2015 Winter – Getting to the Root of Dieback in Lisianthus

Control insect vectors

Excess irrigation, high soil organic matter, and dirty irrigation lines could lead to buildup of algae on the soil surface. Algae are the primary food source for two groups of tiny flies that inhabit the soil surface where spores of soil-borne fungal diseases are located. These flies are known to transmit the disease spores within the crop, so control of these flies will have a significant impact on disease control.

Shore flies (Scatella sp.) look like very small domestic flies, and although their larvae are not thought to actively feed on the plant roots, their role as disease vectors  makes them pests.  If present in high numbers, shore flies can be a nuisance to workers and their feces may cause unsightly marks on the leaves.

Fungus gnats or sciarid flies (Bradysia sp.) look like small mosquitoes and are both a vector for diseases, and cause direct plant damage when their larvae feed on roots and burrow into stems.

For such small insects, they lay a lot of eggs! Females live for about 10 days and each can lay several hundred eggs during their short lives. There can be many generations in a year-round growing environment. Eggs are laid near the soil surface and hatch into small maggots in the soil. Sciarid larvae (3 mm long) are translucent with a dark black head.

Sciarid flies will feed on the roots during this period, while shore flies feed on algae and organic matter. Within two weeks, the fully fed larvae will pupate in the soil and eventually another adult will emerge and the cycle begins again.

Trap adult flies

Fortunately, adult flies of both genera are attracted to yellow sticky traps. This will provide an indication of their presence, although no guidelines exist on the economic thresholds needed before action is taken.

Flies may not be uniformly distributed in a greenhouse. Put traps at soil level and focus on areas where there may be wetter soil conditions, like near irrigation valves. If the traps are the type where parts of the sticky area can be exposed gradually in sections (and the previous week’s sticky section is covered back up again), it is possible to record the rise and fall of adult fly populations. This at least will give the grower an indication of the effectiveness of any crop protection methods.

Control methods for flies, larvae and pupae

Generally, chemical pesticides target specific stages in the life cycle of a pest. In some countries, regular soil applications of diflubenzuron have been used to good effect. Over-reliance on one active ingredient is not recommended for reasons of resistance management.  Combined programmes with Bacillus thuringiensis var israelensis help manage resistance.

A beneficial entomopath-ogenic nematode (Steinernema feltiae) is available com-mercially from Syngenta Bioline (Exhibitline). This species appears to be more effective than S. carpocapsae.  These beneficial nematodes swim in soil water and infect the larvae, causing their death from septicemia in about two days.  Entomopathogenic nematodes are likely to be compatible with both diflubenzuron and Bacillus thuringiensis.  However, check the efficacy of the isolate in low soil temperatures before use. Entomopathogenic nematodes tend to be more expensive than biopesticides made from entomopathogenic fungi.

Biopesticides such as Metarhizium anisopliae ICIPE 69 can infect all stages of fungus gnat and shore fly. Metarhizium is an entomopathogenic (insect-killing) fungus which occurs naturally in the soil, and has been formulated into a biopesticide to be used as either a soil drench (for pupae and larvae) or as a spray against adult flies.  It is easy to use, as it can even be tank-mixed with insecticides or fungicides.
Real Metarhizium 69 is Registered in several African countries and is the product of research from the International Centre for Insect Physiology and Ecology (ICIPE).