Aspects of the ecology of a strawberry crop with special reference to twospotted spider mite (Tetranychidae: Acari)

Abstract

Twospotted spider mite (Tetranychus urticae Koch) and wheat bug (Nysius huttoni White) are present in Canterbury strawberry crops in potentially damaging numbers. Both can be, and are, controlled by chemicals. More wheat bugs were found under plants with dense foliage than under plants with sparse foliage, even after spraying. An improved spray application technique is suggested to overcome this problem.

On strawberry plants in their first season, twospotted spider mite attained higher numbers than on older crops, and the majority were found on mature leaves. The smaller mite populations on older crops (2+ seasons) probably resulted from the production of a feeding deterrent by the plants in reponse to mite feeding. The European harvestman, Phalangium opilio (L.), is shown to include twospotted spider mite in its diet, but does not exert a strong predatory control on mite populations. At the beginning (November and December) and end (February and March) of the berry season, mite dispersion within the plant is strongly contagious but between late December and Early February it is only weakly so. Green's coefficient of dispersion, (s² /x̄ )-1/Σx-1, proved to be a good index of contagion. In these trials twospotted spider mite did not visibly damage strawberry plant tissue or reduce berry yield in the season of infestation. However, heavy mite feeding pressure in one season will reduce the yield in the following season.

A binomial, sequential sampling plan is described. Field tests revealed no significant differences in yield between sequentially monitored plants and those conventionally treated. Fewer acaricide sprays were applied to the sequentially monitored plants.

Foliar applied fertilizers did not improve plant yield, and no changes to total or soluble leaf nutrients or carbohydrates were detected. Following fertilizer application plants treated with high potassium hosted significantly higher mite numbers than the control, plants receiving high nitrogen levels hosted significantly fewer mites.

The introduced phytoseiid, Amblyseius fallacis Garman, established quickly in the field and in the season of release reduced a twospotted spider mite population by 70%. Predator overwintering was poor resulting in no phytoseiid-related twospotted spider mite control in the plots in the season following release.

Some chemicals applied to the crop had a detrimental affect on predator mite longevity, fecundity and egg viability, but fecundity was not permanently reduced.

In Canterbury, twospotted spider mite populations do not reach yield reducing levels on strawberries in the season of infestation. Prudent monitoring, using a binomial, sequentially sampled plan will allow well timed applications of acaricides, resulting in mite populations being kept below the predetermined economic threshold. This reduction in chemical input would be compatible with the introduction of the phytoseiid, Amblyseius fallacis, without the necessity of modifying the spray schedules further.