A review of insect responses to variations encountered in the managed storage environment

Highlights

• Insect responses to environmental cues are discussed in relation to natural and manipulated storage environments.

• Insect responses may be behavioural, developmental or biochemical in nature.

• Resistance or increased tolerance to control measures can arise from all three response categories.

Abstract

In combating infestation in the storage environment the choice of control method and appropriate setting of dose or exposure level needs to take into account how long it is likely to take for the applied measure to reach the site of infestation, and then the local environmental conditions of temperature and humidity before deciding how long is needed for insects to be controlled. Other factors such as the physical properties of the stored product, the nature of the packaging, the time of year and the lighting conditions, may also be of critical importance. This review examines some of the defence mechanisms insects employ and environmental niches they utilize when confronted by control measures.

The responses of insects to natural or induced environmental stimuli may be divided into behavioural and metabolic responses. Behavioural responses include orientation towards more favourable conditions or attractants, and avoidance measures such as retreating from treated surfaces into a refuge or descending a concentration gradient of a repellent gas or fumigant. Metabolic responses include aspects of increased metabolism such as when a toxicant is actively excluded from entering the body or the activation of enhanced detoxification pathways following uptake, and aspects of reduced metabolism such as the shutting down of activity, an induced delay in development prolonging a tolerant stage, or a switch to less active biochemical pathways such as anaerobiosis. The response of insects to physical and chemical gradients, treated surfaces, their temperature-related activity responses, their survival at temperature extremes and survival thresholds in toxic atmospheres are discussed in the context of pest survival and the development of resistance in the storage environment.

Introduction

A basic property of any ecosystem is the process of continual adjustment to maintain stability, a reflection of the universal law of energy transfer from a higher zone to a lower one. All living organisms have survived by adopting strategies for survival in their ecosystem. For mankind the primary strategy has been one of attempting to manage the environment; for insects and mites it has been one of keeping pace with environmental change. To this end the evolutionary trend in insects has been towards small size, a rapid breeding cycle and close links between environmental cues and specific responses. Economy in size leads to economy in the number of cells available to comprise vital organ systems and sensors. The insect nervous system is a wonder of creation in its simplicity and efficiency, enabling the most subtle of environmental stimuli received by a range of finely tuned sensors (Anderson and Hallberg, 1990, Kučerová et al., 2014, Ndomo-Moualeu et al., 2014) to elicit precise metabolic or behavioural responses that are designed to be of potential advantage to the individual.

Many factors are involved in the successful maintenance of a safe storage environment. Environmental conditions can be radically altered by the arrival or increase of insect populations. The application of control measures, whether by using a chemical pesticide or altering the physical environment by manipulation of temperature, humidity or oxygen level, brings about further changes. In any environment upper and lower temperature limits for survival exist for each pest species, in close relationship with those for humidity, individuals differing in their response. Besides taking into account the dosage or application levels needed for effective control, pest management programmes also need to consider temperature thresholds for insect development, mobility and flight which control population growth and dispersal.

The control of insect infestations in food stores has required much effort over the years and remains a major problem in the modern world. Insects and mites can tolerate some extreme conditions much better than mammals or birds. Control intervention by any procedure has the objective of rendering the environment unsuitable for survival of pest species while not endangering humans or livestock. The success of treatments is often challenged by the ability of insects and mites to respond by activating various defence mechanisms. Insects are potentially vulnerable to a wide range of adverse chemical and physical environments but the threshold levels for effective action of these may differ widely from one species to another, between the developmental stages of a particular species and even between individuals of the same species and stage.

In food stores there are limitations in applying measures to combat infestation as the food itself might be adversely affected. For bulk stored food, fumigation has long been chosen as the most practical control option for dealing with an active infestation. Here the key for success is in obtaining an adequate concentration level of the toxic gas for a sufficient time at the actual site the pests are present. This will include penetration to voids and crevices in the retaining structure where local temperature differences may combat the ingress of gas. The difficulty of obtaining an even distribution of a fumigant gas may be compared to the reaching of a target level of temperature in heat treatments, particularly in bulk commodities where heat transfer becomes a major obstacle, but also in food processing facilities where obtaining an even distribution is always a problem. Where structural heat treatments are carried out, the time taken to reach the target temperature at all points always greatly exceeds the time needed to eradicate the incipient infestation which at, say, 50 °C may be less than 1 h (Banks and Fields, 1995, Burks et al., 2000).

The ability of insects to respond to gradients of temperature, humidity and various gas concentrations enhances their chance of survival (Section 2). Other behavioural responses that can be of advantage include the seeking of locations away from treated surfaces such as crevices or food residue layers (Section 3) and activity responses linked to diurnal rhythms and aggregation cues (Section 4). Temperature controls insect mobility in addition to survival and development extremes (Section 5) and there are metabolic responses related to raised temperature and other control procedures (Section 6), the enhancement of which can rapidly lead to pest resistance (Section 7). Metabolic responses include mechanisms excluding uptake of a toxicant, the shut-down of activity or the development of an increased capacity to detoxify or eliminate a toxicant after uptake. Active sites might become desensitised and there are also developmental aspects such as the induced prolonging of a tolerant stage or entry into diapause. In this review the various responses of insects that can increase the potential for survival in managed storage environments are explored.