Is Anything Feasting on Those Seedpods?: Ecology Explorers Protocol for Studying Bruchid Beetles and Desert Legumes

The Sonoran Desert has one of the most diverse native plant and animal species of any desert in the world. Many trees are uniquely adapted to living here and they are part of food webs that include animals and insects also are adapted to desert living. Phoenix is an expanding urban area located within the Sonoran Desert.

One Sonoran Desert plant that has found its way into urban landscapes is the Palo Verde tree. In the desert, this tree supports a community of animals including bruchid beetles. These insects lay their eggs on Palo Verde seedpods and the larvae feed on the seeds.

The interactions between plants and animals can provide vital information on the health of an ecosystem. Ecologists are interested in determining whether the interactions between urban Palo Verde Trees and bruchid beetles is similar or different than their interactions in undisturbed Sonoran Desert communities.

You can provide important data by investigating the Palo Verde trees and bruchid beetle populations in your schoolyard.

What are they?

Bruchid beetles eat the seeds produced by Palo Verde trees. Palo Verde trees are native to Arizona's Sonoran Desert. They also are common in landscaped areas including parks, schoolyards and backyards.

Female bruchid beetles lay their eggs on Palo Verde seedpods. After they hatch, beetle larvae eat the seeds, pupate and emerge as adult beetles through little holes they create in the seed pods.

Why study them?

Plants are at the base of food webs and any change in a plant community can have an impact on all organisms that feed higher up the food web.

We don't really know whether Palo Verde trees growing in a city have different seedpod development as compared to those in the Sonoran Desert.

We also don't know how (or even if!) urbanization of the trees has changed the interaction between bruchid beetles and Palo Verde trees.

There might be many reasons that urban trees will grow differently than desert trees. For example, trees in yards and parks may receive more water and fertilizer which could increase the number of seeds produced.

What do they tell us about our urban ecosystem?

Whether or not a seed will be eaten by a beetle depends on many different things. If a female beetle finds a seed, it must lay an egg on it, and then the bettle larvae must be able to eat the seed and survive. All these factors could be different in the city versus the desert. For example, the city seeds might be more or less attractive to the beetle, just as food prepared in different ways might be more or less appealing to you. Even if the beetle likes the seed the larva may not be able to survive feeding on it. For example a particular seed might be too small or too tough to eat.

CAP LTER ecologists are interested in the impact humans have had on native plants growing in cities and the insects that feed upon them. Your investigations might contribute to this understanding.

What Materials Will You Need?

• Ziplock bags
• Data Sheet
• Pencil
• Magnifying Glass
• Data Sheet

Bruchid Beetles and Seedpods Protocol

1. Choose a site that has either Blue Palo Verde (Cercidium floridum) or Foothills Palo Verde (C. microphyllum.) Palo Verde trees are distinctive because they have green trunks. [photos]

2. Identify your collecting site as either desert or urban.

3. Collect 30 pods from each site. Pods should be collected directly from the trees.

4. Collect 5 pods from 6 different trees. (If you cannot find 6 trees then collect an equal number of pods from each tree for a total of 30.) Be sure to collect the 5 pods from different locations and different heights on the tree.

5. Assign a number to each tree.

6. Put all the pods from one tree in one ziplock bag. If you collect 5 pods from 6 trees then you will need 6 bags (one for each tree) and each bag will have five pods. Put a label in the bag with the site and tree number on it.

7. Once inside your lab, examine each pod, give it a number, and then:

a) count the number of bruchid beetle emergence holes. Bruchid beetle holes are round and 1-2 mm in diameter;

b) count the number of seeds each pod contains.