Insects destructive to bitterbrush flowers and seeds in southwestern Idaho

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Studies were conducted to identify and evaluate the insects associated with flowers and seeds of bitterbrush. A 1.5 % lindane emulsion spray was applied to reduce insect infestation on bitterbrush shrubs but proved to be phytotoxic. A 2% DDT emulsion spray was substituted but also caused some injury.

Six hundred flowers were selected for weekly observation on 10 sprayed shrubs and the same number on 10 unsprayed shrubs. Normal-appearing seeds resulted from 21 to 25% of the flower buds on sprayed and unsprayed shrubs, respectively. Production of fewer normal seeds on sprayed shrubs indicates that the phytotoxic effect of DDT added to or exceeded the effects of insects.

Causes of damage to flowers and seeds were determined by weekly examinations and dissections until June 26, when seed drop began. During the bud and flower stages, a thrips, Frankliniella occidentalis (Pergande), caused most identifiable damage. After seeds had formed and were still attached to the shrubs, a gall midge, possibly Phytophaga sp., led other insects in amount of seed destroyed, followed by the gelechiid, Filalima sp. near sperryi Clarke. Additional, less conspicuous damage possibly may result from punctures made by sucking insects, among which the pentatomid bug, Chlorochroa sayi Stål, was identified as a common seed feeder.