Take a walk on the wild side: Wild plants are better at repelling insect pests than domesticated varieties

Introduction

Humans have domesticated plants by selecting for traits such as better taste and yield, and this selection has changed how crops interact with the environment compared with wild plants. This study seeks to find out if domesticated plants are more vulnerable to herbivorous insects compared to their wild relatives.

Research Process

Researchers performed a series of predator and performance experiments to test how attracted predators and herbivores are to domesticated commercial tomato cultivars, landrace tomato plants (a locally adapted, domesticated variety), and wild tomato plants. The researchers also evaluated how effective the plant’s own indirect defense response was to an herbivore attack. These defenses are called herbivore-induced plant volatiles (HIPVs), which are defense responses which occur in reaction to herbivore saliva, plant damage or egg deposits. These HIPV defenses are able to attract helpful natural enemies and/or repel herbivores.

The study placed tobacco hornworm caterpillars (herbivore Manduca sexta) on domesticated, landrace and wild tomato plants, in cages and in the field. They introduced two “natural enemies” of this herbivore:

1. A stilt bug (Jalysus wickhami), an egg predator

2. A wasp (Cotesia congregata), a larval parasitoid

Both of these insects parasitize and/or attack tobacco hornworm caterpillars. Researchers wanted to see which natural defense response these natural enemies of the caterpillars were more attracted to: the domesticated or wild tomato plants.  

Results

Using an olfactometer (an instrument used to detect and measure odour preferences), the wasp and stilt bug – natural enemies of the herbivore – consistently preferred the chemical signals of wild tomatoes over domesticated cultivars, with landraces in between. Decision speed was also affected by domestication: predators quickly responded to the chemical signals of herbivore damaged wild plants, but were slow to recruit to domesticated varieties.

The study also followed adult hornworm preferences when they were moths. Female moths preferred to lay eggs on the more suitable domesticated plants than on the more toxic wild tomatoes, indicating that insect responses to plant odours vary depending on where the insect is on the food chain. Field trials confirmed odour preference tests: caterpillars taken from wild tomato relatives were more likely to be parasitized than those taken from landraces or domesticated tomatoes.

These results suggest that tomato domestication has reduced how well plants deploy their defense response in attracting helpful predators compared with their wild relatives. This outcome will affect how we understand plant defenses at different levels of the food chain, and also how compatible plants and natural enemies are in defense against herbivores in agriculture.