: Subtropical Agriculture and Environments
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/19/2021
Publication Date: 12/17/2021
Citation: Rivera, D., Noval, J.A., Elliott, L., Penn, H. 2021. Tropical milkweed herbivore and predator dynamics in suburban South Texas. Subtropical Agriculture and Environments: Research Note. 72:16-20.

Interpretive Summary: Tropical milkweed, commonly found in the subtropical United States, is often planted to help preserve monarch butterflies. Given this goal, many homeowners are limited in what they can do to control the pests common on tropical milkweed without harming caterpillars. One solution is to use naturally occurring predators and parasitoids to suppress pest populations through consumption. However, the prevalent tropical milkweed pests and common predators and parasitoid used for control have not been well documented in the Lower Rio Grande Valley as they have in other subtropical states such as Florida. To obtain this information, we studied the occurrence of pests as well as their predators and parasitoids on tropical milkweed planted in a garden and in pots in spring 2020. We found that the most common pests are similar to those in other states – oleander aphids and large milkweed bugs. Additionally, aphids but not milkweed bugs were negatively associated with lady beetles and syrphid fly larvae, indicating that these predators may be effective at suppressing aphid populations. However, we also found that when aphids were present on potted plants rather than in a garden context, fewer predators were available and other pests like mites were more likely to be present.

Technical Abstract: Tropical milkweed is commonly grown in Gulf state gardens to conserve butterflies like monarchs and other pollinators, but the presence of non-caterpillar herbivores may prompt gardeners to treat the plant. Management methods for milkweed herbivores are often limited to non-chemical methods such as biological control, but the natural enemies responsible have not been well studied in the Lower Rio Grande Valley. To address this gap, we documented the arthropod community on tropical milkweed in garden and isolated potted plant settings then analyzed associations between organisms (herbivores and natural enemies) and with branch structures (leaves, flowers, or seedpods). In the garden plot, oleander aphids and large milkweed bugs were the primary herbivores, exhibiting an overlap in seedpod preferences. Natural enemies (lady beetles, syrphid fly larvae, and parasitoids) were positively associated with peaks in aphid but not milkweed bug abundance. The arthropod community experienced similar associations but with reduced natural enemy abundance and diversity on potted plants. Our data indicate that natural enemies like syrphid fly larvae and parasitoids are important for oleander aphid management in the LRGV, but plant isolation may inhibit colonization by certain natural enemies.