Pesticide—including insecticides, fungicides and miticides—are still primarily used to 'control' insect and mite pest populations to prevent damage to the plant or fruit.
However, outbreaks of insect and mite pests following pesticide applications are also known.
The common reasons for insect and mite pest outbreaks following pesticide applications are:
- Elimination of natural enemies (parasitoids and predators) that regulate the pest populations. Without beneficial predators or parasites, pest growth is uninhibited.
- Pesticides may physically alter plant leaf surfaces making them more suitable for colonization by pests (e.g. some sunburn protection products).
- Pesticides may affect the plants nutritional quality or make changes in plant physiology which promotes growth and increases the reproduction of pests. An example would be calcium nitrate on Granny Smith, where the nitrogen enhances two-spotted mite.
- Some pesticides stimulate pests resulting in an increase in the pest's reproduction. This is referred to as insecticide hormoligosis.
Insecticide hormoligosis
Insecticide hormoligosis is not the same as resistance. Hormoligosis is a phenomenon in which reproduction is stimulated in response to exposure of the pest to the pesticide.
The cause of this stimulation is mainly due to the insecticide.
This stimulation leads to increased population growth, and leads to outbreaks of the pest.
To control this outbreak, other insecticide sprays are needed. This often results in an increase in the number of insecticide applications.
This in turn leads to greater selection pressure placed on pest populations and intensifies the potential for resistance.
Insecticide hormoligosis has the effect on insect pests by increasing reproduction rate (e.g. number of eggs laid or offspring produced per female).
Insecticide sprays causing 'flair' (population explosion) of the two-spotted mite (Tetranychus urticae) is well known. There are a number of familiar examples:
- Applications of imidacloprid increased the number of eggs laid by two-spotted spider mite females by 20% to 50%.
- Sprays of carbamates such as carbaryl, and most of the synthetic pyrethroid insecticides, are especially likely to flare mites due to the increased reproduction rate.
These insecticides actually cause the spider mites to reproduce faster.
Facts about two spotted mite (continues next issue)
See this article in Tree Fruit March 2015
