# Initial Sublethal Exposure to an Argentine Bacillus thuringiensis Strain Induces Chronic Toxicity and Delayed Mortality in Alphitobius diaperinus (Coleoptera: Tenebrionidae)

## TL;DR

Exposure to a specific strain of Bacillus thuringiensis weakens beetle larvae over time, causing long-term damage and delayed death.

## Contribution

This study reveals chronic toxicity and delayed mortality in beetles from sublethal exposure to an Argentine Bacillus thuringiensis strain.

## Key findings

- Larvae exposed to the bacterium showed reduced weight, body size, and altered nutritional reserves.
- Surviving insects exhibited significant delayed mortality, indicating long-term irreversible damage.
- The study demonstrates that Bacillus thuringiensis can reduce beetle populations through chronic effects.

## Abstract

Pest control in agriculture and livestock is a constant challenge, particularly when insect pests affect animal production systems. Although agrochemicals have traditionally been the main control strategy, environmentally friendly alternatives are increasingly needed. Bacillus thuringiensis is a widely used bacterium for insect control that acts when ingested and is valued for its safety and target specificity. However, the initial effects of concentrations that do not immediately kill insects but weaken them over time have been little studied in beetle pests. In this work, we evaluated the initial sublethal effects of an Argentine B. thuringiensis strain on Alphitobius diaperinus larvae after 14 days of dietary exposure and followed the insects throughout their life cycle to assess chronic toxicity. Larvae exposed to the bacterium showed significant reductions in weight and body size, altered nutritional reserves, and reduced survival compared with untreated individuals. Even insects that initially survived exhibited significant delayed mortality, indicating long-term irreversible damage. These results demonstrate that B. thuringiensis can reduce beetle populations not only by killing insects directly but also by weakening them through chronic effects, supporting its use as an effective and sustainable biotechnological tool for pest management.

Bacillus thuringiensis is the most extensively studied entomopathogenic bacterium worldwide; however, its sublethal effects on beetles remain poorly characterized. The aim of this study was to evaluate the toxicity of a previously selected Argentine strain of B. thuringiensis on second-instar Alphitobius diaperinus larvae during an initial 14 days of exposure, and to assess its effects at day 14 and throughout the remainder of the life cycle until death. Three treatments were applied: control, LC30, and LC50. Larval, pupal, and adult weight and body surface area were recorded, and nutritional composition was quantified using colorimetric methods. Insect status was monitored every 48–72 h over a total period of 540 days, until the death of the last individual. Among the evaluated variables, statistically significant differences between control and treatment groups were detected in larval area and weight, in the survival analysis and in two nutritional components: total protein and lipid content per larva. Overall, the results demonstrate that initial sublethal exposure to B. thuringiensis induces chronic lethal effects with delayed mortality in A. diaperinus, indicating irreversible physiological damage. This provides valuable information not only for understanding the biology of this insect but also for stakeholders involved in the productive scaling of beetle-targeted bioinputs.