Grubs are the larval form of various beetles, including Japanese beetles, June bugs, and chafers. The months when grubs are most active depend on the specific species, as different grubs have different life cycles and peak activity periods. However, in general, grubs are most active during the spring, summer, and fall when soil temperatures are warmer.
Japanese Beetle Grubs
Japanese beetle grubs are among the most common and destructive grubs in many parts of the United States. The adult Japanese beetles start emerging from the soil and laying eggs in late spring and early summer. The eggs then hatch into grubs that feed on grass roots through the summer and early fall months. Here is a more detailed timeline for Japanese beetle grubs:
- May – June: Adult beetles emerge and begin laying eggs.
- July – August: Eggs hatch into grubs that start actively feeding on roots.
- September – October: Mature grubs move deeper into soil to overwinter.
- November – April: Grubs remain inactive in soil during winter months.
- May: Grubs resume feeding until they pupate into adults.
So in summary, Japanese beetle grubs are most active and damaging during the summer months of July and August. Monitoring for grubs and taking control measures during mid to late summer will be most effective for reducing Japanese beetle grub populations.
June Bug Grubs
June bugs, also called June beetles, have a similar seasonal cycle as Japanese beetles. The adult June beetles emerge from the soil in late spring, mate, and the females lay eggs in the soil. The eggs hatch into grubs in early summer that then feed on grass roots through the summer. Here are the key months:
- May – June: Adult June bugs emerge from soil and mate.
- June – July: Females lay eggs in soil.
- July – September: Eggs hatch and grubs feed actively on roots.
- October – November: Grubs move deeper into soil to overwinter.
- December – April: Grubs remain inactive in soil during winter.
- May: Grubs resume feeding until they pupate into adults.
June bug grubs reach peak activity during mid to late summer, similar to Japanese beetle grubs. The months of July through September are when their root feeding will be most intensive. Treating lawns and gardens for grubs in August or September will help control June bug populations.
Chafer Beetle Grubs
Chafer beetles, including European chafers, northern masked chafers, and other species, also have grubs that damage lawns. Chafer grubs have a similar developmental timeline to other common grub species:
- May – June: Adult chafers emerge from soil and mate.
- June – July: Females lay eggs in soil.
- July – September: Eggs hatch and grubs start feeding on roots.
- October – November: Mature grubs move deeper into soil.
- December – April: Grubs remain inactive in soil during winter.
- May: Grubs resume feeding until they pupate.
So like Japanese beetle and June bug grubs, chafer grubs are most active during the mid to late summer months, reaching peak populations in August and September in many regions. Targeting treatments to control grubs during mid-late summer will help reduce chafer grub damage.
Black Turfgrass Ataenius Grubs
Black turfgrass ataenius is another type of beetle that can cause grub damage. This beetle has two generations per year in warmer climates:
- Spring generation: Grubs active April – June
- Summer generation: Grubs active August – October
So this species has grubs active in both the spring and fall. Applying treatments in early spring and again in late summer may help control black turfgrass ataenius grubs in regions where they are a problem.
Scarab Beetle Grubs
There are also various scarab beetles in the Scarabaeidae family that have grubs that can damage lawns and landscape plants. This includes beetles such as green June beetles, May beetles, and oriental beetles. Their seasonal cycles can vary, but typically:
- Adults emerge in late spring or summer.
- Females lay eggs in soil mid to late summer.
- Grubs hatch within a few weeks and feed through fall.
- Grubs move deeper into soil to overwinter.
- Grubs resume feeding the following spring.
So for many scarab beetle grubs, the peak activity and root feeding occurs in fall, unlike the summer peak for Japanese beetle, June bug, and chafer grubs. August through October are key months for managing scarab grub populations in many cases.
Leatherjackets and Cranefly Larvae
While not technically grubs, the larvae of craneflies and other flies in the family Tipulidae can cause damage similar to grub feeding. The larvae are often called leatherjackets. Their seasonal cycle includes:
- Adults emerge and lay eggs in late summer or fall.
- Eggs hatch within a few weeks.
- Larvae feed on roots through fall and winter.
- Larvae mature the following spring.
- Pupation occurs in spring or early summer.
So the leatherjacket larvae are most active and damaging during the cooler months, primarily fall, winter, and spring. Treating lawns in September or October can help reduce leatherjacket populations.
Summary of Months When Common Grubs are Most Active
| Grub Pest | Months of Peak Activity |
|---|---|
| Japanese Beetle | July – September |
| June Bug | July – September |
| Chafer Beetle | July – September |
| Black Turfgrass Ataenius | April – June and August – October |
| Scarab Beetle | September – November |
| Leatherjackets/Cranefly Larvae | September – May |
As shown in the table, grubs reach peak activity at different times depending on the species. However, late summer to early fall is an overall active period for many grub species. Targeting control treatments for August through October will help reduce populations of most common grub pests.
Conclusion
Grubs can be a serious nuisance pest for lawns and gardens during certain months. Different grub species have varying peak activity periods based on their life cycles and number of generations per year. However, many common grub pests are most active and destructive during the mid to late summer months, especially July through September. Treating lawns or applying preventative measures during this mid-late summer window can help reduce grub populations and damage. Additionally, late fall applications can help control species like black turfgrass ataenius or leatherjackets that are active in autumn and early spring. Knowing the key months when different grub species are prevalent can help focus control efforts for maximum effectiveness. With proper timing of treatments, grub management can be much more successful.
