What are beneficial nematodes?

Nematodes are microscopic roundworms that live in soil all over the world. Some species are entomopathogenic, meaning they actively hunt and kill insect larvae. Entomopathogenic nematodes have been used as beneficial biocontrol agents for decades across hundreds of crops and ornamentals. The use of nematodes in agriculture has been well regarded because it is safe, biodegradable, and compatible with both organic and conventional farming. Steinernema carpocapsae are one of the most used entomopathogenic nematodes used in agriculture.

Nematodes bursting out of a dead fly larvae — Figure 1. *Steinernema carpocapsae* nematodes bursting out of a dead fly larva.

Why Use Nematodes in Mushrooms?

Beneficial nematodes target larvae in the compost and casing where they are most abundant. These nematodes feed exclusively on insect larvae and do not cause damage to mushroom mycelium or fruiting bodies. Applications leave behind no residues and there is no re-entry interval necessary as entomopathogenic nematodes are animals, not chemicals, and pose no threat to human health.

Why S. carpocapsae (Not just S. feltiae)?

Mushroom farms have been using Steinernema feltiae nematodes to control fungus gnat (Sciarid; Lycoriella ingenua) larvae in the compost, but these nematodes are ineffective at killing phorid larvae. Recent work by the Penn State Mushroom Fly Research Team discovered that S. carpocapsae nematodes infect and kill phorid fly larvae in compost (Figure 1). In on-farm trials, rooms treated with S. carpocapsae had an average reduction of 36% in Phorid larval survival compared to untreated rooms (Figure 2).

Comparison of windows in untreated and nematode treated growing rooms — Figure 2. Flies on windows during harvest on equivalent days in untreated growing rooms (left) compared to rooms that received nematode treatments (right).

How long do S. carpocapsae nematodes live in the compost?

Nematodes are notoriously fickle and short-lived organisms. They are particularly sensitive to compost moisture content (too high or too low) and UV light. it was expected that S. carpocapsae nematodes would be longer-lived in mushroom compost given the minimal light and adequate moisture. However, research by the Penn State Mushroom Fly Team discovered that 50% of S. carpocapsae nematodes applied to the compost die within 4-5 days of application. This means that a single application cannot protect the room for an entire crop cycle and repeated applications in short intervals (4-5 days) maintain effective populations.

How should I apply S. carpocapsae nematodes to control mushroom phorid flies?

Luckily, nematodes can be added to a water tank as a powder and can be routinely watered onto the compost. The Penn State Mushroom Fly Research Team has developed new protocols to optimize the use of S. carpocapsae nematodes.

How does mixing with Chlorine impact Beneficial Nematodes?

Nematodes mixed in a 100ppm chlorine solution for about 1 hour showed a 10% decrease in efficacy. For best results, apply nematodes using chlorine-free water whenever possible. However, if chlorine must be used nematodes will still provide suppression of phorid fly larvae in mushroom compost.

How often should I apply S. carpocapsae nematodes?

During moderate infestations, apply 125,000 infective juveniles (IJs) of S. carpocapsae per square foot of growing space just before casing (4 bags of 250 million IJs for 8,000 sq ft). If the compost is not being watered at that time, the nematodes should be applied to the casing layer as soon as possible.

Then, apply 62,500 IJs per square foot every 4–5 days during case hold, following the normal watering cycle (2 bags of 250 million IJs for 8,000 sq ft). If fly populations remain high in that room, an additional application of 125,000 IJs per square foot should be applied through the irrigation system after the first break has finished being picked.

Additionally, if fly numbers are low, growers may choose to only apply nematodes before the compost is cased. Growers can make additional nematode applications in response to spikes on the fly monitors (1-2 days after the spike).

In on-farm trials, growing rooms that followed this program averaged 36% lower phorid larval survival (measured using the emerger:invader ratio) compared to growing rooms not using nematodes when other management practices were similar (Figure 3).