# Effects of Parasitism on the Population Growth of Toumeyella martinezae (Coccidae) in the Presence of Its Mutualistic Ant Liometopum apiculatum (Formicidae) in an Arid Region of Central Mexico

## TL;DR

This study examines how ants and parasitoid wasps influence the population growth of a scale insect pest in a Mexican cactus, finding that ants play a more significant role in sustaining the pest than the wasps.

## Contribution

The study reveals that mutualistic ants are more critical for sustaining the scale insect population than parasitoid wasps in an arid agroecosystem.

## Key findings

- Parasitism rates were higher when ants were present (18.66%) compared to when ants were excluded (5.42%).
- Scale population growth was highest in the absence of parasitoids (λ = 1.532) and negative when ants were excluded (λ = 0.636).
- Ant presence is the primary factor sustaining positive scale population growth, while parasitoids alone are insufficient for effective control.

## Abstract

Pests in agroecosystems reduce crop productivity. Parasitoid wasps are effective biological control agents for agricultural pests; however, their role in natural ecosystems remains uncertain. In the arid regions of central Mexico, the soft scale insect Toumeyella martinezae infests the cactus Myrtillocactus geometrizans, the fruits of which are highly valued locally. This scale insect maintains a mutualistic relationship with ants and is targeted by a parasitoid wasp. We evaluated the effect of the ant presence on the parasitoid’s effectiveness in controlling the scale insect. Using exclusion experiments, we measured parasitism rates and scale population growth under varying combinations of ant and parasitoid presence. Parasitoids reduced scale population growth, but not sufficiently for effective pest control. Ant presence was more important in sustaining scale populations with positive growth rates. Effective management of this pest should therefore focus on reducing the ants’ protective role, with parasitoids used as a complementary strategy.

The soft scale Toumeyella martinezae infests the arborescent cactus Myrtillocactus geometrizans. This scale is, in turn, parasitized by the wasp Mexidalgus toumeyellus and forms a mutualistic relationship with the ant Liometopum apiculatum. This study assessed how ant and/or parasitoid presence influenced parasitism rates and the population growth of the scale insect. Experimental treatments included scale populations with ant access (control) or ant exclusion, and parasitoid exclusion with ant access. Scale population growth rates were estimated using Lefkovitch projection matrices, built based on the individual monitoring of approximately 5400 scales. The average parasitism rate was higher in the “with ants” treatment (18.66%) than under ant exclusion (5.42%). In the absence of parasitoids, the scale population growth rate (λ = 1.532) was 8% higher than in the control treatment (λ = 1.423). Population growth was negative (λ = 0.636) when ants were excluded. These results indicate that interaction with the mutualistic ant is the primary factor sustaining a positive scale population growth. In contrast, the impact of the parasitoid alone is insufficient for effectively controlling the soft scale pest.