Cactus pests. Identification and choice of control

Stem and root mealybugs

Mealybugs produce a whitish waxy secretion and hide in stem recesses, between offsets or beneath tissue remnants. Inspection must reveal the insect's body or developmental stages. Natural areole wool is not evidence of infestation. Honeydew may promote sooty mould. When growth is poor and no insects are present on the stem, the roots and inside of the container are also checked. [1-WWW] [Fleischer and Schütz 1978, pp. 189–192]

Root mealybugs belonging to Rhizoecus are not aphids. EPPO places Rhizoecus falcifer in the family Rhizoecidae. The old broad terms “root aphid” or “root louse” are insufficient to identify a pest. A changed name in a book does not justify identifying every white insect in the root ball as this single species. [17-WWW] [Hinz and Abramowicz 1986, pp. 108–111]

The plant is separated from the collection, accessible insects are removed without wounding the epidermis, and subsequent growth and the root ball are inspected. Heavily infested material of little value can be removed with the contaminated substrate. Greenhouses use, among other agents, the ladybird Cryptolaemus montrouzieri, but effectiveness depends on temperature, population size and mealybug type. It is not an automatic means of controlling all root pests. [1-WWW]

The checked Polish Sanium AL label includes scale insects on ornamental plants indoors and under cover. It specifies spraying, so does not authorise drenching the root ball against root mealybugs. Identifying a “mealybug” also does not remove the need to check the application site, plant species, treatment tolerance and the entire label. [10-WWW]

Spider mites and other mites

Small areas of discolouration, a dull appearance and damage to the young stem surface may indicate mites. The two-spotted spider mite, Tetranychus urticae, is not always red. Webbing may become obvious only with a larger population. The absence of visible webbing does not rule out feeding. A magnifying glass and observation of living stages are needed. Older books grouped different mites under the common name “red spider”. [1-WWW] [Fleischer and Schütz 1978, pp. 189–190]

The predatory mite Phytoseiulus persimilis is used against spider mites under suitable conditions in protected cultivation. Thus, not every red mite is a pest. Higher humidity alone does not eliminate a population, and constantly wetting cacti can create a separate cultivation problem. Chemical protection requires confirmation that the product label includes the relevant mite. The word “insecticide” and an old sulphur recommendation do not establish this. [1-WWW] [10-WWW] [Fleischer and Schütz 1978, pp. 189–190]

Armoured scales, soft scales and aphids

Armoured scales and soft scales differ in the structure of their coverings and their biology. An immobile scale cover must be distinguished from a scar, and an organism sought beneath it. Juvenile stages can move. Aphids more often gather on tender growth, leaves of leafy cacti and buds. Honeydew, shed skins and the presence of insects help distinguish feeding from a colour change alone. [1-WWW] [Fleischer and Schütz 1978, pp. 190–192] [Seneta b.d., pp. 99–104]

Removing small colonies and isolating plants limit the source of further infestation. A contact treatment must reach the organism and may miss hidden stages. Its outcome is therefore assessed through later inspections, and treatments repeated only at the intervals and frequency permitted by the label. Mospilan 20 SP has checked minor uses in ornamental plants, but the current product is intended for professional users. [1-WWW] [10-WWW]

Fungus gnats, thrips and substrate pests

Adult fungus gnats flying above the substrate signal a need to check its moisture and the presence of larvae. Larvae are particularly important around delicate roots and seedlings. Sticky traps help monitor adults, but catching them does not prove that larvae have been eliminated. Substrate hygiene and avoiding a constantly wet surface should suit the seedlings' developmental stage without drying them out. [9-WWW] [Fleischer and Schütz 1978, pp. 192–193]

Thrips can damage young tissues and flowers, and signs of their feeding need to be distinguished from abrasions and mite damage. Root-parasitic nematodes are identified differently from insects. Swellings may justify testing roots, but do not automatically indicate one nematode species. The historically described cactus cyst nematode, Heterodera cacti, is not equated with the root-knot nematode Meloidogyne simply because both damage roots. [Fleischer and Schütz 1978, pp. 192–193] [Seneta b.d., pp. 102–105]

Infested cuttings are not introduced into a collection, and uncleaned containers from an infestation are not reused. Biological methods against larvae and nematodes are selected for the identified organism and substrate conditions. Historical fumigants and nematicides in a book are not instructions for treating a pot occupied by a plant. [9-WWW] [Fleischer and Schütz 1978, pp. 192–193]

Checking the result

A new plant is inspected before joining the collection and observed in isolation. After control measures, further inspections examine new growth, recesses and roots if these were the pest's location. Old scars remain visible even after successful treatment, so the presence of living stages and the appearance of new damage determine the outcome. [1-WWW] [Fleischer and Schütz 1978, pp. 189–193]

A product is selected for the organism, plant, cultivation site and current scope of authorisation. Concentration is not increased beyond the label in response to failure. Before another treatment, identification, plant coverage, hidden stages and the possibility of reintroduction are checked. A detailed update of product names is provided in the table of historical and current plant protection products. [10-WWW] [Seneta b.d., pp. 99–104]

Cactoblastis larvae inside stem segments

The moth Cactoblastis cactorum lays eggs in elongated rows resembling spines. Young larvae enter a stem segment together and feed inside its tissues. After feeding, they leave the plant and pupate in litter or soil. The 2000 review by Zimmermann, Moran and Hoffmann discusses this biology in the context of deliberate introductions against invasive prickly pears. [Zimmermann et al. 2000]

The same feeding behaviour can damage native and cultivated prickly pears. The paper compares successes in suppressing introduced cacti with threats to native flora where the moth has appeared. Predicting outcomes requires considering local hosts, climate and the insect's natural enemies. The article's geographical description of distribution reflects knowledge in 2000. [Zimmermann et al. 2000]